EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY
FACULTY OF ENGINEERING
DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
MSc in OIL AND GAS TECHNOLOGY
MASTER THESIS
EFFECT OF NANOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF WATER
ELISAVET MICHAILIDI B.Sc. Petroleum Engineer
SUPERVISOR:PROF. ATHANASIOS MITROPOULOS
KAVALA2016
EFFECT OF NANONOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF
WATER
by
Elisavet D. Michailidi
Submitted to the Department of Petroleum and Natural Gas Technology,
Faculty of Engineering
in Partial Fulfillment of the Requirements for the Degree of
Masters of Sciences in the Oil and Gas Technology
at the
Eastern Macedonia and Thrace Institute of Technology
APPROVED BY:
Thesis Supervisor: Athanasios Mitropoulos
Committee member:
Committee member:
Date defended: xx.xx.2016
EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY
FACULTY OF ENGINEERING
DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
MSc in OIL AND GAS TECHNOLOGY
MASTER THESIS
EFFECT ON NANOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF WATER
ELISAVET D. MICHAILIDI B.Sc. Petroleum Engineer
SUPERVISOR: PROF. ATHANASIOS MITROPOULOS
KAVALA 2016
EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY FACULTY OF ENGINEERING DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY©2016 This Master Thesis and its conclusions in whatsoever form are the property of the author and of the
Department of Petroleum and Natural Gas Technology. The aforementioned reserve the right to
independently use and reproduce (partial or total) of the substantial content of this thesis for teaching and
research purposes. In each case, the title of the thesis, the author, the supervisor and the department
must be cited.
The approval of this Master Thesis by the Department of Petroleum and Natural Gas Technology does
not necessarily imply the acceptance of the author’s views on behalf of the department.
--------------------------------------------------------------
The undersigned hereby declares that this thesis is entirely my own work and it has been submitted to the
Department of Petroleum and Natural Gas Technology in partial fulfillment of the requirements for the
degree of Masters of Sciences in the Oil and Gas Technology. I declare that I respected the Academic
Integrity and Research Ethics and I avoided any action that constitutes plagiarism. I know that plagiarism
can be punished with revocation of my master degree.
Signature
Elisavet D. Michailidi
ABSTRACT
Nanobubbles are nanoscopic gaseous cavities in aqueous solutions which demonstrate
an extended lifetime. Furthermore, water that is enriched with nanobubbles has
completely different fundamental physicochemical and physicomechanical
characteristics compared with water which not contains nanobubbles. The most notable
characteristic of nanobubbles is their apparent extraordinary longevity, being able to last
for weeks and months. To date, experimental evidence concerning the existence of
nanobubbles is sound; however a theoretical understanding is still lacking. The first step
was to review the literature, trying to present formation theories and explain the
pertinent nanobubble stability; the most notable applications of nanobubbles are also
presented and discussed. The purpose of the dissertation was to elucidate the effects of
nanobubble suspensions, produced with nanobubbles generators, and study the
nanobubble formation, size distribution, coalescence, stability and dynamic behavior.
Consequently, gain insight into the properties of nanobubbles. This study discussed the
effects of bulk nanobubbles on the physicochemical properties of water based on
research results from a variety of experiments. Two different types of NB generators
were used. The “porous plug generator” is an innovative device which was designed in
EMaTTech and is under EPO patent. Hence, it was of vital importance to thoroughly
examine both generators and compare their performance. As it derives from Dynamic
Light Scattering and zeta potential measurements, nanobubbles produced from the
porous plug generator are smaller (≃580 nm) and more stable ( -20 mV for 40 mins of
operation) compared to those produced from the nozzle generator, the mean size of
which is ≃580 nm and their zeta potential is -6 mV. It seems that the porous plug
generator produces ≃750×103 NB/cm2 or 750×106 NB/ml. On the other hand, the
nozzle generator only produces ≃125×103 NB/cm2 or 125×106 NB/ml. Furthermore,
nanobubbles have been found to increase electrical conductivity of the water. Vapor
pressure have been found to increase by 116%.
SUBJECT AREA: Nanotechnology
KEYWORDS: nanobubbles, porous plug generator, stability, bulk, DLS
To my beloved ones…
ACKNOWLEDGEMENTS
After an intensive period of seven months, today is the day: writing this note of thanks is
the finishing touch on my thesis. It has been a period of intense learning for me, not only
in the scientific arena, but also on a personal level. Writing this thesis has had a big
impact on me. I would like to reflect on the people who have supported and helped me
so much throughout this period.
First and foremost, I would like to express my sincere gratitude to my advisor Prof.
Athanasios Mitropoulos for the continuous support of my M.Sc. study and research, for
his patience, motivation, enthusiasm, and immense knowledge. His guidance helped
me in all the time of research and writing of this thesis.
Besides my advisor, I would like to thank Dr. Evaggelos Favvas, for his encouragement
and insightful comments. My sincere thanks also goes to Mr. Georgios Bomis for his
precious help over mechanical engineering issues. He designed and manufactured the
NB generators; so his participation in this project was vital. Of course, I especially thank
my labmate Mrs. Ramonna Kosheleva; for her participation and help during the whole
study.
I thank my fellow classmates and friends in Eastern Macedonia Institute of Technology:
Mr. Fotis Zachopoulos, Mr. Stephanos Kyriakidis, Mr. Aris Mitsis and Mrs. Eleni –
Plousia Kosteroglou, for the stimulating discussions, for the sleepless nights we were
working together before deadlines, and for all the fun we have had during our M.Sc.
I take this opportunity to express gratitude to all of the Department of Petroleum
Engineering faculty members for their help, support and valuable lessons.
Moreover, I would like to thank Dr. Eleni Efthimiadou from N.C.S.R. “Demokritos” for her
valuable help with the Dynamic Light Scattering experiments.
Finally, I must express my very profound gratitude to my parents: Mr. Dimitrios
Michailidis and Mrs. Anastasia Stefanidou and to my beloved aunt Mrs. Anastasia
Kaiafa for providing me with unfailing support and continuous encouragement
throughout my years of study and through the process of researching and writing this
thesis. This accomplishment would not have been possible without them. Thank you.
