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Preface
This presentation is submitted as the summer dissertation project as the major
prerequisite for the Master of Science degree in Chemical Engineering at the
University of Nottingham, United Kingdom. Both laboratory experiments and thesis
construction were conducted under the full supervision of Dr Buddhi
Hewakandamby in the Department of Chemical and Environmental Engineering,
University of Nottingham between June 2010 and September 2010. I hereby declare
that this thesis is a work of my own and has not been previously submitted by me at
another institution for any degree. I also cede copyright of the thesis in favour of
the University of Nottingham, United Kingdom. This piece of work has not been
shared or used by any other persons other than me and my project supervisor, Dr
Buddhi Hewakandamby. Where I have used and quoted the work of other people,
they have been acknowledged and detailed explicitly throughout the entire
presentation.
____________________
Chan Yung Khiong
17th September, 2010
1
Abstract
The drive for the study of droplet coalescence stretches across numerous liquid-
liquid emulsion engineering applications including food, lubricants, pharmaceutical
and oil industry. Such research is also very beneficial for the environmental sectors.
However, liquid-liquid emulsion is complex in nature and its full comprehension is
yet to be achieved. To help break this barrier a step further, emulsion was studied
in this experiment on a smaller scale before escalating to more complicated
conditions. The experiment was composed of the investigation of binary
coalescence dynamics of a pair of kerosene droplets forming at low inlet flowrate
through capillaries in reverse osmosis (RO) water. Throughout the whole
experiment, the evolution of the kerosene droplet coalescence process was
recorded using a high speed video camera and the mean binary coalescence times
were calculated for different conditions. Various concentrations of glycerol were
added into the system to alter the interfacial tension and hence surface free energy
between the kerosene droplets and RO water. Increasing the glycerol concentration
decreased the interfacial tension and hence, surface free energy. Two methods
were employed during the experiment: (i) Non-flow induced and (ii) Flow induced.
For non-flow induced method, the mean binary coalescence time increased with
decreasing surface free energy which was in good agreement with most of the
literature review while the opposite result was found for the induced flow method.
Without the presence of glycerol, the mean coalescence time was higher for
induced flow method than non-induced flow method which drew a conclusion that
an application of force pressing the droplets together yielded higher stability.
However, the addition of glycerol reverses this effect and as a consequence, with
increasing glycerol concentration, the induced flow method reduces the stability of
the droplets and thus coalescence time was lower in comparison with those
obtained from the non-induced flow method. The nature of glycerol on kerosene
droplets when force is not injected is to be investigated further to gain more insight
2
in coalescence aspect. Sceptical results were found on the non-induced flow method
which could be due to errors during experimentation. Further repetitions on the
non-flow induced method are to be performed.
3
Acknowledgement
I would like to take this opportunity to first thank Dr Buddhi Hewakandamby for his
utmost kind assistance and care in facilitating the progress of the experiment and
my thesis. Along the way, he told various motivational short stories of his own life
and shared his wisdom which filled the atmosphere with more than just pure
academics. He was attentive to the various problems encountered for the past 3
months and making matters unworkable seem hopeful by nourishing me with
logical ideas.
I would also like to thank my parents who have been giving me the essential
support and motivation to reach the few final steps of this project with a positive
academic attitude. Without their support, such completion might not have been
successful.
A word of thanks also goes out to Katerina Loizou who shared the same experiment
throughout to cut time the entire summer instead of alternating turns. Together we
worked as a team and she made the long and tedious laboratory hours a bit more
enjoyable through exchanging personal life experiences and cultures. I would like to
thank the laboratory technicians, Phil, Mick, Terry, Fred, Marion and Vikki for getting
the materials I need without any difficulties and hesitations. Appreciation also goes
out to Dr. David Hann and Andy Matthews who made it possible for me to use of the
high speed camera which is the one of the primary elements of the experiment. I
would also like to express my gratitude to Aime for lending me the wonderful
creation of his thermocouple. To Natalia and Anna, thanks for lending the pump.
Also thanks to Nazrul for sharing the bits of information.
Chan Yung Khiong
17th September 2010
Contents
4
Preface.........................................................................................................................1
Abstract........................................................................................................................2
Acknowledgement........................................................................................................4
List of tables and figures..............................................................................................5
1 Introduction...........................................................................................................5
1.1 Quality energy source demand...........................................................................................................................5
1.2 Environmental issues..............................................................................................................................................5
1.3 Lifestyle.........................................................................................................................................................................5
1.4 Relationship between this study and the motivations.............................................................................5
2 Literature review...................................................................................................5
2.1 Mechanisms for two phase separation............................................................................................................5
2.2 Interfacial dynamics of droplet coalescence.................................................................................................5
2.2.1 Interfacial rigidity...................................................................................5
2.2.2 Interfacial mobility.................................................................................5
2.3 Role of surfactants and other surface active agents..................................................................................5
2.3.1 Emulsifiers.............................................................................................5
2.3.2 Demulsifiers...........................................................................................5
2.3.3 Other surface active agents...................................................................5
2.4 Effect of interfacial tension on coalescence...................................................................................................5
2.4.1 The Marangoni Effect.............................................................................5
2.5 Role of interfacial repulsive and attractive forces......................................................................................5
2.6 Effects of hydrodynamics on the coalescence..............................................................................................5
2.7 Drop size on coalescence.......................................................................................................................................5
2.8 Thermodynamics......................................................................................................................................................5
3 Aims and objectives...............................................................................................5
3.1 Aims................................................................................................................................................................................ 5
3.2 Objectives..................................................................................................................................................................... 5
4 Methodology..........................................................................................................5
4.1 Materials....................................................................................................................................................................... 5
4.1.1 Rig.........................................................................................................5
4.1.2 Rig core..................................................................................................5
4.1.3 Attachments..........................................................................................5
5
4.1.4 Delivery..................................................................................................5
4.1.5 Image capturing and video recording....................................................5
4.1.6 Lighting system......................................................................................5
4.1.7 Temperature measurement...................................................................5
4.1.8 Chemicals..............................................................................................5
4.1.9 Others materials....................................................................................5
4.2 Experimental procedures......................................................................................................................................5
4.2.1 Cleaning.................................................................................................5
4.2.2 Set up of experiment.............................................................................5
4.2.3 Method of data acquisition.....................................................................5
4.2.4 Interfacial tension measurements – Pendant drop method....................5
4.2.4.1 Equations....................................................................................................5
5 Results and Discussion..........................................................................................5
5.1 Time evolution of the coalescence process....................................................................................................5
5.2 Effect of induced flow and glycerol concentration.....................................................................................5
6 Conclusion.............................................................................................................5
7 Future work...........................................................................................................5
7.1 Experiment inefficiency and improvements.................................................................................................5
7.1.1 Rig body.................................................................................................5
7.1.2 Rig core..................................................................................................5
7.1.3 Pumps....................................................................................................5
7.1.4 Thermocouple........................................................................................5
7.1.5 Lens.......................................................................................................5
7.2 Environment............................................................................................................................................................... 5
7.2.1 Vibrations...............................................................................................5
7.2.2 Temperature..........................................................................................5
7.3 Data collection............................................................................................................................................................5
7.4 Image processing.......................................................................................................................................................5
7.5 Batch saturated..........................................................................................................................................................5
7.6 Overnight......................................................................................................................................................................5
8 Bibliography..........................................................................................................5
6
List of tables and figures
Table 1. Calculation of kerosene drop dimensions.....................................................5
7
Figure 1. Water in oil emulsion under macroscopic inspection (Mullin, 2006)............5
Figure 2. Oil spillage in the sea (Solutions, 2002)......................................................5
Figure 3. Emulsion used in manufacturing drugs (Pharma, 2009)..............................5
Figure 4. Sequential mechanisms governing phase separation. Adapted from
Paunov (2008)...........................................................................................................5
Figure 5. Ostwald Ripening involving droplet’s molecules (Bfigura, 2007).................5
Figure 6. Effect of adding surfactants on interfacial tension......................................5
Figure 7. Picture showning Marangoni stress in a wine glass.....................................5
Figure 8. Successive stages of the Marangoni flow in a wine glass............................5
Figure 9. Total free energy required for coalescence (Paunov, 2008)........................5
Figure 10. Components of the resultant net energy..................................................5
Figure 11. Rig body with cover, protrusion and walls.................................................5
Figure 12. Laboratory experiment set up...................................................................5
Figure 13. Dissembled rig core with cover.................................................................5
Figure 14. Rig core with needles. Figure 15.Top view of rig core.................5
Figure 16. Rig core immobilized by Perspex Glass.....................................................5
Figure 17. Rig core in rig body. Figure 18. Capillary attachments................5
Figure 19. Pump 1. Figure 20. Pump 2...................................5
Figure 21. Pump 1 calibration graph..........................................................................5
Figure 22. Pump 2 calibration graph..........................................................................5
Figure 23. High speed camera Phantom v12.1..........................................................5
Figure 24. High speed camera bird eyes view...........................................................5
Figure 25. Dedocool lighting system..........................................................................5
Figure 26. Thermocouple...........................................................................................5
Figure 27. LabVIEW temperature measurement........................................................5
Figure 28. Thermocouple simulation..........................................................................5
Figure 29. Temperature calibration equation.............................................................5
Figure 30. Schematic diagram of rig..........................................................................5
Figure 31. Image of the kerosene droplet required for the calculation of the
interfacial tension......................................................................................................5
Figure 32. Dimensions of the kerosene drop needed to be determined for interfacial
tension determination................................................................................................5
Figure 33. Droplets evolution at 0s............................................................................5
Figure 34. Droplet evolution at 0.256s.......................................................................5
Figure 35. Droplet evolution at 1.101s.......................................................................5
Figure 36. Droplet evolution at 1.726s.......................................................................5
Figure 37. Droplet evolution at 1.731s.......................................................................5
Figure 38. Droplet evolution at 1.732s.......................................................................5
8
Figure 39. Droplet evolution at 1.733s.......................................................................5
Figure 40. Droplet evolution at 1.734s.......................................................................5
Figure 41. Droplet evolution at 1.735s.......................................................................5
Figure 42. Droplet evolution at 1.736s.......................................................................5
Figure 43. Droplet evolution at 1.740s.......................................................................5
Figure 44. Droplet evolution at 1.741s.......................................................................5
Figure 45. Droplet evolution at 1.742s.......................................................................5
Figure 46. Droplet evolution at 1.743s.......................................................................5
Figure 47. Droplet evolution at 1.753s.......................................................................5
Figure 48. Droplet evolution at 1.754s.......................................................................5
Figure 49. Droplet evolution at 1.757s.......................................................................5
Figure 50. Droplet evolution at 1.758s......................................................................5
Figure 51. Droplet evolution at 1.797s.......................................................................5
Figure 52. Droplet evolution at 1.820s.......................................................................5
Figure 53. Interfacial tension per area against % concentration v/v glycerol.............5
Figure 54. Induced flow cumulative percentage against coalescence time graph for
0% glycerol................................................................................................................5
Figure 55. Induced flow individual percentage against coalescence time graph for
0% glycerol................................................................................................................5
Figure 56. Induced flow cumulative percentage against coalescence time graph for
0.5% glycerol.............................................................................................................5
Figure 57. Induced flow individual percentage against coalescence time graph for
0.5% glycerol.............................................................................................................5
Figure 58. Induced flow cumulative percentage against coalescence time graph for
1.0% glycerol.............................................................................................................5
Figure 59. Induced flow individual percentage against coalescence time graph for
1.0% glycerol.............................................................................................................5
Figure 60. Induced flow cumulative percentage against coalescence time graph for
5.0% glycerol.............................................................................................................5
Figure 61. Induced flow individual percentage against coalescence time graph for
5.0% glycerol.............................................................................................................5
Figure 62. Induced flow cumulative percentage against coalescence time graph for
10.0% glycerol...........................................................................................................5
Figure 63. Induced flow individual percentage against coalescence time graph for
10.0% glycerol...........................................................................................................5
Figure 64. Induced flow mean coalescence time against glycerol concentration.......5
Figure 65. Non-induced flow cumulative percentage against coalescence time graph
for 0% glycerol...........................................................................................................5
9
Figure 66. Non-induced flow individual percentage against coalescence time graph
for 0% glycerol...........................................................................................................5
Figure 67. Non-induced flow cumulative percentage against coalescence time graph
for 0.5% glycerol........................................................................................................5
Figure 68. Non-induced flow individual percentage against coalescence time graph
for 0.5% glycerol........................................................................................................5
Figure 69. Non-induced flow cumulative percentage against coalescence time graph
for 1.0% glycerol........................................................................................................5
Figure 70. Non-induced flow individual percentage against coalescence time graph
for 1.0% glycerol........................................................................................................5
Figure 71. Non-induced flow cumulative percentage against coalescence time graph
for 5.0% glycerol........................................................................................................5
Figure 72. Non-induced flow individual percentage against coalescence time graph
for 5.0% glycerol........................................................................................................5
Figure 73. Non-induced flow mean coalescence time against glycerol concentration.
..................................................................................................................................5
1 Introduction
Emulsions are crudely dispersions of fluids in another immiscible fluid phase. In the
context of this project, only liquid-liquid emulsions are considered. Such typical
occurrences are complex in nature and they appear in various aspects of
engineering and science applications. For this reason, emulsion remains worthy to
be understood through research to yield benefits in the world of engineering and
science. In the next few sub-chapters, several major emulsions related industries
are elaborated.
10
1.1 Quality energy source demand
To date, exploitation of crude oil to meet the insatiable global energy demand in the
petroleum industry has advanced to the point where the era of large fields with both
high quantity and satisfactory quality crude oil is at the brink of exhaustion. The
most crucial issue when focusing on existing large oilfields is to increase the
recovery rate. This can however be connected with typical flow assurance problems
which is often related to the multiphase transportation issues of the crude oil. At
many oil fields, the co-production of water from wells is in most cases substantial
which can be up to an extent of 50% to 70% during extraction (Ali & Alqam, 2000).
It is also known that in the petroleum industry, the issue of more than 95% of the
crude oil emulsions which are in the form of stable water-in-crude oil emulsion
exists as shown in Figure 1 (Xia et al., 2004) (Gaaseidnes & Turbeville, 1999).
Through the extensive experiences in the petroleum industry, water-in-crude oil
emulsions has nevertheless been seen to be encountered at many stages during
drilling, producing, transporting and processing of crude oils. Such emulsions are
extremely stable and viscous materials that increase pumping and transportation
expenses, cause corrosion of pipes, pumps, production equipment and distillation
columns and poison downstream refinery catalysts (McLean et al., 1998) which adds
to the already costly extraction process due to rise in demand (EIA, 2010) and oil
resource exhaustion (Petroleum, 2007). Recent studies also have shown that a
significant portion of the operating cost associated with the daily oil production is
spent on the oil-water separation (Li & Gu, 2005) (Crossdale et al., 1999). Moreover,
the performance and properties of crude oils are to a great degree determined by
the presence of water (Poznyshev, 1982) (Likhterova et al., 2003). Minimizing the
water levels in the crude oils by oil dehydration method can reduce pipeline
corrosion dramatically, improve crude oil quality and maximize pipeline usage for
transportation (Xia et al., 2004). Crude oil free from water is mandatory for pipeline
flow and refinery operations. The emulsion ageing tends to increase its stability; the
11
breaking must be carried out as soon as possible in the production facility close to
the well (Kilpatrick & Spiecker, 2001).
Figure 1. Water in oil emulsion under macroscopic inspection (Mullin, 2006).
1.2 Environmental issues
In both offshore and onshore oil processing, one of the most challenging
environmental problems today is the effective removal of stable oil deposits from
water sources such as reservoirs, oceans and seas which is essentially necessary for
industrial and domestic consumption. These are known as oil-in-water emulsions
stabilised by naturally occurring surfactants. Offshore oil spillage as shown in Figure
2, runoff oil released during oil well extraction and oily wastewater from other
industries such the recycling industry and water desalination are undesirable events
contribute to this concern. The oil content which has low biodegradability must be
separated as much and quick as possible from the aqueous phase to prevent mass
ecological problems.
12
Figure 2. Oil spillage in the sea (Solutions, 2002).
1.3 Lifestyle
Daily products in the olden days have shifted from simple to sophisticated genres
with variety if improvements in the new millennium. As such, the demand for such
astonishing materials has increased tremendously. One of the sources for such
complexity often can be subjected to the incorporation of numerous components
into a product so that the benefits of each constituent may be garnered
simultaneously for product enhancement. This is often desirable due to the
resultant change in tactile properties, for instance in mayonnaise and other foods
which are oil-in-water emulsions.
Alternatively, there may be a more sophisticated purpose such as when an oil-
soluble material must be delivered to an aqueous environment. In this case, the
material may first be dissolved in an organic liquid which in turn is dispersed in an
aqueous medium to create the emulsion. Such schemes find application in drug
delivery as shown in Figure 3. To create dispersion they must inherently be
insoluble in one another and so a third component usually must be added to
prevent them from phase separating. Other products include, lubricants, cosmetics
and paints (Venugopal & Wasan, 1983). Such technology is often the correct entry
for emulsion applications. A difficulty that arises in such applications however is the
incompatibility of the materials making up the emulsion.
13
Figure 3. Emulsion used in manufacturing drugs (Pharma, 2009).
1.4 Relationship between this study and the motivations
In a nutshell, there is an endless list of activities that make use of the emulsion
technology to function in the designated way. While effective separation of
unwanted oil dispersions from water and water from crude oil has long been a
challenging technical task in the petroleum, oil spills and water industries, such
emulsions or dispersions are useful in the food and pharmaceutical industries.
At present, there exist several oil-water separation methods which are the current
state of the art technologies. However, most of them have rather limited
applications in separating oil form the produced fluids due to their large capital and
or operating costs, low separation efficiency and or slow separation process – long
residence time. Therefore, some significant improvements in oil water separation
techniques are still required in practice. This applies to the food and pharmaceutical
and other industries as well to improve the quality of the products.
During these industrials operations, the emulsion drops are subjected to flow fields
which may cause them to break or coalesce. The simultaneous occurrence of the
two phenomena controls the drop size distribution, which in turn has significant
effect on the processing conditions and characteristics of the product. The addition
of components such as surfactants can cause the inversion of different phases.
Moreover, the lifetime of emulsions may vary from seconds to minutes to hours to
14
days to weeks to months and to years depending the nature of the surfactants,
nature of both fluids and their volume ratio. Despite the large amount of work
devoted to this issue, predicting the emulsions lifetime still remains a challenge.
It is necessary to go back to the science of the process to understand the emulsion
stability mechanisms from both macroscopic and microscopic points of view.
Understanding the underlying principle mechanisms of one type of emulsion with
regards to its stabilization strength and formation can improve the design of the
phase separator equipments and demulsifying/ emulsifying agents or techniques so
that effective separation of the dispersed phase or emulsification can be made
successful. Generally, an improved understanding of coalescence behaviour is a
prerequisite for better engineering and thus would potentially allow one to improve
formulations, quality, stability, process yields and stability during product shelf-life,
through engineering of interfaces, bulk rheology, and process.
In particular, it is the interfacial dynamics between the droplets that is of great
interest in this project. The interfacial dynamics involve the mechanisms that
governed the probability of coalescence between the droplets and are the very first
several things to look at, in the beginning of an emulsion research.
2 Literature review
In the field of liquid-liquid emulsion, the behaviour of coalescence in many forms
such as between a pair of drops, a pair of bulk phases, many drops, drop and bulk
phase have been increasingly popular and continuously studied theoretically and
experimentally in detail for more than 50 years now. Investigators have conducted
15
experiments in many possible ways with either pure chemicals or in the presence of
added components that affect the interfacial properties. The effects of varying
interfacial tension, hydrodynamics and other conditions such as temperature and
pH levels were investigated to confirm the proposed coalescence mechanisms and
to characterize the coalescence behaviour of various liquids systems.
2.1 Mechanisms for two phase separation
From the visible perspective regardless of the presence of surfactants which is
discussed later in Section 2.2, an emulsion will eventually phase separate given
sufficient time without introducing external forces or energy. Figure 4 shows the
generally accepted time evolution of an emulsion breaking occurrence (phase
separation). The four mechanisms responsible in destabilizing the emulsion:
Flocculation – The droplets form aggregates of two or more drops. The inter-
particular distance between the droplets is strongly diminished due to a net
attraction between the droplets.
Creaming – Due to the difference in the density between the dispersed phase
and continuous phase, this results in the formation of a dispersed phase
concentration gradient in the mixture.
Coalescence – This significant occurrence involves specifically the amalgamation
of two drops.
Ostwald ripening – In more microscopic terms as shown in Figure 5, the diffusion
of droplet’s molecules through the medium causes small droplets to decrease in
size and disappear while large drops grow. It was found that this process
actually takes place prior to coalescence (Schmitt & Leal-Calderon, 2004).
16
Figure 4. Sequential mechanisms governing phase separation. Adapted from
Paunov (2008).
Figure 5. Ostwald Ripening involving droplet’s molecules (Bfigura, 2007).
However with respect to the field of this research project, only the coalescence
stage is of concern. Therefore, the interaction between the two droplets at very
close proximity to each other especially at dynamics at the interface is reviewed in
this section.
2.2 Interfacial dynamics of droplet coalescence
According to an early theory developed by Smoluchowski (1917), two drops merge
and coalesce immediately upon collision straightforwardly entailing that there is no
resistance to coalescence imparted by the thin liquid film trapped between the
drops (Mitra & Ghosh, 2007). However, in practice, this theory is only applicable
only when the continuous phase has very low viscosity and there is no surface
active species present in the medium that can stabilize the drops. Also, the
aforementioned theoretical work by Smoluchowski is only applicable to non-
deformable particles (Danov et al., 1993). However, many studies now also include
deformable droplets where the viscosity of the dispersed phase or droplets is low.
17
It is generally accepted that the coalescence behaviour of approaching emulsion
droplets is said to be controlled by the dynamics of the film between their surfaces.
Zapryanov et al (1983) reported that for both types of oil/water emulsion,
coalescence between two droplets occurs in three specific steps:
(1) Approach of the droplets through the continuous phase where the droplets
eventually come into contact driven by the applied forces of the flow field or
induced flow;
(2) Deformation of the droplets by flattening of their contact surfaces to form a thin
film between them which starts to drain through a process commonly known as film
drainage;
(3) Thinning of this film to a critical thickness below which the droplets coalesce due
to intermolecular forces like van der Waals forces become dominant and cause the
film to rupture followed by the formation of the bridging or merging of the two
droplets.
Danov et al (1993) and Aarts and Lekkerkerker (2008) supported the sequential
mechanisms listed above and investigated the dependence of external forces on
the interaction of the droplets such as van der Waals, electrostatic, hydrodynamics
and also the energy for the deformation of the droplets which are also studied by
many researchers which are discussed later. Auflem (2002) also reported that the
main mechanisms that influence coalescence are film drainage and film rupture
which are considered acceptable mechanisms in the vast literature. Despite such
logical hypothesized mechanisms, the coalescence probability depends only on the
details on the drainage of the film between the droplets.
However, with regards to the film thinning occurrence, many have thought that this
film should be nevertheless ‘invisible’ to the naked eye until Pu and Chen (2001)
investigated on the surprising phenomenon of ‘jumping’ coalescence of two largely
separated water droplets of unequal size under microgravitation where essentially
18
no external force acts which defies largely against the film thinning theory. In their
work, coalescence was successful even when the continuous phase film was seen to
be very obviously larger than even the size of the drops. It was concluded that the
coalescence driving force within the liquid drops under microgravitation to be
responsible for the jumping phenomenon but no further investigation was
attempted at such unusual phenomenon under microgravitation. However, quite
recently, results obtained from a study by Kim and Longmire (2009) might be able
to lead clues to the coalescence under microgravitation. They concluded the vortex
rings within colliding drops must be oriented such that they induce streaming flow
with a strong component toward the centerplane and the drops must collide with
sufficient inertia such that they deform significantly, increasing the velocity
magnitude in the streaming flow and also if the large velocities in the streaming
flow approach the center plane, then they can induce a faster outflow in the thin
film between the drops.
2.2.1 Interfacial rigidity
Often in many emulsion studies, surface rigidity plays as one of the most important
dynamics that governs the coalescence probability (Groeneweg et al., 1993).
Surface rigidity reflects the tendency of the interface to deform as the two drops
approach each other. In the aspect of forces in the vicinity of the interacting drops,
it is obvious that the separation distance between the two facing surfaces at which
the deformation takes place, increases and decreases with attraction and repulsion
respectively i.e. the higher the attractive force between the droplets, the larger the
distance at which deformation starts to occur. Perhaps, the drainage of the thin film
may be analyzed by means of a force balance comprising the force exerted on the
droplets by the flow field and the resistance to drainage due to the viscous flow in
the film.
Ivanov et al (1985) observed that often in the beginning of the deformation the
drop caps acquire a bell-shaped form called a dimple. Ivanov and Dimitrov (1988)
19
added that this film thins with time and at a certain critical thickness ruptures as
observed by Zapryanov et al (1983). Chesters (1991) concluded that the rigidity of
the colliding interfaces of the drops governs the thinning rate of the liquid film to its
critical thickness. Specifically, the interfacial rigidity determines how much the
colliding interfaces will flatten and is influenced by drop size and interfacial tension.
If the film can be thermodynamically stable, the thinning stops at the equilibrium
thickness where the disjoining pressure in the film equilibrates the capillary
pressure in the drops. In the case of thermodynamically unstable films which always
rupture: then the film lifetime depends mainly on the rate of thinning and the
critical thickness. If the interface is undeformable, which means that the pressure in
the liquid film is lower than the Laplace pressure inside the drops, the film can
easily be drained.
2.2.2 Interfacial mobility
The interfacial rigidity is often related to the interfacial mobility. The flow in the film
is coupled to the flow inside the approaching particles via the mobility of the
interfaces, which is especially relevant for pure fluids. Abid and Chesters (1994)
observed that the interfacial mobility is governed by the tangential stresses exerted
on the film by the drops. Saboni et al (1995) found that this tangential stress
depends on the viscosity ratio of the dispersed and continuous phases. There are
three types of interfacial behaviour during coalescence identified based on the
dispersed/continuous phase viscosity ratio: (1) immobile interfaces, when the
dispersed phase viscosity is much higher than the continuous phase one, or when
surfactants are present that retard the drainage of the liquid film; (2) fully mobile
interfaces, when the continuous phase viscosity is very large compared to that of
the dispersed phase; (3) the most commonly encountered partially mobile
interfaces, when the viscosity ratio is moderate (Chesters, 1991).