…
TABLE OF CONTENTS
PREFACE ..................................................................................................................... 18
1. CHAPTER1 INTRODUCTION ............................................................................... 19
1.1 INTRODUCTORY PARAGRAPH .................................................................... 19
1.2 PURPOSE OF THE STUDY ............................................................................ 19
1.3 STRUCTURE OF THE DISSERTATION ......................................................... 19
1.4 DEFINITION OF TERMS ................................................................................. 20
1.4.1 BASIC PRINCIPLESOF GAS CAVITIES .................................................. 20
1.4.2 PROPERTIES OF WATER ....................................................................... 22
1.4.3 WATER PHASE DIAGRAM ...................................................................... 25
2. CHAPTER 2 THEORETICAL BACKGROUND ..................................................... 27
2.1 INTRODUCTION ............................................................................................... 27
2.2 STABILITY OF NANOBUBBLES ..................................................................... 27
2.3 NANOBUBBLE FORMATION .......................................................................... 31
2.4 APPLICATIONS OF NANOBUBBLES ............................................................. 32
2.4.1 FLOTATION .............................................................................................. 32
2.4.2 WATER TREATMENT .............................................................................. 33
2.4.3 ELECTROCHEMICAL ANTIFOULING ...................................................... 34
2.4.4 MEDICAL APPLICATIONS ....................................................................... 34
2.4.5 OIL RECOVERY ....................................................................................... 35
2.4.6 AGRICULTURAL & BIOLOGICAL APPLICATIONS ................................. 36
3. CHAPTER 3 EXPERIMENTAL PROCEDURE ..................................................... 37
3.1 INTRODUCTION ............................................................................................. 37
3.2 NANOBUBBLE GENERATION........................................................................ 37
3.2.1 POROUS PLUG GENERATOR ................................................................ 37
3.3 POROUS PLUG CHARACTERIZATION ......................................................... 40
3.3.1 PRINCIPLES OF SCANNING ELECTRON MICROSCOPY ...................... 40
3.4 OPTICAL AND CONFOCAL MICROSCOPY ................................................... 42
3.4.1 CONFOCAL MICROSCOPE ...................................................................... 42
3.5 DYNAMIC LIGHT SCATTERING MEASUREMENTS ...................................... 43
3.5.1 PRINCIPLES OF DYNAMIC LIGHT SCATTERING ................................... 43
3.5.2 DYNAMIC LIGHT SCATTERING EXPERIMENTAL PROCEDURE .......... 45
3.6 ZETA POTENTIAL ........................................................................................... 46
3.6.1 ELECTROPHORETIC LIGHT SCATTERING INSTRUMENTATION ......... 47
3.6.2 Ζ POTENTIAL EXPERIMENTAL PROCEDURE ....................................... 49
3.7 VAPOR PRESSURE ........................................................................................ 49
3.7.1 EXPERIMENTAL PROCEDURE ............................................................... 49
4. CHAPTER 4 RESULTS AND DISCUSSION ........................................................ 51
4.1 INTRODUCTION................................................................................................ 51
4.2 TYNDALL EFFECT .......................................................................................... 51
4.3 SIZE DISTRIBUTION ....................................................................................... 53
4.3.1 SIZE OF MNB AS A FUNCTION OF TIME ................................................ 53
4.3.2 SIZE OF NB AS A FUNCTION OF TEMPERATURE ................................ 58
4.4 ZETA POTENTIAL ........................................................................................... 59
4.5 OPTICAL & CONFOCAL MICROSCOPY ........................................................ 61
4.6 POROUS PLUG CHARACTERIZATION .......................................................... 63
4.7 VAPOR PRESSURE MEASUREMENTS ......................................................... 64
4.8 CONDUCTIVITY MEASUREMENTS ............................................................... 66
4.9 EFFECT ON BIOLOGICAL MATTER; THE CASE OF PLANTS ...................... 67
5. CHAPTER 5 CONCLUSIONS ............................................................................... 71
5.1 CONCLUSIONS .................................................................................................. 71
5.2 FURTHER RESEARCH ................................................................................... 73
6. ABBREVIATIONS – INITIALS ............................................................................ 75
7. REFERENCES ....................................................................................................... 77
LIST OF FIGURES
Figure 1.1 (A) the tetrahedral structure of a single water molecule with the oxygen atom
in the center and the two hydrogen atoms in two corners of the tetrahedron.(B) Ball and
stick model of a water molecule[11]. ................................................................................ 22
Figure 1.2 Water phase diagram ................................................................................... 26
Figure 2.1Basic schematic representation of the Ostwald ripening process .................. 28
Figure 3.1 Schematic representation of the main components of the Nanobubble
Generating Device, and their interconnection. ............................................................... 38
Figure 3.2 Schematic representation of G1, consisting of two rotary pumps connected in
series ............................................................................................................................. 39
Figure 3.3 Pre-chamber, collecting liquid from pumps 1 and 2 ...................................... 39
Figure 3.4 Generator's sample collection tank ............................................................... 40
Figure 3.5 Schematic of a SEM ..................................................................................... 41
Figure 3.6 Principle of confocal microscopy .................................................................. 42
Figure 3.7 Brownian motion, relation of particle size to speed of movement ................. 44
Figure 3.8 Typical Dynamic Light Scattering setup ........................................................ 45
Figure 3.9 Figure depicting the EDL on a negatively charged particle. .......................... 47
Figure 3.10 Schematic showing the instrumentation of ZP measurement by
electrophoretic light scattering ....................................................................................... 48
Figure 3.11 Vapor Pressure experimental configuration ................................................ 50
Figure 4.1 Presence of Tyndall scattering in a sample containing micro-nanobubbles . 51
Figure 4.2 Tyndall scattering is observed in the first two samples which contain
nanobubbles. The phenomenon cannot be observed to the last sample (right) which is
simple water .................................................................................................................. 52
Figure 4.3 Tyndall effect. in colloidal solution light beam is visible. This is due to the
particles (in this case bubbles) absorb light energy and then emit it .............................. 52
Figure 4.4 Size - Time Diagram for Porous Plug 10 min Sample .................................. 53
Figure 4.5 Size - Time Diagram for Porous Plug 20 min Sample .................................. 54
Figure 4.6 Size - Time Diagram for Porous Plug 30 min Sample .................................. 54
Figure 4.7 Size - Time Diagram for Porous Plug 40 min Sample .................................. 55
Figure 4.8 Size- Time Diagram for Vibrating Generator Samples ................................. 55
Figure 4.9 Size distribution of NB produced by porous head (blue) and nozzle (green)
generators. At the left: The auto-correlation coefficient (ACF) diagram in the same
colors. ........................................................................................................................... 57
Figure 4.10 Size-Production Time Diagram for Vibrating and Porous Plug Generator . 57
Figure 4.11 Nanobubble size as a function of temperature for the porous plug (P40) and
nozzle (V40) generators after 40 minuntes of operations ............................................ 58
Figure 4.12 Zeta potential as a function of time for the porous plug generator samples 59
Figure 4.13 Zeta potential as a function of time for the vibrating generator samples .... 60
Figure 4.14 Effect of production time on zeta potential for the porous plug (blue) and
nozzle generator (red)................................................................................................... 61
Figure 4.15 Optical microscopy images for the nozzle (left) and porous plug generators
(right). Both of the samples were taken after 40 mins of operation. .............................. 62
Figure 4.16 Confocal Microscopy Image of a Nanobubble Sample, with fluoresceine .. 63
Figure 4.17 Scanning Electron Microscope images from the sintered porous plug at
X140 (up) and X40 (down) ............................................................................................ 64
Figure 4.18 Vapour Pressure of NB samples, produced from the porous plug generator
at different temperatures; 20 oC (blue), 30 oC (red) and 40 oC (green) ......................... 65
Figure 4.19 Electrical Conductivity as a function of production time ............................. 66
Figure 4.20 Distribution of ions at and near the gas-water interface in an aqueous
solution of electrolyte. The electrolyte ions are attracted to the interface and create the
electrical double layer. .................................................................................................. 67
Figure 4.21 Left: Oat seeds watered with oxygen nanobubbles; Middle: Oat seeds
watered with atmospheric air nanobubbles; Right: Oat seeds watered with normal water
...................................................................................................................................... 68
Figure 4.22 Left: Soya seeds watered with oxygen nanobubbles; Middle: Soya seeds
watered with atmospheric air nanobubbles; Right: Soya seeds watered with normal
water ............................................................................................................................. 68
Figure 4.23 Wheat plant dry weight as a function of time. ............................................ 69
LIST OF TABLES
Table 1.1The electronegativity of hydrogen and the Group 16 elements, and the
molecular weight (MW), melting points (MP) and boiling points (BP) of their hydrides[14]
...................................................................................................................................... 23
Table 1.2 Surface tension of water as a function of temperature[15] ............................... 24
Table 3.1 Samples for DLS measurements ................................................................... 45
Table 3.2 Samples for ζ potential measurements .......................................................... 49
Table 4.1 NB size as a function of temperature ............................................................. 58
PREFACE
The hereby presented dissertation entitled “The effect of nano-bubbles on the
physicochemical properties of water” was submitted to the Department of Petroleum
and Natural Gas Technology, Faculty of Engineering, Eastern Macedonia & Thrace
Institute of Technology in Partial Fulfillment of the Requirements for the Degree of
“Master of Philosophy in Petroleum Engineering”. This dissertation is based on research
upon the physicochemical properties of nanobubbles in aqueous solutions. The
research took place at the Department of Petroleum Engineering, EMaTTech using the
facilities of Hephaestus Advanced Laboratory. Part of the research, and more particular
the Dynamic Light Scattering and confocal microscopy experiments, were conducted at
the National Center for Scientific Research, NCSR Demokritos. The work includes an
extended literature review, examining the topic in detail. The research was conducted
during the period of June 2015-November 2016.
CHAPTER 1: INTRODUCTION
Elisavet D. Michailidi - 19 - 2016
1. CHAPTER1
INTRODUCTION
1.1 INTRODUCTORY PARAGRAPH
Nanobubbles are nanoscopic gaseous cavities in aqueous solutions which demonstrate
an extended lifetime. Furthermore, water that is enriched with nanobubbles has
completely different fundamental physicochemical and physicomechanical
characteristics compared with water which not contains nanobubbles. The most notable
characteristic of nanobubbles is their apparent extraordinary longevity, being able to last
for weeks and months. Existing theories, however, predict that they should dissolve
extremely quickly. Thus, to fully exploit their potential benefits, major developments are
needed in the science underpinning their existence and behavior. In this direction, over
the last few years, the emerging field of microbubble and nanobubble (MNB)
technologies have drawn great attention due to their physicochemical properties and,
as a matter of fact, their applications in many fields of science and technology.
1.2 PURPOSE OF THE STUDY
The overall aim of this dissertation is to study both experimentally and theoretically the
underlying mechanisms by which these nanobubble dispersions come to exist and
persist, and explain some of their unusual properties. The purpose is to elucidate the
effects of nanobubble suspensions, produced with nanobubbles generators, and study
the nanobubble formation, size distribution, coalescence, stability and dynamic
behavior. Consequently, gain insight into the properties of nanobubbles. This
dissertation discusses the effects of bulk nanobubbles on the physicochemical
properties of water based on research results from a variety of experiments.
1.3 STRUCTURE OF THE DISSERTATION
The work is structured as follows: Chapter 1 is the introductory chapter, defining the
research problem and the purpose of the study. Moreover, it presents some basic
principles and definitions. The second chapter establishes the theoretical background
of the study. Chapter 3 thoroughly described the experimental procedures that were
followed. The outcomes of the research are presented and discussed in Chapter 4,
entitled “Results & Discussion”. Finally, the conclusions are presented in Chapter 5.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 20 - 2016
1.4 DEFINITION OF TERMS
1.4.1 BASIC PRINCIPLESOF GAS CAVITIES
In order to have a thorough understanding of the topic, it is considered important to
define the basic principles pertaining multiphase systems and more precisely, gas
bubbles.
Bubbles are defined as gas filled cavities with internal equilibrium pressures at least that
of the external environment. Each bubble is surrounded by an interface with diversified
properties than the bulk solution[1]. Bubbles are formed when a pure homogeneous
liquid undergoes a phase change. Gas bubbles are formed when the amount of
dissolved air in a solution exceeds the saturated solubility. Saturated solubility is the
amount of gas that eventually dissolves in a solution in equilibrium state[2].
Νotwithstanding, this quantity varies depending on the type of solution, type of gas,
temperature, and pressure.