20
2.3 Role of surfactants and other surface active agents
Previously the information garnered only indicated the interfacial dynamics without
specifying whether the dynamics of the coalescence behaviour are the
consequences of the addition of surfactants or otherwise. The role and types of the
surfactants should be clearly understood beforehand.
According to Bancroft (1913) rule, a liquid containing the surfactant becomes the
continuous phase. In other words, when dispersing equal volumes of liquids for
instance oil and water, the emulsion obtained is oil-in-water if the surfactant is more
soluble in water and vice versa.
Surfactants are substances that alter the interfacial tension between the two or
more phases of liquids and by altering the interfacial tension; this can influence the
coalescence behaviour very much which is discussed in Section 2.4. However, it is
important to distinguish that there are two types of surfactants – emulsifier and
demulsifier. Many articles have confused readers where the authors did not
specifically draw the line between emulsifiers and surfactants; they indicate that
surfactants are emulsifiers and the other way around. Perhaps it is crucial to review
the simple definitions of the types of surfactants available in the chemical industry.
It remains very important to identify the surfactant type because different
categories have different effects (Pichot et al., 2010).
Surfactants commonly are divided into two families – (i) emulsifiers and (ii)
demulsifiers. An emulsifier when added into a mixture of oil and water stabilizes the
emulsion. Hence, emulsions are dispersions of two immiscible liquids, kinetically
stabilized by the action of a surfactant or specifically an emulsifier. The function of
the demulsifier can only work to break the emulsion which is already stabilized. In
other words, demulsifier can only be added to a mixture which is already emulsified
or stabilized by natural occurring or induced emulsifiers. Hence demulsifiers are
emulsion breakers that separate for instance, crude oil emulsion into distinct oil and
21
water phases (Mikula & Munoz, 2000). However, demulsifiers can also be added to
emulsion free of emulsifiers to facilitate phase separation.
2.3.1 Emulsifiers
In the absence of emulsifiers, drops aggregate rapidly as a consequence of the van
der Waals force. In their presence, the emulsifier particles adsorb to the interface of
the drops creating a barrier that decelerate aggregation and Ostwald ripening
preventing the higher probability of drops coalescing. Hence, they favour the
occurrence of immobile interfaces, delaying the drainage of the intervening film
between flocculated drops. Depending on their interfacial properties, these films
can drain and rupture after a period of time significantly longer than that of without
the presence of emulsifiers or, remain stable for long periods of time (Vrij, 1964)
(Vrij & Overbeek, 1968). Conversely, emulsifiers lower the interfacial tension of
these films, favouring the appearance of surface oscillations and holes which
happens to reduce the drainage rate (Ivanoc et al., 1999). Hodgson and woods
(Hodgson & Woods, 1969) investigated the effect of additions of SDS on the
coalescence of oil toluene drops in water. Most of the surfactants used by the
aforementioned researchers previously are regular emulsifiers which can be found
in a handbook by Mukerjee and Mysels (1971). Chen and Pu (2001) investigated on
the addition of an emulsifier concentration and found out that the binary
coalescence time was increased under microgravitation. They concluded that a film
is apparent when sodium sulphate dodecyl (SDS) was present at a higher level
concentration. This film prevents the coalescence of the droplets. There are other
works that used regular emulsifiers to investigate on the coalescence behaviour of
various liquid systems and yielded similar outcome (Bazhlekov et al., 2000)
(Chesters & Bazhlekov, 2000) (Binks, 2002) (Yeo et al., 2003) (Chevaillier et al.,
2006) (Giribabu & Ghosh, 2007) (Sacanna et al., 2007).
22
2.3.2 Demulsifiers
Based on the amount of work done on liquid coalescence using surfactants,
compared with emulsifiers, the work done using demulsifiers was not as extensive.
Few researchers developed similar scientific hypothesis that supported the
replacement of the emulsifiers by demulsifiers at the interfaces and the
consequence of this is the increased facilitation of coalescence. Demulsifiers are
surfactants found to develop high surface pressure at the crude oil-water interface
and promote phase separation (Bhardwaj & Hartland, 1994). It was known that
during the demulsification process, the effective demulsifiers replace the emulsifiers
adsorbed on the oil water interface (Kang et al., 2004). Deng et al (2005) added
that the presence of demulsifiers results in replacement of rigid film of natural
crude oil emulsifiers by a film which is conducive to coalescence of water droplets.
When two drops approach each other due to external forces, the thickness of the
intervening continuous phase film decreases. The shear stress associated with
drainage tends to concentrate the emulsifier molecules outside the film and their
concentration inside the film is lowered. Thus an interfacial tension gradient is set
up with high interfacial tension inside the film and low tension outside the film. Such
phenomenon is discussed in more detail in Section 2.4.1. The demulsifier molecules
migrate from the surface adjacent to the interface where the demulsifier molecules
are adsorbed in the spaces left by the emulsifier molecules in the film so the
interfacial tension falls quickly. Once the surface layer is depleted, demulsifier
molecules have to diffuse from the bulk which is a slow process and therefore the
interfacial tension falls more slowly. Ultimately the interface becomes saturated and
the equilibrium interfacial tension is reached. As indicated by the variation in
interfacial tension with time, the rate of adsorption of the demulsifier at the crude
oil water interface is much faster than that of emulsifiers in the crude oil. Adsorption
of the demulsifier reverses the interfacial tension gradient and enhances film
drainage. Ultimately, a stage is reached when the film becomes very thin and due
23
to the proximity of the dispersed phase the van der Waals forces of attraction
dominate and the droplets coalesce. More presently, Abdurahman and Yunus (2009)
confirmed with higher concentration of the demulsifier used, the droplet size was
larger than that of with lower concentration. It was also observed that increase in
demulsifier will accelerate the coalescence of droplet faster. But regardless of
concentration levels, the presence of a demulsifier is characterized by very short
initial coalescence time. Demulsification was also studied by Borges et al (2009),
Rondon et al (2006) (Rondon et al., 2008) (Hung et al., 2007) (Bhardwaj & Hartland,
1994) (Miller & Bohm, 1993)
Figure 6. Effect of adding surfactants on interfacial tension.
2.3.3 Other surface active agents
Other surface active agents have been used as well. In crude oil emulsions, it has
been known that they are stabilized by naturally present substances such as
asphaltenes and resins which behave like emulsifiers (Makhonin et al., 1979).
Authors McLean et al (1998) were reported that, asphaltenes and resins adsorb at
the water-oil interfaces and form interfacial films that confer stability against phase
separation or in other words these surfactants provide steric hindrance to droplet-
droplet coalescence which are similar to functions of the regular surfactants or
specifically emulsifiers. The stability of water in oil emulsions in petroleum systems
24
using the asphaltenes were also investigated by other researchers (Kumar et al.,
2001) (Sjoblom et al., 2003) (Sullivan et al., 2007). Such mechanism was also
similarly described by Sundararaj (1995) with polymeric materials (Borrell & Leal,
2008) which also act as surfactants and they are called compatilizers. Electrolytes
(Li & Slattery, 1988), salts like sodium chloride NaCl (Koh et al., 2000) (Mitra &
Ghosh, 2007) and even gelatine (Lobo, 2002) were also used as surfactants to
investigate coalescence behaviour or droplets and yielded the same stabilization
effects as emulsifiers.
The paradigm that indicates that the water-in-oil emulsions are stabilized by
asphaltenes often seems to be a gross oversimplification without any specific
details regarding the true observable contribution of asphaltenes. Because of this,
Czarnecki and Moran (Czarnecki & Moran, 2005) created a model to explain the
mechanism of water-in-oil emulsion stabilization in petroleum systems in order to
provoke further potential discussions. Their model suggested that only a small sub
fraction of asphaltenes and not all is being used as the stabilization. There appears
to be another chemical responsible for the stabilization which is a low molecular
weight surfactant material. The competition for the oil/water interface between
these two substances is based on the difference of their adsorption kinetics where
the asphaltenic material adsorbs slowly and irreversibly and forms rigid skins while
the other material adsorbs faster. Abdurahman and Yunus (2009) also investigated
the dependency of co-surfactants. For instance, by increasing the ratio of resin
concentration to asphaltene concentration effectively increases the water
separation from crude oil emulsion while an increase in the asphaltene
concentration in the crude oil decreases water separation rate. The effect of resin is
that it solubilises the asphaltenes into the oil phase minimizing asphaltene
interaction with the water droplets and hence more coalescence.
25
2.4 Effect of interfacial tension on coalescence
Previously, we have seen only the decrease of magnitude of the interfacial tension
of fluid systems by addition of surface active or surfactants. In this section, the
effect of altering the interfacial tension on the coalescence behaviour is elaborated
in more detail.
Hartland and Wood (1973) concluded that with decreasing interfacial tension, the
film drainage rate will be reduced, which leads to longer coalescence times. They
also found that during the coalescence of a liquid drop with a flat interface the
drainage rate decreased with a decrease in interfacial tension and applied force or
with an increase of the drop volume. Li and Slattery (1988) confirmed that the
coalescence time of nitrogen bubbles increased when the surface tension was
decreased by changing the concentration of sodium chloride in the aqueous
continuous phase. So far, many researchers have only found the effect of
decreasing the interfacial tension of binary liquid/liquid systems and gas/liquid
systems by addition of emulsifiers on the coalescence frequency (Nielsen et al.,
1958) (Charles & Mason, 1960) (Charles & Mason, 1960) (Mackay & Mason, 1963)
(Mackay & Mason, 1963) (Vrij & Overbeek, 1968) (Hodgson & Lee, 1969) (Hodgson
& Woods, 1969) (Burrill & Woods, 1973) (Chen et al., 1998) (Dreher et al., 1999)
(Chen & Pu, 2001) (Ghosh, 2004).
On the other hand, Abdurahman et al (2007) found that by using many demulsifiers,
which did not only decrease the interfacial tension, but increased the interfacial
tension and promoted separation of oil and water phases. Surprisingly, Wang et al
(2009) found that by decreasing the interfacial tension, the coalescence time
decreases which is a contrast to the present literature. Such unusual findings are
not very common in the literature.
26
2.4.1 The Marangoni Effect
Several researchers investigated into more detail with regards to the mechanisms
responsible for the low probability of coalescence in the presence of certain
emulsifiers. The most commonly accepted idea is that the flow out of the thin film
produces a gradient in the surfactant concentration, with the lowest concentration
at the centre of the film and the highest ones near the edge of the film (Yoon et al.,
2007). This yields the Marangoni stresses that are hypothesized to immobilize the
interfaces within the film, and thus slow the drainage process (Aversana et al.,
1995) under micrograviation. This was also previously observed in a recent
dissertation by Yoon (2006). Other previous researchers also yield such
phenomenon (Mackay & Mason, 1963) (Allan & Mason, 1961). With surfactant
concentrations being the highest at the edge this yield the surface tension at the
edge of the film to be the lower while the surface tension at the centre of the film is
highest. According to the Marangoni effect which was first observed by James
Thomson (1855), and later in more detail by Marangoni (1865) suggests that the
film moves from region of low interfacial tension (emulsifiers concentration is high)
to high interfacial tension (emulsifiers concentration is low) causing a film at the
centre of the point of contact between the drops.
However, Dai , Graham and Leal (2008) showed that during coalescence, the
interface in the thin gap actually exhibits a significant degree of mobility which
explains that at least to a certain extent, the fact that the assumption of a
completely immobilized interface is not in agreement with their experimental
conclusions. Hence the unexpected new result is that the role of Marangoni effect
on the coalescence process does not occur via immobilization of the interface within
the thin gap region, but rather is due to its effects on the hydrodynamics outside
the thin film. In particular, Marangoni stresses immobilize the drop interface outside
the thin film, and this increases the total external hydrodynamic force that pushes
the drops toward each other. This, in turn increases the degree of flattening and
27
dimpling of the thin film, and it is primarily this change that slows the film drainage
process and thus increases the required drainage time prior to coalescence.
To help understand the Marangoni mechanisms better, it is best to demonstrate the
appearance of the wine tears.
Figure 7. Picture showning Marangoni stress in a wine glass.
Stage 1 – Alcohol is first spread across a curved dish.
Stage 2 – As evaporation occurs everywhere along the free surface, a film can be
seen around the outer edges of the dish.
Stage 3 – The film seems to spread out further. The alcohol concentration in the thin
layer is thus reduced relative to that in the bulk owing to the enhanced surface area
to volume ratio. As surface tension decreases with alcohol concentration, the
surface tension is higher in the thin film than the bulk. The associated Marangoni
stress drives up flow throughout the thin film
Stage 4 – The alcohol thinly spread across the curved dish climbing up to reach the
top of the dish where it accumulates in a band of fluid that thickens.
28
Stage 5 – Eventually the thick band becomes gravitationally unstable and releases
the tears of wine. Marangoni flows are created successfully as tensions draw the
liquid towards the centre of the dish.
Stage 1 Stage 2
Stage 3 Stage 4
Stage 5
29
Figure 8. Successive stages of the Marangoni flow in a wine glass.