Nucleation is typically defined[3] as:“A process by which a cluster of molecules of one
phase forms in the presence of a bulk phase which has been moved from phase
equilibrium to a metastable region by a change in temperature, pressure, or composition
(in the case of multicomponent systems)”.
Nucleation is the onset of a phase transition in a small region of a medium. The phase
transition can be the formation of a bubble in a liquid or of a droplet in saturated vapour.
There are two main types of nucleation models: homogeneous nucleation and
heterogeneous nucleation.Homogeneous nucleation will be discussed in this section, as
it is the type of nucleation that takes place during bubbles formation in bulk.
Homogeneous nucleation takes place in a liquid phase without the prior presence of
additional phases[4, 5]. It is a consequence of the distribution of thermal energy among
the molecules comprising a volume of liquid. Because some molecules will be more
energetic than others, random processes will occasionally produce groupings of higher
energy molecules. If the average energy is high enough, such a grouping of molecules
represents an inclusion consisting of gas and vapour in the bulk of the liquid. A gas
bubble will dissolve in an undersaturated solution and the effect of surface tension will
cause it to dissolve in a saturated solution. In supersaturated solutions, a bubble can be
in equilibrium because the tendency for the bubble to dissolve due to surface tension is
opposed by the tendency for the bubble to grow by diffusion of gas into it. This
CHAPTER 1: INTRODUCTION
Elisavet D. Michailidi - 21 - 2016
equilibrium is unstable; the bubble will grow or dissolve depending on whether the
perturbation increases or decreases the bubble’s radius relative to its equilibrium
radius[6]. Therefore, a liquid would be free of bubbles after a short period of time. This
does not imply that gas bubbles could not serve as cavitation nuclei. It does imply,
however, that in order for gas bubbles to serve as cavitation nuclei, they must be
stabilized at a size small enough to prevent their rising to the surface of the liquid, yet
large enough so that they will grow when exposed to negative pressure as low as a few
bars. In other words, a stabilization mechanism must exist for a gas bubble before it can
act as a cavitation nucleus[7].
The formation of gas bubbles and their subsequent rise due to buoyancy are very
important fundamental phenomena that contribute significantly to the hydrodynamics in
gas-liquid reactors. The rise of a bubble in dispersion can be associated with possible
coalescence and dispersion followed by its disengagement from the system[2]. The
phenomenon of bubble formation decides the primitive bubble size in the system (which
latter attains an equilibrium size), whereas the rise velocity decides the characteristic
contact time between the phases which governs the interfacial transport phenomena as
well as mixing.
The degree of saturation next to a bubble depends on the gas pressure within the
bubble. Smaller bubbles have higher internal pressure and release gas into under-
saturated solution whereas larger bubbles grow by taking up gas from supersaturated
solution.
According to the widely-accepted Young-Laplace equation, the pressure in the interior
of gas bubbles is inversely proportional to their diameter, with excess pressure. The
Young–Laplace equation describes the equilibrium pressure difference sustained
across the interface between two static fluids, such as water and air, due to the
phenomenon of surface tension.
The Young-Laplace equation can be expressed in its simplified form as follows (Eq.
1.1.):
2P
R
Eq. 1.1
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 22 - 2016
Where:
ΔΡ=Pvap-Pliq are the pressure inside (vapor phase) and outside (liquid phase) of the
bubble, respectively, and γ is the surface tension.
The relative terminal rising velocity of a single gas bubble, moving into a liquid phase, is
determined by its size, by the interfacial tension, by the density and viscosity of the
surrounding liquid.
1.4.2 PROPERTIES OF WATER
As the main purpose of this dissertation is to examine the effect of bulk nanobubbles on
the physicochemical properties of water, the most important properties will be analysed
in this chapter.
The structure of isolated water molecules is well-known. The oxygen atom has six
valence electrons and each hydrogen atom has one, such that the two hydrogen atoms
form covalent bonds with the oxygen leaving two lone pairs of electrons on the oxygen
(Fig. 1.1). The length of the O–H bond is 1 A and the angle between the bonds is
104.5o, or very close to the angle between the vertices of a regular tetrahedron (109o).
Although the structure appears trivial, the physicochemical properties of water are far
from simple[8-10].
Figure 1.1 (A) the tetrahedral structure of a single water molecule with the oxygen atom in the
center and the two hydrogen atoms in two corners of the tetrahedron.(B) Ball and stick model of a
water molecule[11]
.
Based on electronegativity, the electrostatic surface of water is associated with a dipole
with a partially negative oxygen atom and partially positive hydrogen atoms. (Fig. 1).
The polarity of each water molecule results in an attraction between it and other water
molecules, resulting in formation of a hydrogen bond. Hydrogen bonds are relatively
CHAPTER 1: INTRODUCTION
Elisavet D. Michailidi - 23 - 2016
strong (∼5–40 kJ/mol) compared to van der Waals interactions (∼1–10 kJ/mol) but
much weaker than covalent bonds (∼200–1000 kJ/mol). Intermolecular
hydrogenbonding of water leads to enhanced molecular cohesion.
Due to this molecular cohesion, water behaves differently than the hydrides in its group.
Water is primarily a liquid under standard conditions, which is not predicted from its
relationship to other analogous hydrides of the oxygen family (Group 16) in the periodic
table (Table 1.1), which are gases such as hydrogen sulfide. The elements surrounding
oxygen in the periodic table, nitrogen, fluorine, phosphorus, sulfur and chlorine, all
combine with hydrogen to produce gases under standard conditions.
It has been estimated that if water did not possess this extensive cohesion such that it
behaved more like other group 16 hydrides, then its boiling point (BP) would be about
−90 ◦C or almost 200 ◦C lower than the actual value. The electronegativity of the
heavier Group 16 elements, i.e., sulfur, selenium and tellurium, is much lower than that
of oxygen, and close to that of hydrogen. Thus, their hydrides are unable to form
hydrogen bonds[12]and consequently both their melting point (MP) and BP are much
lower than that of water (Table 1.1). Hydrogen bonds also affect other physicochemical
properties of liquid water, such as its dielectric constant (ε 78.5 at 25 ◦C), density (1.000
g/ml at 3.98 ◦C), surface tension and heat of vaporization (40.65 kJ/mol), making them
all higher than expected[13].
Table 1.1The electronegativity of hydrogen and the Group 16 elements, and the molecular weight
(MW), melting points (MP) and boiling points (BP) of their hydrides[14]
Name Symbol Electronegativity Group
16 hydrides
MW MP (oC)
BP (oC)
Oxygen O 3.5 H2O 18 0 100
Sulfur S 2.5 H2S 34 -85 -60
Selenium Se 2.4 H2Se 81 -66 -41
Tellurium Te 2.1 H2Te 130 -49 -2
1.4.2.1 SURFACE TENSION
The cohesive forces among liquid molecules are responsible for the phenomenon of
surface tension. In the bulk of the liquid, each molecule is pulled equally in every
direction by neighboring liquid molecules, resulting in a net force of zero. The molecules
at the surface do not have the same molecules on all sides of them and therefore are
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 24 - 2016
pulled inwards. This creates some internal pressure and forces liquid surfaces to
contract to the minimal area. Surface tension can be defined in terms of force or energy.
Another way to view surface tension is in terms of energy. A molecule in contact with a
neighbor is in a lower state of energy than if it were alone (not in contact with a
neighbor). The interior molecules have as many neighbors as they can possibly have,
but the boundary molecules are missing neighbors (compared to interior molecules) and
therefore have a higher energy. For the liquid to minimize its energy state, the number
of higher energy boundary molecules must be minimized. The minimized quantity of
boundary molecules results in a minimal surface area.
Surface tension, usually represented by the symbol γ, is measured in force per unit
length. Its SI unit is newton per meter.
Water has a high surface tension of 71.99 mN/m at 25 °C, caused by the strong
cohesion between water molecules, the highest of the common non-ionic, non-metallic
liquids.A table, with the surface tension of water at various temperatures is given below
(Table 1.2):
Table 1.2 Surface tension of water as a function of temperature[15]
CHAPTER 1: INTRODUCTION
Elisavet D. Michailidi - 25 - 2016
1.4.3 WATER PHASE DIAGRAM
The liquid-vapour phase boundary in the diagram summarises the variation in the
vapour pressure of liquid water with temperature.
The solid-liquid boundary represents the variation of the melting point with pressure. Its
very steep slope indicates that the pressure changes required to noticeably affect the
melting point are enormous. The line also has a negative gradient up to about 2000
atm, which is highly unusual, indicating as it does that an increase in pressure lowers
the melting point. The reason behind this behaviour can be traced to the fact that ice
has a larger molar volume than liquid water close to its freezing point. (Due to the
hydrogen bonding between water molecules in the solid which enforces a fairly open
cage-like structure.) Raising the pressure thus makes it more favourable for the solid to
transform into the liquid, as it can reduce its volume (and the pressure acting upon it) by
doing so.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 26 - 2016
Figure 1.2 Water phase diagram
CHAPTER 2: THEORETICAL BACKGROUND
Elisavet D. Michailidi - 27 - 2016
2. CHAPTER 2
THEORETICAL BACKGROUND
2.1 INTRODUCTION
To date, experimental evidence concerning the existence of nanobubbles is sound;
however a theoretical understanding is still lacking. The purpose of this chapter is to
review the literature, trying to present formation theories and explain the pertinent
nanobubble stability. Furthermore, the most notable applications of nanobubbles are
presented and discussed.
2.2 STABILITY OF NANOBUBBLES
As it was mentioned in the introduction, it is observed that nanobubbles demonstrate an
extremely long lifetime, ranging to several weeks[16]or even months[17]. This fact has
triggered the interest of the researchers and a large number of studies discuss the
reasons and the phenomena behind this extraordinary property.
Taking into consideration the classical thermodynamics, a paradox seems to appear in
systems containing nanobubbles[18, 19]due to the fact that the longevity of nanobubbles
is not in accordance with the the Young-Laplace Law.
Obviously, since the pressure inside gas bubbles is inversely proportional to their
diameter, microscopic bubbles have large internal pressure. This implies that the air
inside the nanobubble cannot be in equilibrium with the atmosphere.
2P
R
Eq. 2.1
Where:
ΔΡ=Pvap-Pliq are the pressure inside (vapor phase) and outside (liquid phase) of the
bubble, respectively, and γ is the surface tension.
Thus, it would be expected that bubbles which their diameter is in the nanoscale (<1
μm), dissolve immediately, within a few microseconds, in favor of larger ones according
to the phenomenon of Ostwald ripening[20] (Figure 2.1.).
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 28 - 2016
Figure 2.1Basic schematic representation of the Ostwald ripening process
However, nanobubbles demonstrate an extended lifetime compared to larger diameter
bubbles and can be observed in aqueous solutions even after several weeks.
For small diameter bubbles, which are approximately perfect spheres due to surface
tension dominant effect of on their shape, Stokes equation[21] provides a reasonably
accurate description. (Eq.2.2.):
2
18
g dR
Eq. 2.2
Where: R = rise rate (m∙s-1), ρ = density (kg∙m-3), g = gravity (m∙s-2), d = bubble
diameter (m) and μ = dynamic viscosity (Pa∙s).
Hence, micro and nano bubbles have a very slow rise rate.
One of the most dominant theories, suggest that the stability of the nanobubbles is
caused by the fact that their surface is charged. The nanobubble gas/liquid interface is
charged, introducing an opposing force to the surface tension, so slowing or preventing
their dissipation. Each nanobubble is surrounded by a double layer[22, 23]. It is believed[24]
that the developed double layer plays a critical role in the formation and stability of
nanobubbles in aqueous solutions by providing a fairly high repulsive force, which
prevents inter-bubble aggregation and coalescence of the stable bubbles.