However, it seems that the Marangoni effect is only applicable to emulsifiers which
happen to cause the increased stability of drops. With respect to demulsifiers, no
such effect was reviewed. Perhaps, with demulsifiers, the reason for the promotion
of coalescence frequency can be due to their natural structure which is beyond the
scope of this project. Previously, it was mentioned that the most demulsifiers
decrease the interfacial tension of liquid systems which is in common with
emulsifiers. With this, if the Marangoni stresses hold true, the demulsifiers replace
the spaces left by emulsifiers as the thin film is being squeezed out so quick that
the concentration of demulsifiers at the centre of the film is very large compared to
that at the edge of the film and hence the reverse of interfacial gradient which
promotes coalescence. However, this does not apply to demulsifiers which increase
the interfacial tension which was observed by Abdurahman et al (2007) because it
would defy the coalescence role of demulsifiers.
2.5 Role of interfacial repulsive and attractive forces
The coalescence behaviour of emulsion droplets upon collision depends on the
interplay between these two types of forces. If the total interaction is very repulsive,
the droplets can rebound; if the intermolecular repulsive forces can hold the film at
an equilibrium separation, the droplets may flocculate; and if the total interaction is
attractive, the film will become thinner and finally rupture, i.e. the droplets
coalesce. Coalescence requires rupture of a thin liquid film between the drops. In
most real-life applications there are effects associated with the thinning of the liquid
film and interfacial repulsion originated from Derjaguin Landau Verwey Overbeek
(DLVO) and non-DLVO forces. Such DLVO forces are the van der Waals and
electrical double layer forces which have received much studies (Chen, 1985)
whereas non-DLVO forces are attractive depletion steric and repulsive structural
forces (Kumar et al., 2001) which on the other hand did not receive much attention.
30
The condition for rupture is the critical film thickness such that the attractive force
which is the van der Waals forces must be larger than the electrostatic repulsive
force and the Marangoni effects as discussed in Section 2.4.
This is illustrated in Figure 9 where the total free energy of the particles to be at
very minimal distance must be at most at zero Joules. k is the Boltzmann constant
and T is the temperature. D is the distance between the two surfaces of the drops
facing each other and U is the free energy of the particles.
Figure 9. Total free energy required for coalescence (Paunov, 2008).
Figure 10. Components of the resultant net energy.
31
The red line in Figure 9 shows the net energy which is composed of the double layer
repulsive force and the van der Waals attractive force as shown in Figure 10. Hence
in order for coalescence to commence, the net energy curve must be below zero
value at the vertical axis. Many researchers failed to take into account their
significance when characterising the model for coalescence behaviour. Because
other relevant forces play an important role in determining the stability of emulsion
droplets, neglecting them (van der Waals) will unavoidably lead to deviation from
the real situation (Greene et al., 1994). Chen (1985) confirmed that the van der
Waals disjoining pressure destabilized the film, whereas the electric double layer
stabilized it using a model that describes the film profile evolution between two
equal sized drops and predicts the film stability, time scale and film thickness given
only the radius of the drops and the required physical properties of the fluids and
surfaces.
2.6 Effects of hydrodynamics on the coalescence
While some researchers focussed on coalescence under stagnant flow, several other
researchers investigated droplet coalescence under flowing conditions. For
instance, Hartland and Wood (1973) found that during the coalescence of a liquid
drop with a flat interface the drainage rate decreased with applied force. Many
research groups have studied theoretically and numerically the film drainage
process under constant approaching velocity or constant driving force (Stergios et
al., 1991) (Chesters, 1991) (Jeelani & Hartland, 1993) (Saboni et al., 1995) (Rother
et al., 1997) (Nemer et al., 2004). Their results indicate that the film drainage rate is
mainly controlled by the interfacial tension, the viscosity ratio between the
dispersed and suspending phases and the external force. These factors have
profound effect on the drainage rate, which often depends on the deformability and
tangential mobility of the droplet surface as discussed previously. Wang et al (2009)
criticized that most of the models assumed simple boundary conditions, such as
32
constant interaction force or constant approach velocity. In reality both will vary
during the collision of a drop with a flat interface or with another drop.
Al-Mulla and Gupta (2000) concluded that using a Coutte device at lower shearing
rate favoured coalescence. Similarly, Nandi et al (2001) indicated that their
surfactant stabilized emulsions were prepared in a stirred tank and low shear rates
prevented drop breakup and at high shear rates, the emulsions were stabilized
further. However, Nandi et al (2005)extended their shearing experiments with
surfactant-less liquid mixtures and found that without surfactants, higher shearing
rates promoted coalesce. Chen and Tao (2005) also found that by increasing the
stirring intensity it increases the oil in water emulsion stability. Previously, Leal
(2004) added the film drainage is slowed down upon addition of emulsifiers in a
four-roll mill where the binary coalescence of a pair of droplets were exposed to
external shearing flows. Dai and Leal (2008) validated the film drainage and rupture
using polymeric materials in which the experiment was based on the coalescence of
a pair of equal size drops undergoing head-on collision in a biaxial linear flow under
constant linear velocity. They also investigated that there is an equally important
effect which is due to the increasing hydrodynamic force pushing the drops together
causing the film to be more strongly deformed into a dimpled configuration which
further slows the film drainage process. They also confirmed the validity of the
Marangoni effects such that the immobilization of the interface within the thin film
as expected and the force pushing the drops together is increased by the
Marangoni immobilization of the interface outside the thin film. This caused the thin
film to be much more dimpled and deformed than it is in the absence of surfactant
at the same capillary number and the more dimpled film shapes slow the rate of
film drainage. An observation holds true in this respect in the opposite manner in
which, Bremond and Bibette (2008) investigated the high affinity of a pair of
emulsion drops to favour coalescence under decompression or rather, separation.
Briefly, the experiment was undergoing first expansion and then decompression of
33
a flow of trains of emulsion droplet pairs passing through a coalescence chamber.
No coalescence was observed until the decompression of the downstream droplet of
the pair. It was also observed that at this instant, both droplets form a pair of facing
nipples in the contact area prior to coalescence. The separation term is used
because the action of decompressing the downstream droplet leads to acceleration,
causing it to move away faster from the upstream droplet. Gary leal (Baldessari et
al., 2007) (Borrell & Leal, 2008) (Yang et al., 2002) studied the stability of film
drainage with respect to flow induced coalescence. They investigated on the growth
of disturbance in relation with the film drainage phenomenon.
Contrarily in the same perspective, the demulsifying experiment conducted by
Abdurahman and Yunus (2009) on different stirring rates concluded that with higher
stirring rates and hence, larger turbulence and high shear rates, the average
droplet size was larger. This relatively means that shear induces higher drainage
rates. However, despite the obvious introduction of shear stress into the medium,
no significant observation was made on the flow patterns.
The capillary number, which is the ratio between viscous and interfacial forces,
characterizes the droplet deformation and plays an important role in determining
whether two droplets coalesce upon their collision. It was found that the
coalescence behaviour of two droplets in a simple shear flow strongly depends on
the capillary number. Coalescence only occurs at a capillary number that is lower
than a critical value. Also in a simple shear flow, Loewenberg and Hinch
(Loewenberg & Hinch, 1997) numerically studied the collision between two
deformable droplets. They found that the collision behaviour is dictated by capillary
number and predicted that, for flow with capillary number very much less than 1,
coalescence tends to occur when the droplets are pressed together, whereas for
capillary number ~ 0 the tendency for coalescence reaches its maximum when the
droplets begin to separate in the extensional quadrant. The latter behaviour was
experimentally verified by Guido and Simeone (1998). In a different flow fashion
34
where a pair of equal size drops was on a head-on collision, the findings were very
similar (Yoon et al., 2007). Leal (Leal, 2004) investigated on the effect of capillary
numbers on the coalescence of the droplets and added that coalescence requires
very ‘gentle’ collisions, i.e., collisions at low capillary numbers.
A simplistic view of the coalescence process begins with the observation that the
thin layer of fluid that separates the two drops once they collide must become thin
enough for van der Waals attraction to destabilize the film to produce film rupture.
During the whole collision process, the net force on each drop is actually zero
assuming that they are neutrally buoyant. However, when they are in close
proximity, it is convenient to think of the net force as being the sum of two equal
magnitude but oppositely directed forces; one the hydrodynamic force due to the
external flow which tends to push the drops together when they first come into
apparent contact, and the other the lubrication force due to the extra pressure in
the thin film. From this point of view, the question of whether a pair of drops can
coalesce is then the question of whether the film between them thins sufficiently
before the drops rotate to the orientation where the external force changes from
pushing the drops together to pulling them apart. Leal (Leal, 2004) investigated
such phenomenon in a study of the effect of the capillary number on droplet
coalescence in four roll mill device which generates flow to the surrounding fluid.
The device is advantageous because there is a stagnation point at the geometric
centre where coalescence and drop breakup experiments can be conveniently
carried out and monitored. It was found that at a capillary number larger than the
critical capillary number, the pair of droplets was first in a horizontal orientation.
Then the orientation varied such that there was an increase in inclination of the
droplets and the droplets were getting closer to each other. However, at an angle of
45 degrees, the droplets move away from each other due to the influence of the
external forces. In contrast, at a capillary number lower than the critical capillary
number, the droplets coalesced at an angle of inclination of only 10 degrees. It is
35
also important to note that at horizontal level, after the collision, the droplets
rotated afterwards. Such are glancing collision, where the two drops rotate in the
flow from the point of initial contact to the point where they hydrodynamic force
along the lone of centres changes sign and the drops begin to separate. The most
important finding here was that the influence of the capillary number. Hu et al
(2002) studied the collision between two droplets of equal size in a simple shear
flow. They showed that, for a fixed initial position and viscosity ratio, the minimum
separation between two droplets is solely determined by the capillary number.
2.7 Drop size on coalescence
It is commonly accepted that with large drops the large contact area reduces the
film drainage rate resulting in larger coalescence time (also called rest-time)
compared to small drops. Results produced by Dreher et al (1999) show that the
coalescence time depends almost linearly on drop size. Moreover, Mackay and
Mason (1963) found that for easily deformable drops, coalescence time and drop
stability increase with drop size and was later validated by Chen el al (1998).
Hartland and Wood (Hartland & Wood, 1973) also found that during the coalescence
of a liquid drop with a flat interface the drainage rate decreased with an increase of
the drop volume.
2.8 Thermodynamics
Surface free energy is another term in the aspects of thermodynamics for interfacial
tension. There information regarding the interfacial mechanisms which can still be
considered to be under proposition or being hypothesized because there is no
concrete or credible proofs that can defend its existence. However, the only basis
that we can still rely on is the change of the surface free energy which changes with
the curvature of the droplets. The curvature of the droplets is related to the
interfacial tension and this is not only available in the literature but also logical and
visible to the eye. By incorporating thermodynamics, one can visualize the
36
coalescence behaviour in terms of the period of coalescence by changing the
interfacial tension. With this and the established numerical equations of
thermodynamics through Gibbs free energy, which can link the surface curvature to
the concentration of surfactants, then there is solid verification that the presence of
surfactants can increase or decrease the coalescence time.
Following the conclusions obtained for published articles, the behavior of a system
under the influence of a surfactant is to be observed. The surfactant is expected to
lower the interfacial tension of the system, keeping in mind that the molecular
weight of the surfactant to be used must be low, so that coalescence is enhanced.
Thermodynamics can be utilised to speculate the behavior as shown below:
Helmholtz equation:
F=U-TS (1)
Differential of Helmholtz equation at constant temperatures :
dF=dU-TdS (2)
Internal energy:
dU=TdS+μdn+g dA (3)
Combining equations (2) and (3) :
dF=μdn+g dA.
(4)
where, F= Helmholtz’s Free energy, U= internal energy T= temperature, S=
entropy, μ= chemical potential, n= number of species,γ=interfacial tension and A=
surface area.
37
As a conclusion, lowering the interfacial tension and keeping the system constant,
will result to the decrease of dF, thus the energy required to create a system after
the spontaneous energy transfer from the environment has taken place, is lowered
and coalescences is favored.
3 Aims and objectives
3.1 Aims
The aim of this project is to investigate the coalescence behaviour of oil-in-water
emulsion under the influence of surfactants and the presence of stagnation in the
vicinity of the region of coalescence. It is crucial to identify that the coalescence
process be it successful or unsuccessful, to be free of external forces other than the
generation/induction of flow to allow the growth of the droplets. This will involve the
study of the interactions between the close proximity facing surfaces of a pair of oil
droplets suspending in water undergoing coalescence or otherwise through a high
speed camera. Observations of the physical changes of the interface between the
pair of oil droplets are to be validated with the available literature to further support
or criticize the suggested mechanisms and wherever possible, draw new
conclusions.
With these contributions, more understanding of the coalescence behaviour can be
gained. This is beneficial to both the science and engineering fields in the context of
better design for emulsion breaker devices or techniques and the synthesis of
better and more efficient demulsifying chemicals.