Many authors claim that the gas bubbles in aqueous media always develop negative
charges on their surfaces, which suggests that cations (protons) are more likely
hydrated and therefore have a tendency to stay in the bulk aqueous medium, whereas
the smaller, less hydrated and more polarised anions tend to adsorb on the bubbles’
surfaces. However, this specific adsorption has not been fully explained, and its
existence has not been universally accepted
CHAPTER 2: THEORETICAL BACKGROUND
Elisavet D. Michailidi - 29 - 2016
It is also claimed that zeta potential possesses a key role on nanobubble stability. The
zeta potential of a bubble is an important factor in many engineering applications, as it
determines the interaction of the bubble with other materials such as oil droplets and
solid particles[25-27]. Zeta potential assists in predicting long-term stability in colloidal
system[28]. If all the particles in suspension have a high zeta potential (negative or
positive), then they confer stability (the suspension or dispersion will resist
agglomeration)[29]. On the other hand, if the particles have a low potential then they tend
to come closer and will flocculate[30]. Zeta potential can be determined using simple
Smoluchowski equation. The zeta potential is generally negative and varied depending
on the kind of gas introduced[31]. The negative zeta potential value could be explained
by the excess of hydroxyl ions (OH−) relative to hydrogen ions (H+) at the gas–water
interface in pure water[32, 33]. The charging mechanism of nanobubbles was also
believed to be due to preferential adsorption of OH− in electrolytic solution[34]. Ushikubo
et al.[24, 35] found that bubbles with zeta potential >30 mV were much more stable than
those with <30 mV. They suggested that the stability of nanobubbles is mainly due to
the magnification of electrostatic repulsive forces caused by overlapping electrical
double layers of the neighboring bubbles.
Apart from the zeta potential, Ohgaki et al.[36] suggested that the stability of a
nanobubble is strongly related to hydrogen bonding at water–gas interface. They
reported that the surface of the nanobubble contains hard hydrogen bonds that may
reduce the diffusivity of gases through the interfacial film. More recently, Wang, Liu, and
Dong[37] (2013) reported that the surface of a nanobubble is kinetically stable and the
water–gas interface is gas impermeable. A reduction in surface tension of the vapor–
liquid interface reduces the Laplace pressure (Eq. 1.1.) and increases nanobubble
stability. Tolman and others predict a decrease of the surface tension for large curvature
on small scales[38-41]. Specifically, Tolman calculated theoretically that the surface
tension in drops should decrease significantly at small sizes.
In agreement with the hypothesis of a lower internal pressure of nano-bubbles[42] based
on molecular simulation data, concluded that there are too few vapor atoms inside
nano-bubbles, so the interior gas pressure would not be high enough to support the
force balance of a nano-bubble. The authors stated that the internal pressure should be
much lower than predicted by Y-L equation, so it should not be valid for nano-bubbles.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 30 - 2016
Instead, the liquid–gas interface should play an important role in the stabilization of
nano-bubbles.
Another theory from Lohse and Weijs[43]claims that the anomalous stability of
nanobubbles comes from the slow rate of dissolution of gas into a surrounding liquid
already saturated with it.
However, physical chemist Vincent Craig[44] at the Australian National University in
Canberra disagrees, saying that “this theory wouldn’t explain the stability of
nanobubbles produced by electrolysis, where it is unlikely that significant
supersaturation occurs. Also, they predict the bubbles should be steadily shrinking over
time. But as far as I know nobody has seen that – although with pinning it might not be
obvious. An obvious experiment is to measure nanobubble volume over regular
intervals and compare this to the theory.”
A pertinent article by Ducker suggests that a film of water insoluble contaminant at the
vapor–liquid interface decreases the surface tension and increase contact angle of
nanobubbles. Additionally, the layer of water insoluble contaminates acts as a barrier to
diffusion of gases from the bubble, further increasing nanobubble lifetime.Despite the
sufficient theoretical and experimental validation of the contamination mechanism, a
number of areas need to be investigated before this stabilization approach is taken as
standard.
The experiments of Oshikubo et al[35]. indicated that the gas supersaturation condition
of the water can reduce the gas transfer rate from the bubble to the liquid. However, this
factor should be not the only one that can explain the nano-bubble stability, since nano-
bubbles should exist even near or after reaching the saturation equilibrium. Other
contribution to the nano-bubble should be related to the electrical charge at the bubble
surface, as indicated by the ζ -potential measurements. The high absolute value of ζ -
potential could avoid the bubble coalescence by the creation of repulsion forces. At a
high absolute ζ -potential, the electrical charged particles tend to repel each other,
avoiding aggregation of particles in a colloidal dispersion. In the case of a bubble
dispersion, the high ζ-potential could create repulsion forces that would avoid the
coalescence of bubbles and contribute to the stabilization of the bubbles.
Seddon et al.[45, 46] (2011) provided a model for this remarkable nanobubbles stability to
bulk dissolution. Their argument is that the gas in a nanobubble is of Knudsen type.
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Elisavet D. Michailidi - 31 - 2016
This leads to the generation of a bulk liquid flow which effectively forces the diffusive
gas to remain local. The model appears to be in good agreement with experimental
atomic force microscopy. Thus, nanobubbles last for so long because gas molecules
inside them do not escape into the main liquid, but instead hitch a ride on a circular.
Thus, because nanobubbles are so small, a gas molecule would be able to travel from
one side of a bubble to the other, without colliding with any other gas molecule. The
other is that a gas molecule sticking to the surface inside the bubble is most likely to
leave the surface in the perpendicular direction.
2.3 NANOBUBBLE FORMATION
While many theories have been developed concerning the formation of nanobubbles on
hydrophobic surfaces, the discussion about the formation of NB in bulk is still limited.
The formation of nanobubbles is often achieved when the homogenous liquid phase
undergoes a phase change caused by sudden pressure reduction below a critical value,
which is known as cavitation[47]. The cavitation is commonly induced by the passage of
ultrasonic wave (acoustic cavitation), or by high pressure variations in the flowing liquid
(hydrodynamic cavitation)[48-50]. Nanobubbles can be induced by ultrasonication[51, 52].
Nanobubbles can also be generated by means of chemical reactions such as
electrolysis[53]. Venturi-type generator has been widely used for the generation of
nanobubbles based on hydrodynamic cavitation mechanism[48, 50, 54]. The Venturi
system is composed of three main parts, i.e. inflow, tubule, and tapered outflow[55].
Reduction of pressure in Venturi tube can be achieved by increasing liquid velocity in
the conical convergent zone of the tube due to narrow diameter[50]. In the Venturi-type
generator system, both gas and liquid are passed simultaneously via the Venturi tube to
generate the bubble. When pressurized fluid is introduced in the tubular part, the liquid
flow velocity in the cylindrical throat becomes higher whereas pressure becomes lower
compared to the inlet section, thus resulting in cavitation[50]. Ahmadi and Khodadadi
Darban (2013)[54] generated the nanobubbles with a mean diameter of 130-545 nm via
Venturi tube based on hydrodynamic cavitation mechanism. Fan et al. were able to
generate nanobubbles of a mean diameter with less than 50 nm using Venturi tube. Kim
et al. generated nanobubbles with a mean diameter of 300–500 nm via ultrasonication
using palladium-coated electrode. Oeffinger and Wheatley (2004) generated
nanobubbles with a mean diameter of 450–700 nm via ultrasonication of a mixed
surfactant solution with regular purging using octafluoropropane gas. Cho et al. (2005)
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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produced nanobubbles with a mean diameter of 750 and 450 nm by ultrasonification in
pure water and adding surfactant, respectively. More recently, Wu et al. (2012)
successfully generated nanobubbles with a mean diameter less than 500 nm using a
batch high-intensity agitation based on hydrodynamic cavitation. At present, many
nanobubble generators are commercially available in the market for both laboratory and
pilot-scale experiments.
In the current study, a novel nanobubble generator was developed and described at the
next chapter. The generator utilizes a porous plug through which gas-liquid mixture is
forced, leading to the formation of MNB.
2.4 APPLICATIONS OF NANOBUBBLES
The growing significance of nanotechnology as well as the special properties of
nanobubbles has drawn huge attention in many sectors due to their wide range of
applications, including mine industry, medical applications, food processing and
wastewater treatment. Some of the most prominent applications will be analyzed in this
section.
2.4.1 FLOTATION
Solid–liquid separation is the primary step in any wastewater treatment system that can
be achieved by various techniques[56, 57]. Flotation is widely accepted as the most
reliable and practical separation method used for removing suspensions that contain
fats, oil, and grease mixed with low-density organic suspended solids and colloids[58].
The separating mechanism is based on adsorption of gas bubbles (while rising) upon
the surface of finely suspended particles, which reduces the effective specific gravity of
the particles and makes the contaminants rise up to the surface (increase rising velocity
of contaminants)[59]. This technique is often used to separate extremely fine particles or
globules from the solution, which do not possess a significant settling rate[58].
Tsai et al.[60] investigated the nanobubble flotation technology (NBFT) with coagulation
process for the cost-effective treatment of chemical mechanical polishing (CMP)
wastewater in both laboratory and pilot scale flotation reactors. They reported that the
application of NBFT with coagulation increased the wastewater clarification efficiency by
40%, compared with the conventional coagulation/flocculation process. The authors
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Elisavet D. Michailidi - 33 - 2016
also suggested that the operating as well as chemical costs required for NBFT with
coagulation process was much lesser than those in conventional coagulation process.
Nanobubbles form on the surface of hydrophobic particles with higher contact angle (θd)
and ultimately increase Fp and Fe , creating suitable conditions for bubble–article
attachment ). In addition, presence of fine bubbles, particularly nanobubbles and
microbubbles, can reduce Fd and considerably decreases the detachment force[61, 62].
Thus, application of nanobubbles on surfaces of coarse hydrophobic particle increases
the adhesion forces of bubble–particle[63], i.e. Fp and Fe , and decreases the detachment
force; Fd.. The probability of bubble–particle detachment can be explained by the
following equation:
1
1d
at
de
PF
F
………………………………………………………………………………………Eq. 2.3
where Pd is the bubble–particle probability, Fat and Fde are the total attachment force
and detachment force, respectively.
2.4.2 WATER TREATMENT
In the past few years, more and more attention has been given to the potential
applications of the MBs and NBs for water treatment due to their ability to generate
highly reactive free radicals. Recently, MBs/NBs have been used for detoxification of
water[64], while it has been reported that air and nitrogen MBs/NBs can enhance the
activity of aerobic and anaerobic microorganisms in submerged membrane bioreactor.
Evidence shows that nitrogen MBs/NBs cannot only be applied for water and
wastewater treatment, but also for fermentation, brewing as well for human waste
treatment. MBs/NBs have been found to catalyze chemical reactions, and enhance the
detoxification efficiency, thereby improving the efficiency of chemical treatment of water.
The main purpose of water pretreatment is to reduce biological, chemical and physical
loads in order to reduce the running costs and increase the treated water quality. In this
context, air MBs/ NBs as a pretreatment means has been shown to be highly beneficial
for downsizing the water/wastewater treatment plants and improving the quality of
product water.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 34 - 2016
2.4.2.1 DEGRADATION OF ORGANIC POLLUTANS
MBs generated through hydrodynamic cavitation have been employed for degradation
of various organic compounds. Nanobubble technology has been applied for the
degradation of surfactants coupled with UV irradiations[65]. In this line, Tasaki et al.[66]
investigated the effect of 8-W low-pressure mercury lamp in the presence of
nanobubbles (diameter: 720 nm) for the decomposition of sodium
dodecylbenzenesulfonate (SDBS), as a model compound in aqueous solution. The
degradation experiments were conducted with and without nanobubbles using ozone
lamp (185–254 nm). Their experimental results showed that both oxidation and
mineralization rate of SDBS were significantly enhanced under 185-254-nm irradiation
when coupled with nanobubbles. They reported the SDBS removal is effective in the
integrated nanobubbles/vacuum UV system, and observed 99.8% oxidation of SDBS
and 76.8% total organic compound removal after 24 h of irradiation.
2.4.3 ELECTROCHEMICAL ANTIFOULING
The electrochemical formation of nanobubbles at conductive surfaces has recently been
applied to the prevention of surface fouling and de-fouling surfaces. The studies by Wu
et al.[67] show that proteins can be removed from a surface by electrochemically forming
nanobubbles at the surface. Additionally, pre-existing nanobubbles reduce the
absorption of proteins at a surface.