38
3.2 Objectives
The behaviour of these coalescences is variable due to the presence of factors such
as external hydrodynamic forces and surfactants. The specific contributions made
by each factor with regards to the coalescence behaviour are unpredictable. Hence
there are many potential investigations available to investigate the dependence of
each factor on each other. The objectives of this project are to:
Investigate the effect of different surfactants on the coalescence frequency
Investigate on the effect of surfactant concentration on the interfacial tension of
the system
Investigate on the relationship between interfacial tension and coalescence
frequency
Investigate the effect of induced and non induced flow on the coalescence
frequency
39
4 Methodology
This section brings about the details in which how the experiment was carried out,
what materials were used and their sources and what were measured. The
experiments of binary drop coalescence were carried out using the experimental set
up as shown in Figure 1. Figure 2 shows the whole experimental set up. For step by
step procedures, this is available in the Appendix.
4.1 Materials
4.1.1 Rig
The coalescence cell or the rig shown in Figure 11 is a vertical rectangular acrylic
cell with a square cross section of side length 26 cm and height 25 cm. The rig is
made out of square and rectangular pieces of Perspex glass glued together using
thermal glue and let dried. The bottom centre of the rig has a protrusion made out
of Perspex glass which is also glued to the bottom. The protrusion is made such that
the rig core can be inserted without any mobilisation. The rig has a cover to prevent
the entry of dust particles during experiment.
40
Figure 11. Rig body with cover, protrusion and walls.
41
42
Figure 12. Laboratory experiment set up
4.1.2 Rig core
The rig core consists of several components combined together including a 4
screws, cover, needles and connectors. The rig core was a cube of sides 7cm with a
vertical cylindrical hollow centre which was designed to fit in through the protrusion
in the rig as shown in Figure 13. Two Perspex glasses of dimensions are cut and
inserted into the bottom of the rig to prevent any movement of the rig core during
experiment as shown in Figure 14.The rig core also had four V-shaped grooves to fit
in the plastic needles such that the rig core cover is pressed firmly with screws on
top of the rig core body to lock in the plastic needles as shown in Figure 14 and 15.
By adjusting the tightness of the screws, this adjusts the levels of the tip of the pair
of needles. The needles were bent at 90o in the mechanical workshop as accurate
as possible. The outer diameter of the needles was 0.966mm. The tip of the vertical
needles should be aligned with each other horizontally and in line with each other
so that droplets can be produced equally with sides touching each other.
Figure 13. Dissembled rig core with cover.
Figure 14. Rig core with needles. Figure 15.Top view of rig core.
43
Figure 16. Rig core immobilized by Perspex Glass.
4.1.3 Attachments
Drops were formed through two polyethylene capillaries with outer diameter 0.7mm
attached to the needle on one end and the other to the syringe connector as shown
in Figure 16. Each capillary was connected to the inlet of the needle at one end and
the other end to the syringe connectors shown in Figure 17. Hence when the rig
core with its attachments was placed within the rig, care was taken not to damage
or apply pressure on the vulnerable capillaries. The cover was slid open and a thin
film of cling paper is used to cover the opened part of the cover to prevent entry of
dust particles as shown in Figure 18.
Figure 17. Rig core in rig body. Figure 18. Capillary attachments.
44
4.1.4 Delivery
Two different infusion pumps were used due to lack of resources: (i) Pump 1 and (ii)
Pump 2 as shown in Figures 19 and 20 respectively. Due to this reason and the
unequal inefficiencies of both pumps, pump calibration was necessary. The pump
calibration was carried out such that the true mean volumetric flowrate was
calculated for a particular pump setting flowrate. The methodology of the pump
calibration can be viewed as the volume of liquid discharged per unit time for a set
of pump setting flowrates (0.1ml/hr – 1.0ml/hr). The liquid used for pump calibration
was RO water. 5mL syringes were inserted securely into the infusion pumps and
connected to the syringe connectors as shown in Figures 18 and 19. The desired
volumetric flowrate of the pump is checked through the pump calibration curve
obtained from pump calibration. For instance, if 0.5ml/hr was needed to induce flow,
then the corresponding pump setting flowrate at the x-axis of the pump calibration
graph with error bars as shown in Figures 21 and 22 was used.
Figure 19. Pump 1. Figure 20. Pump 2.
This configuration allows equal and steady delivery in each syringe of low inlet
flowrates required in this work. Throughout the experiment, a mean setting flowrate
of 0.5ml/hr was used corresponding to setting flowrates of 0.63ml/hr for Pump 1 and
0.62ml/hr for Pump 2.
45
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.0
0.2
0.4
0.6
0.8
1.0f(x) = 0.535393994884959 x² + 0.471063185898291 x
Mean real setting flowrate (ml/hr) against Pump 1 setting flowrate (ml/hr)
Pump 1 setting flowrate (ml/hr)
Mea
n re
al se
ttin
g flo
wra
te (m
l/hr
)
Figure 21. Pump 1 calibration graph.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
f(x) = 0.0678819933664271 x² + 0.729006575325994 x
Mean setting flowrate (ml/hr) against Pump 2 setting flowrate (ml/hr)
Pump 2 setting flowrate (ml/hr)
Mea
n se
ttin
g flo
wra
te (m
l/hr
)
Figure 22. Pump 2 calibration graph.
46
Syringes were purchased from the Medical School University of Nottingham.
4.1.5 Image capturing and video recording
The coalescence was captured with a high speed video camera (Phantom v12.1)
placed in front of the rig, as shown in Figures 23 and 24 which has a CMOS sensor
with a maximum resolution of 1280 × 800 pixels. The average recording rate used
throughout the coalescence process was 1000 pps, while even higher recording
rates up to maximum 6200 pps at maximum resolution (1280 × 800 pixels) were
necessary to capture the formation of the liquid bridge between the two drops as
soon as coalescence started.
Figure 23. High speed camera Phantom v12.1.
Figure 24. High speed camera bird eyes view.
4.1.6 Lighting system
Back lighting for the camera was provided by Dedocool lighting system which was
composed of a spotlight and a motor as shown in Figure 25. The spotlight was
47
placed behind of the rig to allow light to pass through the camera where the camera
was focussing. Dedocool lighting system was able to avoid generating temperature
gradients within the rig. The spotlight was clamped with its position adjusted to
obtain the best image and videos possible in terms of contrast, colour and
brightness. Baking paper was clamped when necessary to provide a translucent
environment of the lighting to capture clear images and videos when the frame rate
used was at maximum. This was necessary because at maximum frame rate, the
exposure to light was very low and so the spotlight is switched on at its brightest
level. To balance the brightness, the baking paper was used.
Figure 25. Dedocool lighting system.
4.1.7 Temperature measurement
The temperature measurement was done using a thermocouple designed,
programmed and fabricated by Aime as shown in Figure 26. The temperature
detector is a metal which is attached at the side of the rig submerged in the
mixture. Calibration of the thermocouple was necessary for accurate temperature
measurements. Calibration was performed to obtain an equation shown in Figure 29
which relates the voltage and the temperature by measuring different temperatures
(0oC – 100oC) of mixtures of the ice cubes and boiling water.
48
Figure 26. Thermocouple.
Figure 27. LabVIEW temperature measurement.
A voltmeter was used to measure the temperature in terms of voltage. The
thermocouple is then inserted into the rig attached to the walls of the rig and
49
submerged in the liquid mixture. The temperature was read off on the computer
screen using LabVIEW as shown in Figure 26. The program was composed by Aime
as shown in Figure 27.
Figure 28. Thermocouple simulation.
This equation is to be inserted into LabVIEW. LabVIEW was used as the program to
measure the temperature during the experiment.
0.9 1 1.1 1.2 1.3 1.40
20
40
60
80
100
120
f(x) = 258.155577299412 x − 255.439359099804
Temperature (oC) against Voltage (V)
Voltage (V)
Te
mp
era
ture
(o
C)
Figure 29. Temperature calibration equation.
50
4.1.8 Chemicals
Kerosene was sourced from Sigma Aldrich and used as the dispersed phase in all
the experiments. The continuous phase was either pure RO water or RO water with
different surfactants glycerol or sodium dodecyl sulphate (SDS) concentrations.
Glycerol was purchased as 200mL bottles from Boots while SDS powder was
obtained from Sigma Aldrich.
4.1.9 Others materials
25L PVC container
Glassware beakers (200mL, 500mL and 5L)
Measuring cylinder
Pipette and pipette filler
Propanol (cleaning solvent)
Methanol (cleaning solvent)
Latex gloves
4.2 Experimental procedures
This section will give specific treatment of major experimental procedures so that in
future if this work is to be repeated, results can be compared to verify the validity of
the results obtained in this current experiment.
4.2.1 Cleaning
The investigation of surface science requires very clean and pure system without
any contamination. Any contamination will affect results and disallow comparison
between repeated results which leads to waste of time. Hence extreme care was
taken when preparing prior to setting up of the experiment. Below is a list cleaning
procedures that was performed and is required to be followed for future
experiments:-
51
Latex sterilised gloves were worn throughout the whole experiment. They are
rinsed with methanol and RO water prior to making any contact with the
experimental materials.
Before the start of each experiment the rig was strongly sprayed and rinsed
several times with tap water from the laboratory room. Care was taken to
remove any oily stains or dust or solid particles in the rig by spraying strongly
the tap water. The rig is then filled with tap water and left overnight to pull out
the contamination particles remained as residues in the rig.
The rig core body, cover and the Perspex glass pieces were also washed and
sprayed with tap water and dipped into the rig of tap water to allow any residual
oily stains to float and rise to the surface.
All connectors, needles and screws were dipped in methanol in a beaker under
sonification thermal bath at least twice for 10 minutes before rinsing them with
RO water and leaving them to dry in the fume cupboard. The methanol dissolved
any residual stains attached to these smaller components and the RO water
washes the dissolved particles away.
The capillaries were filled and washed with methanol, RO water and air in that
order for several times using new clean syringes. The capillaries were then
curled into a small beaker where they were washed with methanol and rinsed
with RO water.
Stirrer is to be rubbed and rinsed with methanol, RO water and dry in fume
cupboard.
All glass beakers were strongly sprayed with tap water to remove oily stains and
then with methanol and tap water and rinse with RO water and left dry in the
fume cupboard.
Pipette is washed with RO water several times.
52
4.2.2 Set up of experiment
The setting up of the experiments was started off with the cleaning procedures
mentioned in the previous Section 4.2.1. Once the equipments were cleaned, RO
water was brought to mix and saturate with 100mL of kerosene in two 5L glass
beakers. This was done so that there is no diffusion within the liquids during
experiments. The beakers were left for at 12 hours sealed with cling film paper to
prevent entry contamination in the fume cupboard.
The rig was to be emptied of the overnight tap water and rinsed with RO water. The
rig core was set up with the needles, connectors and tubes. Care was taken when
handling the needles without touching the tips. The distance separating the needles
was taken to be around 2.8mm, measured using a vernier calliper. It was very
important that the needles are secured tightly with the tips aligned horizontally in
line with each other as shown schematically in Figure 30. The needles were made
also vertically parallel to each other.
The excess kerosene on floating on top of the saturated RO water in the 5L beaker
was removed out carefully using a pipette and placed in a beaker. The syringes
were then filled with the saturated kerosene. The syringes were also rinsed with the
relevant aqueous phase before the experiment. Once the rig core and the
connectors are inserted into the rig as shown in Figure 17, the syringe connectors
are attached to the syringes located at the infusion pumps. Care was taken not to
touch the capillaries inside the cell after the cleaning. The saturated RO water was
then fed into the rig slowly without splashing on the rig core while saturated
kerosene was delivered to both capillaries by the syringes through pumping. Air
within the syringes was discharged by continuous pumping of the saturated
kerosene.
53
Figure 30. Schematic diagram of rig.
4.2.3 Method of data acquisition
Pairs of kerosene drops were formed at the tips of the needles through the
capillaries and settled on top of the saturated RO water in the cell. At the low flow
rates used the drops formed in each capillary were well separated from subsequent
ones and did not interfere with each other and with the coalescence process. It was
made sure that minute organic phase was generated so that it does not affect the
coalescence process for instance 20% of the rig is covered with kerosene.
The coalescence process in the rig was carried out using two methods:-
54
i. Non-induced flow
ii. Induced flow
For both methods, the binary coalescence time was recorded using the camera
software Phantom Showcase. The kerosene inlet flowrate used throughout the
whole experiment was 0.5mL/hr (mean pump flowrate – refer to Figures 20 and 21)
According to Ban et al. (2000), the coalescence time is defined as the difference
between the time the two drops come into contact and the time the two drops
‘begin to coalesce’. ‘Begin to coalesce’ can also refer to the instant at which the
bridge between the two drops is formed. It is important to define illustratively the
point at which the two drops come into contact and also the point at which the two
drops ‘begin to coalesce’. This is because later in the experiments, there was
difficulty in judging the point where the two drops start come into contact.
The difference between the two methods was that for non-induced flow, the flow of
kerosene in the syringe was such that it was stopped at the instant just before the
two drops come into contact for the first time. The binary coalescence time for this
method was measured at this instant until the formation of the bridge was first
witnessed clearly on the camera software depicted on the laptop screen.
As for the induced flow method, the flow of kerosene was maintained at 0.5ml/hr
until the end of the coalescence process or the drops leave the needles due to
unsuccessful coalescence. Hence, if coalescence occurred, the binary coalescence
time was calculated from the point at where the drops first contact until the
formation of the bridge occurs.
For the two methods, glycerol was added at 4 different concentrations v/v of 0.5%,
1.0%, 5.0% and 10.0% to compare the system when glycerol concentration was
zero.
For each glycerol concentration, 10-15 pairs of drops were formed. Between the
generations of each pair, 5 minutes were given to allow the flow patterns to
55
disperse so that during the coalescence process, the environment was maintained
as a stagnant one so that there is no potential shear or disturbance that can affect
the coalescence of the pair of droplets. 5mL of kerosene in a syringe was more than
sufficient to generate data for all concentration. It was also noted that the change in
the liquid level height in the rig was insignificant and did not affect the coalescence
process. At the end of the experiment, the rig was emptied and cleaned to repeat
the same batch of experiments. This procedure allowed consistency in the system
for comparison of results.
The coalescence between two drops can be affected by factors such as external
vibrations and temperature gradients (Charles & Mason, 1960) (Davies, 1992). Due
to the limitations of the experiment, it was unable to avoid any such effects; no
thermostatic environment was available and also vibrations were inevitable. Despite
the thermocouple was available to monitor the temperature change of the system,
if a large deviation from the constant temperature of the system occurs, there was
nothing that could be done to remove this occurrence. There were also insufficient
benches to place equipments that produce vibrations. However, despite such
disturbances, they were attempted to be minimized to as low as possible.
4.2.4 Interfacial tension measurements – Pendant drop method
One of the ways to measure the surface tension is by using the pendant drop
method which was used in this project. The pendant drop method can be used to
determine the static surface and interfacial tensions of liquids. is probably the most
convenient, versatile and popular method to measure interfacial tension between
emulsion. It involves the determination of the profile of a drop of one liquid
suspended in another liquid at mechanical equilibrium. It was important to eliminate
any vibration that can disrupt the still.
The measurement of the interfacial tension requires certain dimensions of the
kerosene droplet in the medium. It was relatively difficult to measure the
56
dimensions of the oil droplet because there was no close access to it where the
droplet can be measured off directly using ruler. Hence, measurements of the oil
droplet are to be made through the use of the camera and an image processing
software called GNU Image Manipulation Program (GIMP) which is freely distributed
over the internet (GIMP, 2001-2010). The image of the drop in the medium was as
shown in Figure 31 was taken by Phantom Camera Control software while GIMP
calculates the dimensions necessary for the interfacial tension to be measured by
converting pixels to distances in SI units. For each glycerol concentration, this was
done several times to account for the error in consistency. Hence the average
interfacial tension was used. A droplet was generated each time slowly at 0.5ml/hr
and the volume of the droplet at which the droplet is detached from the tip of the
needle is recorded. The droplet was regenerated again at about 95% of the
recorded volume. The flow was then stopped and the droplet was monitored for at
least minute before the image of the oil droplet in the aqueous phase was taken.
Figure 31. Image of the kerosene droplet required for the calculation of the
interfacial tension.
57
4.2.4.1 Equations
There are various methods of calculating the boundary tensions from the pendant
drop profiles, but the method of Andreas, Hauser and Tucker (Andreas et al., 1938)
is the most commonly used. With this method, the equatorial diameter de and the
diameter ds in a selected plane, which is located by measuring vertically from the
vortex a distance equal to de are measured as shown in Figure 6. The ratio of the
two diameters ds/de is designated as S, the drop shape; the quantity 1/H is a
function of S. The interfacial tension Υ is calculated by the equation (1):
Υ=g ρ de
2
H (1)
where g is the acceleration due to gravity and ρ is the difference in density of the
two phases.
The relationship between S and 1/H is relatively important to characterize all the
profiles of the oil droplet. The range of calculated values of S for 1/H has been
extended (Fordham, 1948) (Mills, 1953) (Stauffer, 1965). Misak (Misak, 1968) stated
that there are 5 equations that can relate 1/H with S for 5 corresponding ranges of S
that are sufficiently accurate to be used in the most exacting interfacial tension
calculations :
For S = 0.401 to S = 0.46
1H
=( 0.32720S2.56651 )−0.97553S2+0.84059S−0.18069 (2)
For S> 0.46 to S = 0.59
1H
=( 0.31968S2.59725 )−0.46898S2+0.50059S−0.13261 (3)
For S> 0.59 to S=0.68
58
1H
=( 0.31522S2.62435 )−0.11714 S2+0.15756S−0.05285 (4)
For S> 0.68 to S=0.90
1H
=( 0.31345S2.64267 )−0.09155S2+0.14701S−0.05877 (5)
For S> 0.90 to S=1.00
1H
=( 0.30715S2.84636 )−0.69116 S3+1.08341 S2−0.18341S−0.20970 (6)
The dimensions were calculated and shown in Table 1. The density of glycerol was
800kg/m3.
Figure 32. Dimensions of the kerosene drop needed to be determined for
interfacial tension determination.
59
Table 1. Calculation of kerosene drop dimensions.
Conc. v/v glycerol de ds S=(ds/de) H ρDispersed VROwater VGlycerol ρContinuous ρ γ
% mm mm kg/m3 m3 m3 kg/m3 kg/m3 kg/m2s2
04.52
3 2.711 0.599381 0.818622 800 8000 0.0 998.2 198.2 0.04859
0.54.51
2 2.715 0.601729 0.827047 800 8000 40.0 999.5 199.5 0.04818
1.04.50
1 2.718 0.603866 0.834761 800 8000 80.8 1000.8 200.8 0.04781
5.04.51
1 2.788 0.618045 0.887051 800 8000 421.0 1011.3 211.3 0.04755
10.04.46
8 2.810 0.628917 0.92846 800 8000 888.9 1024.4 224.4 0.04733
5 Results and Discussion
5.1 Time evolution of the coalescence process
The time evolution of the coalescence process of two kerosene drops is depicted
from Figures 33-52 at the highest frame rate of 6200pps at maximum resolution.
From Figure 33, it can be seen that there is a visible thin gap between the two
drops and the point of contact is defined in Figure 35. Although in other perception,
the ‘dark bridge’ that occurred at Figure 35 seemed relatively like a shadow, but
generally, this point is the start of the binary coalescence time. The bridging of the
drops starts at Figure 37 and this represents the point where the binary
coalescence time stops. The evolution of this coalescence process corresponded to
a binary coalescence time of about one second which happened very rarely
throughout the period of the experiment. All other figures show the aftermath of the
bridging process which leads to coalescence of the two droplets. The bridge gets
larger in size until it becomes the same size as the two droplets. The formation of
nipples can be seen on the two opposing sides of the two droplets. They deformed
such that the nipples disappear and reappear; the frequency of the sudden
expansion and contraction of the coalesced droplets gets smaller with time until it
settles on top of the needles as shown as Figure 50.
60
5.2 Effect of induced flow and glycerol concentration
Figures 53-63 and 64-71 illustrate the cumulative coalescence time distributions
and also the individual coalescence time distributions during binary kerosene drop
coalescence at the same centre-centre capillary separation distance of 2.8mm for
the two methods – (i) Induced flow and (ii) Non-induced flow respectively. In a
general perspective, the coalescence times for all experiments were not as
randomly distributed as shown in the figures. The interfacial tension per unit area
was found to decrease with increasing concentration of glycerol as shown in Table 1
and Figure 53 which was also the same result as obtained by Wang et al (2009).
From Figure 53, the trend decreased sharply at lower concentration. At higher
concentrations of glycerol, the decrease in interfacial tension per unit area was
reduced. Glycerol was used in this experiment to confirm the work of Wang et al
(2009) because it was surprising to notice that with glycerol, the interfacial tension
decreased but the coalescence time decreased which was a contrast to the current
literature. However, this work proved that their findings were nevertheless
substantial and I further extended their work without inducing flow to monitor the
coalescence trend.
From the results obtained, it can be seen that for induced flow, the coalescence
time decreases with increasing concentration of glycerol which is in agreement with
the work done by Wang et al. (2009). The only difference between this work and
theirs was that they had oil as the continuous phase and deionised water as the
dispersed phase while in this work, it was the opposite. Despite this phase inversion
issue, the results remained comparable with that of Wang et al’s because both
experiments investigated the same interface between water and oil under the
influence of glycerol. However, the result was a contrast for non-induced flow. For
non-induced flow, increasing the concentration of glycerol led to stabilization of the
drops and is in very good agreement with the vast literature. At 10% concentration
v/v glycerol, the non-induced method had 0% coalescence and this was the reason
61
for the missing graphs for the non-induced method for 10% concentration v/v
glycerol.
But however, comparing the coalescence time between between the two methods
at 0% glycerol, the non-induced flow actually promotes coalescence which is in
good agreement with the literature review where some authors mentioned that with
lower force, coalescence frequency was enhanced. But as the concentration of
glycerol is increased, such effect was reversed. One logic reason could be the
inaccuracy of the measurement of the binary coalescence time for the non-induced
flow. As it was difficult to judge the first point of contact between the two drops,
most of the readings in the data collection could be wrong. This can be possible
because it may have been overlooked that they made their first contact but in
reality they did not and this could be the reason for the increased stability of the
drops. Neglecting this error by assuming that the drops did made their first contact;
another error that arises could be the contamination of the liquid system by solid
particles that may have entered the liquid system unnoticeably during the
experiment. Putting aside the human error aspects for such trends, in scientific
terms and together with equipment deficiencies, such trend can be possible, given
that glycerol behaves like regular surfactants which happened to have been
stabilizing emulsion and drops in liquids. When the pumps were switched off, due to
the relaxation of the syringes, the flow might still be induced at very low flowrate.
This will keep pushing the drops together, sweeping away glycerol particles away
from the centre of the gap between the drops. When this happens, the interfacial
tension is highest at this centre and Marangoni effects are induced thus increasing
the drainage time. However, this is a defying statement for the induced flow
method where the drainage time was decreased. So far, glycerol has not been
identified as a general emulsifier or demulsifier in the literature. But however,
Griffin (1954) did found that the HLB (hydrophilic-lipophilic balance) value of
glycerol is 11.09. This indicated that specifically glycerol is an oil in water
62
emulsifier. Perhaps with this statement, the results obtained from this experiment
can be held true coupled with several other explanations such as the flows within
the drops.
Simply, with glycerol as an oil-in-water emulsifier, for non-induced flow, with
increasing concentration of glycerol, this would lead to stabilization. For 0% glycerol
concentration, the result obtained from induced flow method is in good agreement
with other researchers (Mackay & Mason, 1961) (Chen et al., 1998) (Dreher et al.,
1999) as discussed in the literature review. Inducing flow produced force which
presses the drops together and causes the continuous film to drain which depends
on the capillary pressure and on van der Waals forces given by Δp=( 2ΥR )+(AH6 π h3
)
(Marrucci, 1969) where AH is the Hamaker constant. This was also observed by
Wang et al. (2009). On the aspects of flowrates, the experiments performed only
used non-induced flow and induced flow at 0.5ml/hr. It was found that with an
increase in the drop diameter and hence radius which was caused by induction of
flow, this decreases the overall pressure and lead to larger coalescence times.
Borreal and Leal (2004) (2008) also indicated that as the inlet flow rate increases,
the effect of drop size on coalescence time diminishes. It can be concluded that an
increase in the inlet flow rate for the same capillary separation distance leads to an
increase in the coalescence time. Furthermore, Borreal and Leal (2008) also
indicated that with increasing inlet flowrate, Q, the flat film area a between the two
drops increases as suggested by the equation 1 for coaxial drop coalescence where
ao is the initial value of the flat film area which is equal to 0 for drop-drop
coalescence. The increase in area will therefore increase the time for the film to
drain to its critical value thus increasing the coalescence time
a=√ tQ2 πR+ao2 (1)
63
Another factor that affects the coalescence behaviour in this experiment was the
solubility of a component into the opposite phase which allows mass transfer to
occur. Nielsen et al (1958) found that coalescence taking place in mutually
saturated phases always results in longer coalescence times compared to the
unsaturated ones. Charles and Mason (1960) also had similar findings on the same
aspect with different liquids. Wang et al (2009) found that coalescence times
increased obviously in the saturated mixtures which were in agreement with Nielsen
et al. However, in the current experiment, the saturation method was different from
Wang et al’s. Their saturation method included glycerol within the mixture of the oil
and aqueous phases whereas the current experiment had the saturation of water
with oil only. Wang et al used glycerol in the saturation process and this halted all
possible mass transfer. However, in this experiment, when glycerol is transferring to
the oil phase due to unsaturation with glycerol in the first place, then a
concentration gradient of the glycerol within the drop can appear. The film region
between the pair of drops is easily saturated as the area involved is very small
compared to the rest of the continuous water phase. As a result there would be a
high glycerol concentration on outer edge of the drops that could give rise to
Marangoni flows that will help film drainage (Davies, 1992). In the current
experiment, such explanation could render validity for the decreased coalescence
time when the mixture is not saturated with glycerol and this allowed mass transfer.
Wang et al (2009) however, found that this was not the case. This suggests that
mass transfer alone cannot fully account for the increased coalescence efficiency in
the presence of glycerol in water. The addition of glycerol tends to make the
interface to deform easier and the non uniform distribution of surfactants on the
interface has been shown to cause fusion of drops because of fluctuations
generated at the interfaces (Evans & Wennerstrom, 1999). This was also observed
by Dickinson (1992) where thermal fluctuations were known to cause coalescence
of drops. Finally, it was noted that the hydrophobic part of glycerol within the oil
64
phase is not long enough to generate steric interactions that would prevent drops
from approaching very closely and would delay coalescence (Tadros, 1996).
Figure 33. Droplets evolution at 0s.
Figure 34. Droplet evolution at 0.256s.
Figure 35. Droplet evolution at 1.101s.
Figure 36. Droplet evolution at 1.726s.
Figure 37. Droplet evolution at 1.731s.
Figure 38. Droplet evolution at 1.732s.
Figure 39. Droplet evolution at 1.733s.
Figure 40. Droplet evolution at 1.734s.
65
Figure 41. Droplet evolution at 1.735s. Figure 42. Droplet evolution at 1.736s.
Figure 43. Droplet evolution at 1.740s.
Figure 44. Droplet evolution at 1.741s.
Figure 45. Droplet evolution at 1.742s.
Figure 46. Droplet evolution at 1.743s.
Figure 47. Droplet evolution at 1.753s.
Figure 48. Droplet evolution at 1.754s.
Figure 49. Droplet evolution at 1.757s.
66
Figure 50. Droplet evolution at 1.758s.
Figure 51. Droplet evolution at 1.797s. Figure 52. Droplet evolution at 1.820s.
67
Figure 53. Interfacial tension per area against % concentration v/v glycerol.
68
0 1 2 3 4 5 6 7 8 9 100.04650
0.04700
0.04750
0.04800
0.04850
0.04900
Interfacial tension per area (kg/m2s2) γagainst % concentration v/v glycerol
Percentage concentration % v/v glycerol
Inte
rfa
cia
l te
nsi
on
pe
r a
rea
(k
g/
m2
s2)
Induced method
69
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 4200
20
40
60
80
100
Cumulative percentage (%) against Coalescence time (s) for 0% concentration
v/v glycerol
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
%
Figure 54. Induced flow cumulative percentage against coalescence time graph for 0% glycerol.
0 5 10 15 20 25 30 35 40 45 50 60 70 80 90 100 200 300 4000
5
10
15
20
25
30
Individual percentage of coalesced drops (%) against Co-alescence time (s) for 0% concentration v/v glycerol
Coalescence time (s)
Ind
ivid
ual
per
cen
tage
of c
oale
sced
dro
ps
(%)
Figure 55. Induced flow individual percentage against coalescence time graph for 0% glycerol.
70
71
0 20 40 60 80 100 1200
20
40
60
80
100
Cumulative percentage (%) against Coalescence time (s) for 0.5% concen-
tration v/v glycerol
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
(%
)
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 320
20
40
60
80
100
Cumulative percentage (%) against Co-alescence time (s) for 1.0% concentration
v/v glycerol
Series2
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
(%
)
0 10 20 30 40 50 60 70 80 90100
11005
10152025303540
Individual percentage of coalesced drops (%) against Coalescence time (s) for 0.5%
concentration v/v glycerol
Series1
Coalescence time (s)
Ind
ivid
ua
l p
erc
en
tag
e o
f co
ale
sce
d d
rop
s (%
)
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10010511005
10152025303540
Individual percentage of coalesced drops (%) against Coalescence time (s) for 0.5%
concentration v/v glycerol
Coalescence time (s)
Ind
ivid
ua
l p
erc
en
tag
e o
f co
ale
sce
d d
rop
s (%
)
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 320
20
40
60
80
100
120
Cumulative percentage (%) against Coalescence time (s) for 1.0% concen-
tration v/v glycerol
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
(%
)
Figure 56. Induced flow cumulative percentage against coalescence time graph for 0.5% glycerol.
Figure 57. Induced flow individual percentage against coalescence time graph for 0.5% glycerol.
Figure 58. Induced flow cumulative percentage against coalescence time graph for 1.0% glycerol.
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 300
5
10
15
20
25
30
Individual percentage of coalesced drops (%) against Coalescence time (s) for 1.0%
concentration v/v glycerol
Coalescence time (s)
Ind
ivid
ua
l p
erc
en
tag
e o
f co
ale
sce
d d
rop
s (%
)
Figure 59. Induced flow individual percentage against coalescence time graph for 1.0% glycerol.
72
73
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 210
20
40
60
80
100
120
Cumulative percentage (%) against Coalescence time (s) for 5.0% concen-
tration v/v glycerol
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
(%
)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200
5
10
15
20
25
30
Individual percentage of coalesced drops (%) against Coalescence time (s) for 5.0%
concentration v/v glycerol
Coalescence time (s)
Ind
ivid
ua
l p
erc
en
tag
e o
f co
ale
sce
d d
rop
s (%
)
0 5 10 15 20 25 300
20
40
60
80
100
120
Cumulative percentage (%) against Coalescence time for 10.0% concentration
v/v glycerol
Series2
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
(%
)
0 1 2 3 4 5 6 7 8 9 100
102030405060708090
Mean coalescence time (s) against Percentage concentration (%) v/v
glycerol
Percentage concentraion v/v glycerol (%)
Me
an
co
ale
sce
nce
tim
e (
s)
Figure 60. Induced flow cumulative percentage against coalescence time graph for 5.0% glycerol.
Figure 61. Induced flow individual percentage against coalescence time graph for 5.0% glycerol.
Figure 62. Induced flow cumulative percentage against coalescence time graph for 10.0% glycerol.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 260
4
8
12
16
20
Individual percentage of coalesced drops (%) against Coalescence time (s) for 10.0%
concentration v/v glycerol
Series1
Coalescence time (s)
Ind
ivid
ua
l p
erc
en
tag
e o
f co
ale
sce
d d
rop
s (%
)
Figure 63. Induced flow individual percentage against coalescence time graph for 10.0% glycerol.
Figure 64. Induced flow mean coalescence time against glycerol concentration.
Non-Induced flow method
740 5 10 15 20 25 30 35 40 450
20
40
60
80
100
120
Cumulative percentage (%) against Coalescence time (s) for 0 % concentration
v/v glycerol
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
(%
)
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 400
5
10
15
20
25
Individual percentage of coalesced drops (%) against Coalescence time (s) for 0 %
concentration v/v glycerol
Coalescence time (s)
Ind
ivid
ua
l p
erc
en
tag
e o
f co
ale
sce
d d
rop
s (%
)
0 20 40 60 80 100 120 140 160 1800
20
40
60
80
100
120
Cumulative percentage (%) against Coalescence time (s) for 0.5% concen-
tration v/v glycerol
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
(%
)
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 16005
10152025303540
Individual percentage of coalesced drops (%) against Coalescence time (s) for 0.5%
concentration v/v glycerol
Coalescence time (s)Ind
ivid
ua
l p
erc
en
tag
e o
f co
ale
sce
d d
rop
s (%
)
0 20 40 60 80 100 120 1400
20
40
60
80
100
120
Cumulative percentage (%) against Coalescence time (s) for 1.0% concen-
tration v/v glycerol
Coalescence time (s)
Cu
mu
lati
ve
pe
rce
nta
ge
(%
)
0 10 20 30 40 50 60 70 80 90 100 110 1200
5
10
15
20
25
Individual percentage of coalesced drops (%) against Coalescence time (s) for 1.0%
concentration v/v glycerol
Coalescence time (s)Ind
ivid
ua
l p
erc
en
tag
e o
f co
ale
sce
d d
rop
s (%
)
0 20 40 60 80 100 120 140020406080100120
Cumulative percentage (%) against Co-alescence time (s) for 5.0% concentration v/v glycerol
Coalescence time (s)Cumulative perce
ntage (%)
0 10 20 30 40 50 60 70 80 90 100 110 120 13005101520253035
Individual percentage of coalesced drops (%) against Coalescence time (s) for 5% concentration v/v glycerol
Coalescence time (s)Individual percen
tage of coalesced d
rops (%)
Figure 65. Non-induced flow cumulative percentage against coalescence time graph for 0% glycerol.
Figure 66. Non-induced flow individual percentage against coalescence time graph for 0% glycerol.
Figure 67. Non-induced flow cumulative percentage against coalescence time graph for 0.5% glycerol.
Figure 68. Non-induced flow individual percentage against coalescence time graph for 0.5% glycerol.
Figure 69. Non-induced flow cumulative percentage against coalescence time graph for 1.0% glycerol.Figure 70. Non-induced flow individual percentage against coalescence time graph for 1.0% glycerol.Figure 71. Non-induced flow cumulative percentage against coalescence time graph for 5.0% glycerol.Figure 72. Non-induced flow individual percentage against coalescence time graph for 5.0% glycerol.
75
0 1 2 3 4 5 60
10
20
30
40
50
60
Mean coalescence time (s) against Per-centage concentration (%) v/v glycerol
Percentage concentration (%) v/v glycerol
Me
an
co
ale
sce
nce
tim
e (
s)
Figure 73. Non-induced flow mean coalescence time against glycerol concentration.
6 Conclusion
In this work, the work of Wang et al (2009) was extended such that non-induced
flow coalescence method was introduced to compare the results obtained with
induced flow. The induced flow method yielded very similar trends with the work of
Wang et al. It was found that with increasing concentration of glycerol, the
interfacial tension decreased and hence the surface free energy. Decreasing the
interfacial tension by increasing the glycerol concentration on both methods
showed opposite results. For non-induced method, the binary coalescence time
increased and at 10% glycerol concentration v/v, zero coalescence was observed.
For induced method, the binary coalescence time decreased favouring coalescence
with increasing glycerol concentration. In another perspective, at 0% glycerol
concentration, the non-induced flow resulted in lower coalescence time than that of
induced flow. This result was in good agreement with the vast literature such that
and introduction of force or shear stress increases the coalescence time enhancing
stability. It can be concluded generally that the glycerol acts as a demulsifier for
induced flow method while an emulsifier when the flow was not induced. However,
the results yielded by the non-induced flow remain sceptical due to human and
experimental errors. Due to time limitation, further investigation prohibited from
investigating repeatedly on the non-induced flow method. Hence, with this result,
the work of Wang et al can remain substantially concrete while further repetitions
should done on the non-flow induced method.
76
7 Future work
7.1 Experiment inefficiency and improvements
The design of the rig is a simple one but with various small degrees of inadequate
design approaches. The surroundings also contribute to large variations of results
obtained from the experiments. This section will detail the necessary improvements
needed to be applied onto the rig and procedures if a more accurate reproducible
work should be done to compare future results based on the similar basis.
7.1.1 Rig body
7.1.1.1 Thermal glue
The rig body is made up of polyethylene plastic which is very inert and does not
contribute any effects to the liquid systems. However, the rig is made by attaching
the edges of the pieces of polyethylene plastics together using thermal glue. During
the fabrication of the rig, the excess thermal glue was removed carefully without
leaving any residual attached to the internal parts of the rig body. However, this
was not achievable and there remained a small amount of excess dried thermal
glue at the joined edges of the plastic pieces which are significantly visible to the
naked eye. They appear to be flaky and light. This could be the reason for the
appearance of very tiny solid particles suspended in the liquid mixture. As seen
before in the literature review, solid particles do appear to stabilize the emulsion
droplets preventing them from coalescing. The thermal glue is shown in Figure.
7.1.1.2 Acetone and methanol contamination
Apart from the thermal glue issue, there were visible stains from acetone and
methanol contamination which were mistakenly used for cleaning the rig previously.
However, it was not advisable as acetone left several white translucent patches of
stains in the rig. It might have damaged the rig as the white patches were not
removable. Methanol is not advisable because of the chemical reactions with the
77
polyethylene plastic pieces. These unknown stains may not be inert and could have
potentially affected the coalescence experiment in many ways.
7.1.1.3 Protrusion cork
The protrusion which is made of polyethylene plastic to partly immobilise the rig
core is inserted and attached securely through a hole drilled at the centre bottom of
the rig body using thermal glue. The protrusion has a hollow space running vertical
through its centre. Hence the protrusion can be viewed as a solid annular cylinder
attached to the bottom of the rig body at its bottom. Obviously the hollow space will
drain the liquid mixture away and hence the cork was inserted at the bottom of the
protrusion to cease any flow of liquid out of the rig. An inert rubber cork is shaped
and inserted very tightly at the bottom of the hollow space in the protrusion.
Despite this, it may have accumulated very minute amount of kerosene or glycerol.
It was found that overnight kerosene will lead to higher stability of the kerosene
droplets. The leftover glycerol or kerosene may deposit and being trapped between
the tiny spaces between the cork and internal sides of the cylindrical protrusion.
They may have altered in their physical properties overnight. Moreover, such
trapped kerosene liquid is extremely difficult to remove once inserted tightly and
securely into the bottom of the hollow space. During washing the hollow space with
the inserted cork were strongly sprayed with RO water and the cork was not
removed for a more considerable clean. The issue of thermal glue is also similar to
that of Section 5.1.1.1.
An improvement for this is a solid cylinder instead of the annular one. The solid
cylinder will cease all potential leakage. However, the attachment of the solid
cylinder requires thermal glue.
7.1.1.4 Slanting of rig core
This is a visual problem for the camera to focus. The rig core is a cubical solid made
out of polyethylene plastic with a vertical cylindrical hollow space at the centre for
78
insertion of the protrusion. When fitted through the protrusion, the cube should lie
flat horizontally on the bottom of the rig body. However this was not the case. It
appeared that the rig core is tilted an angle less than 10o on the right. This problem
was thought to be the cause of the unequal size fitting between the cylindrical
hollow space of the rig core and the protrusion. More specifically, the diameters of
the protrusion and the cylindrical hollow space of the rig core at near bottom are
not similar. Obviously for the rig core to fit through the protrusion, the diameter of
the protrusion should be less than 1% larger than the cylindrical hollow space of the
rig core. However, it appeared that near bottom, the right side of the protrusion at
near bottom was not fabricated at the diameter as top part; like an appearance of
very small outgrowth that is invisible to the eye as shown in Figure.
The problem that this discrepancy in design can cause is the inaccuracy of image
processing of the droplets to obtain the correct interfacial tension. Due to time
limitation, there were no amendments made neither the protrusion nor the
cylindrical hollow space of the rig core. Instead, when taking images or recording
videos of the coalescence process, the camera position was adjusted to match the
inclination of the rig core, and hence the inclination of the needles, so that the tip
needles are correctly aligned horizontally and the height vertically. However, in the
videos and images, the droplets might look a bit less than 90o vertically due to
gravity and buoyancy effects.
Improvement is to eradicate the unequal shape of the protrusion with respect to the
cylindrical hollow space of the rig core.
7.1.1.5 Rig cover and entry of experimental objects
The purpose of the rig cover is to prevent the entry of unwanted solid particles
during the experiment. However, throughout the whole experiment, the cover was
79
left opened as shown in Figure. The reason for this is to allow the capillaries or tube
and the metal temperature detector to pass into the rig body. However, the gap left
by the position of the rig cover may introduce dust particles into the rig and
contaminate the liquid mixture despite the fact that these dust particles may be
light and only affects the top section of the liquid mixture. Larger solid or liquid
particles may gain entry by accident such as coughing and sneezing.
An improvement is to close the exposed top of the rig body with the rig cover
completely. Three small holes can be drilled on top of the cover for the passage of
the capillaries and metal temperature detector into the system. The size of the
holes should be small enough to just fit these objects to reduce the chance of
particles entering and contaminating the system. Sealing these holes can be
difficult because these objects are washed often.
7.1.1.6 Mobility of capillaries and thermocouple
The capillaries and thermocouple were both partly submerged in the liquid mixture.
During experiment it is essential that these objects remain stagnant to go accordant
with the basis of the experiment – no external force to disrupt the coalescence
process. From Figure, it can be seen that the capillaries and thermocouple were
stable clinging onto the top edges of the rig body. However, from experience, these
objects moved significantly due to relaxation, in this case the capillaries. The
thermocouple is attached to a BLUTACK on the top wall of the rig body and due to
the inelasticity of the BLUTACK, the thermocouple fell and moved several times in
the run. The BLUTACK contaminated the system nevertheless and the experiment
has to be redone.
To avoid such experimental slip, it is recommended that a special locking system
with screws and locks that can be custom built on the rig body to secure these
objects to cease any chance of mobility that can disturb the system.
80
7.1.1.7 Plates
The plates were cut to prevent further mobilisation of the rig core. However, more
precision and accuracy of the dimension were needed to immobilise the rig core in a
correct form and not by tilting as shown in Figure.
7.1.1.8 Mixing
Surfactants are added in by removing the rig body cover and slowly pouring the
surfactants while stirring. It is not advisable to pour at a large rate; very small rate
is needed to sufficiently mix well. The stirrer which is a metal spatula is washed and
rubbed with methanol to remove stains and rinse with RO water. Removing the rig
body cover introduces particles into the system.
7.1.2 Rig core
7.1.2.1 Placement of needles
The rig core comes with two parts – the rig core body and the rig core head as
shown in Figure. On the top sides of the cubical rig core body, V-shaped or
triangular grooves were cut off from the rig core body. The function of the grooves
is to allow the cap needles to sit on them while the rig core head presses on them
and screwed to fix and hold the positions of the needles securely. The grooves were
cut accurately to say the least as shown in Figure. However, the problem arises
when adjusting the needles.
The pair of needles was initially straight as shown in Figure and they were bent
using hands on a strongly held vertical metal pole. Such method was very crude but
due to limitations of the experimental sources, it was the only possible way to get
as much similarly bent as possible. Due to limited amount of such needles, only the
best few were chosen for the experiment. As shown in Figure, needles were bent
differently at least. The diameters of the needles were checked using a vernier
calliper instrument.
81
Because the needles are not generally similar in terms of the bent areas,
they were difficult to place in line with each other. The needles should be
vertical parallel to each other and their tips should be horizontally in line
with each other. Nevertheless such difficult task was always completed
through different levels of screw tightness. Despite this, to the naked eye
this may seemed to be aligned but if they were to be accurate measured
their relative positions, they might be not. Such accurate position was to
allow accurate determination of the coalescence time of the two droplets. In
front of the camera view, apart from the needles being vertically parallel and
the tips horizontally aligned, another criterion for accurate ‘measurement’ of
the coalescence behaviour is that when the droplets come in contact, the
droplets should be seen pressed against each other side by side without one
side behind of the other. Their circumference at the point of touching should
be like Figure and not in Figure.
There were experimental sessions where the droplets were generated as shown in
Figure which is not desirable. A major improvement for this is to replace or replicate
a similar needle of the same shape and size.
Another factor is the reproducible positions for every repetition of experiment.
Because every component has to be dissembled for thorough cleaning, upon
reassembly of the components, the general position which reflects the angle of
inclination, distance between the two needles and height will be different at least to
the one tenth of a centimetre.
82
7.1.2.2 Connectors
The connectors from the rig core come from the needles. As shown in Figure, the
connectors were not as tight as it seemed. Because the experiment requires
periodical stirring when adding surfactants, to mix well, care has to be taken to
avoid contacting the connectors. The connector to the needle is not of screw type;
slide, slot, twist and tighten type as shown in Figure and hence they are very
susceptible to the slightest disturbance introduce such as the spatula slightly
contacted it and displaced its initial position which can cause potential leakage of
the oil phase when loosen. It is rather difficult to tighten the loosen connector
without introducing contaminants into the system. When such event occurred, the
whole experiment is to be redone.
The solution to such issue is to use a screw type connector which can be very
secure.
7.1.2.3 Overall design of rig core
The rig core design was simplified one but however, the fact that it has a many
components linked to it made it more exposed to accidental errors. Perhaps to
repeat the experiment using the same methodology, the design of the rig core has
to be significantly improved. For instance, instead of locking the positions of the
needles by pressing and screwing the rig core head, a rig core can be designed
such that the needles can be inserted through a fix hole within the rig core without
any difficulty.
7.1.2.4 Capillaries
The capillaries used are made of polyethylene and they are fragile and susceptible
to sharp bents. Hence pressuring the capillaries should be avoided. One advantage
of this tube is its inert material and its internal section is visible enough to detect
undesirable solid particles from entering the system. The capillaries were more than
83
70 cm which is unnecessarily too lengthy and difficulty arises during cleaning
because even after properly cleaned, the skin of the capillaries are easily
contaminated upon contacting surfaces like the bench or laboratory coat. Hence
more caution has to be taken when handling the capillaries because part of them
was submerged into the liquid mixture. Any contamination from other particles such
as sand or oil particles can ruin the consistency of the system.
7.1.3 Pumps
Due to lack of pump resources, two different infusion pumps were used instead of
the same ones. From the pump calibration figures, it can be seen that at the same
rate, the two different pumps generated two different pump rates. Calibration was
necessary to find the true volumetric flowrate due to inefficiencies of the
instruments. It is suggested that two new similar pumps to be purchased and the
calibration from the two pumps should give equivalent true volumetric flowrate at a
given setting.
Often the pumps give obstacles for a clear coalescence process. This happened
because upon the release of the coalesced droplets or unwanted droplet from each
needle due to unsuccessful coalescence, the pumps were simultaneously stopped to
prevent the growth of droplet, but this did not happen. Even when the pumps had
stopped, the growth of the droplets from the two needles remained continuous
despite the growth rate was slow enough to allow the flow patterns to disperse
away under the basis of time 5 minutes before making first contact. However, in the
middle of the allowance time, say 2.5 minutes, the size of the droplets could be
large enough to cause force of attraction and repulsion between them. However,
despite the efforts in making changes to the pumps, such occurrence was inevitable
throughout the whole experiment.
84
7.1.3.1 Controlling the growth rate
The volumetric flowrate of the two pumps were set to the true volumetric flowrate
of 0.5ml/hr and this corresponds to 0.63ml/hr and 0.68ml/hr for red pump and white
pump respectively look at Figures. When generating the droplets, the size or
specifically the volume of the droplets has to be equal for consistent measurement.
However, difficulty arises when using the experimental pumps. Efforts were made to
generate the droplets volume before contacting each other. If the one droplet is
seen to be outgrowing the other, the flowrate generating the larger droplet is
stopped and then switched on when the smaller droplet comes to the same size as
the larger droplet. The method used in this is very inaccurate as it seemed to be.
The droplets size were always monitored regularly throughout the experiment by
observing the growth rate and the size of the droplets by placing a ruler horizontally
on the computer screen which shows the live magnification of the droplets in the
rig, and then move the horizontal ruler vertically and stop when the tip of one
droplet is found. If the tip of the droplet is not aligned horizontally with this tip, then
the flowrates are adjusted such that the tips are aligned horizontally as shown in
Figure. Again this method show very little accuracy in determining the equality of
both volumes. The volumetric flowrates must be equal to one another.
Fix volumes of droplets can be generated but such function is only available in the
red pump but not on the white one. Solution is to get the same pumps preferably
new.
Overall the pumps should be control by a controller such that one switch will trigger
the motors of the pumps and will induce equivalent flow at low flowrates. Previously
a three way valve was used but had been a failure due to the unequal flowrates
from two sides as shown in Figure.
85
7.1.4 Thermocouple
The fabrication of the thermocouple is complex and was completed successfully by
Aime. The components of the thermocouple were of detectors as shown in Figure
which are programmed to function as temperature measurer. Calibration was
necessary to obtain correct temperature measurements using the instrument.
However, throughout the experiment, the sensitivity of the temperature was very
high as shown in Figure. The x- axis can be taken as per seconds while the y-axis is
the temperature. The range of temperature variation was large from 19oC- 28oC.
And it was impossible for temperature to rise in an instant given that the room
temperature did not vary as such. The sensitivity of the thermocouple was too
large.
7.1.5 Lens
A more desirable lens is such that it can focus clearly on the interface with higher
resolution.
7.2 Environment
7.2.1 Vibrations
The vibrations were caused by 4 objects, the 2 pumps, cold light and fume
cupboard. The vibration from the pump is caused by the clicking of the motor that
turns the screws which pushes the syringe to induce flow. The motor of the cold
light also produces vibration. The fan in the fume cupboard produces the vibration.
Due to the lack of benches, the pumps and cold light were put on the same bench
as the rig. The fume cupboard is turned on continuously. These vibrations will
nevertheless affect the surroundings of the droplets.
An improvement for this would be to place the pumps and light on different and
separate benches. The fan from the fume cupboard should be switched off when not
using.
86
7.2.2 Temperature
The coalescence process is influenced by the change in temperature. Slight
temperature change can change the interfacial tension and cause inconsistency
with the coalescence behaviour. The only source of temperature gradient is the cold
light. A transparent plastic glass should be placed in front of it to absorb the heat
radiated from the light.
7.3 Data collection
To collect the binary coalescence time, for the first experiment where only touching
was allowed and no flow induced. Such method was tedious because it is very
difficult to define the point at which the surfaces touch and then wait for the
coalescence to occur without inducing flow as shown in Figure. This method of data
collection is very inaccurate. Moreover, in Section, the pumps had relaxation and
often generated slower growth rates even after the pumps were switched off. In
Figure, the dark shadow which forms at this point is taken as the point of contact.
When this occurs to the naked eye, the stopwatch begins to run until the bridging
appears. Due to the limitation of the camera software, the camera was unable to
record very high frame rates for a larger period of an event. For instance, at frame
rate 6200pps, it can only record 3.589 seconds with maximum resolution. Maximum
resolution and frame rate are desirable to obtain the point where the contact and
bridging occur. However, throughout the experiment, the videos and images were
recorded using 1000pps which allowed 22 seconds of recording.
However for the second part of the experiment, this was simpler because the flow is
induced. When the shadow starts to appear, the recording is started and the
stopwatch starts until the droplets coalesced. It was not necessary for the exact
time at which the shadow appears because the difference between the exact time
at such event occurs and the point at which the observer notice its occurrence and
simultaneously starts the stopwatch should be a fraction of a second.
87
This issue is the same as the bridging and the full coalescence of the droplets. We
humans can see Figure as a whole and stopped the time when this occurs but not
Figure (bridging). The time difference between the two should also be a fraction of a
second. Hence the accuracy in such cases is not necessary with respect to the
coalescence time recorded.
It is important that only the middle side of the surface touch of the droplets touch
and not other parts.
7.4 Image processing
The quality of the image and videos are very important especially with regards to
the formation of the bridge
7.5 Batch saturated
When carrying out the experiment for one day, it is recommended that it is
important to use just one syringe batch enough for the experiment. 5mL was
sufficiently enough to produce up to 15 droplets for 0, 0.5, 1.0, 5.0 and 10.0 %.
7.6 Overnight
It is recommended to carry out one whole batch of experiments before proceeding
to repeat the second batch to perform reproducible data or result. The saturated RO
water is prepared 12 hours before the experiment starts.
88
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