2.4.4 MEDICAL APPLICATIONS
Micro/nanobubbles have wide applications in both disease diagnosis and therapy. By
comparison with microbubbles (1–999 μm), nanobubbles have smaller carrying
capacity. Where the fine bubble quite often leaves the system having transferred only a
small fraction of its “cargo”, the microbubble, due to its small carrying capacity and high
transfer rates (surface area per unit volume increases with decreasing diameter),
frequently becomes capacity-limited[68, 69] and, inevitably, transfer with microbubbles is a
transient process, whereas the historic models of gas–liquid transfer processes assume
pseudo-steady state[70]. With nanobubbles, transfer rates are enormous, yet in fact, the
delivery of bulk materials with nanobubbles is expensive due to the low volumetric flow
rates and gas phase holdups, as well as the energy cost of their production. Yet, even
nanobubbles expensively produced with high energy density techniques have high
transfer efficiencies and performance. Nevertheless, quite a lot of the biomedical uses
CHAPTER 2: THEORETICAL BACKGROUND
Elisavet D. Michailidi - 35 - 2016
of nanobubbles stem from their ability to deliver materials in a controlled fashion. The
migration of nanobubbles can be directed by ultrasonic fields, and their interfaces can
be loaded with surfactant materials that are held by high interfacial affinity. Watanabe et
al. [71] illustrate this paradigm beautifully by delivering genetic materials with ultrasound
— a non-viral “vector” for genetic engineering. Controlled release of materials can be
achieved optically with plasmonically induced nanobubble rupture[72]. Nanobubbles are
also very useful for ultrasound imaging, providing the compressible surface for reflection
of the irradiated waves. Imaging of tumors by a mixture of nanoparticles and
nanobubbles, stabilized by block co-polymers, has proven a successful technique for
medical diagnostics. For biomolecular separations, very small bubbles, termed aphrons,
have shown particularly useful extraction properties. Lye and Stuckey[73], for instance,
report large mass transfer coefficients in the extraction of erythromycin using colloidal
liquid aphrons, an emerging technique for the recovery of microbial secondary
metabolites, such as antibiotics in pre-dispersed solvent extraction (PDSE) processes.
Protein separations are typically by interfacial affinity, rather than phase transfer. For
instance, Noble et al.[74] showed that protein affinity to foams was largely chemical
interactions, but to gas aphrons, electrostatic and hydrophobic interactions dominated.
Most current applications for nanobubbles are high value added activities – such as
drug delivery – for which the cost of production is a secondary issue.
2.4.5 OIL RECOVERY
Oil recovery could benefit from gas-lift technology if sufficiently small microbubbles can
be generated in situ downhole[75], driving such innovations as new pump cavitation
mechanisms[76].
Microbubble CO2 can be generated by injecting CO2 through special porous filters
attached to borehole casing or gas tubing. When injecting microbubble CO2 into saline
aquifers, dissolution of injected CO2 into formation water can be accelerated up more
than 20%, compared to conventional CO2 injection. As a result, microbubble CO2
injection will minimize the free CO2 fraction in the subsurface and consequently
contribute to the long term safety of large-scale CO2 storage. Microbubble CO2 injection
will also lead to effective use of pore space within the reservoirs. P-wave velocity and
resistivity changes obtained when injecting CO2 in microbubble and normal bubble sizes
into artificial brine-saturated porous sandstones indicate more pore water displaced by
the injected microbubble CO2 in terms of sweep efficiency. Combined effects of
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 36 - 2016
enhanced dissolution and sweep efficiency in microbubble CO2 injection can reduce the
potential risk of CO2 leakage from the subsurface and enable us to store more CO2 in in
same reservoirs compared to normal CO2 injection as well as economic benefit of
achieving higher oil recovery.
2.4.6 AGRICULTURAL & BIOLOGICAL APPLICATIONS
Recently, the application of MNB technology in biological processes has been focused
upon considerably. Water that contains MNBs has been reported to accelerate the
growth of plants and shellfish and has also been used in the aerobic cultivation of yeast.
The air micro-bubble supply resulted in a better cultivation of oysters (Heterocapsa
circularisquama) in terms of size and taste[31].. Kurata et al.[77], who applied oxygen
micro-bubbles in an osteoblast cell-culture system, reported greater alkaline
phosphatase activity, which was related to increased osteoblastic cell activity. Park and
Kurata[78] found that fresh weights of micro-bubble treated lettuces were 2.1 times
greater than those of the macro-bubble treated lettuces, when grown under a similar
dissolved oxygen (DO) concentration. Ushikubo et al.[79] showed that when barley
coleoptile cells were floated in water after the generation of oxygen MNBs, cytoplasmic
streaming rates inside the cells were accelerated. Moreover, nanobubbles (NBs) may
provide a transport mechanism for gas delivery to a membrane or cell and thus affect
trans-membrane proteins or the membrane structure. Both effects considerably alter the
cell function[45, 80].
Liu et al[31]., concluded that the germination rates of barley seeds dipped in water
containing NBs were 15–25 percentage points greater than those of seeds dipped in
distilled water, which verified the clear effect of NBs on the physiological activity.
According to NMR results, the number of NBs had a positive correlation with the T2
value of the water, which indicated that NBs could increase the mobility of the water
molecules in bulk. These results suggested that NBs in water could influence its
physical properties, which provides an explanation for the effect of NB promotion on the
physiological activity of living organisms. The other explanation is that negatively
charged NBs may influence the bioelectric field of plants, which is strongly related to
their elongation growth. After completely understanding NBs' ability to promote plant
growth is achieved, the manipulation of NBs will provide an efficient and cost-effective
approach for the cultivation of hydroponic vegetables and allow the development of a
new technology in agriculture applications.
CHAPTER 3: EXPERIMENTAL PROCEDURE
Elisavet D. Michailidi - 37 - 2016
3. CHAPTER 3
EXPERIMENTAL PROCEDURE
3.1 INTRODUCTION
3.2 NANOBUBBLE GENERATION
Two MNB generators, each one based on different principle, were used in the present
study in order to produce nanobubbles in water. The first generator uses a porous plug
head through which nanobubbles are produced, while the second generator utilizes a
nozzle through which the gas-liquid mixture is ejected.
Both generators were developed at EMaTTech and will be described below.
3.2.1 POROUS PLUG GENERATOR
Figure 3.1 is a schematic illustrating the main components of the Nanobubble
Generating Device, and their interconnection. The system consists of three generators
connected in series.
Gas and a liquid are introduced to G1 [100] to produce a MB-containing liquid. The
liquid is fed to G2 [200] where it passes through a porous material [201], generating
MNB. These are stored in G2-Tank [400] and can be collected for various applications,
into G2-Tank [400]. The MNB-containing liquid from this tank can be circulated back to
G1 or pumped to G3 [300]. In G3 the liquid is compressed at 150bar and oscillated back
and forth through a fractal porous material [301] to generate an UNB-containing liquid.
This can be collected in Tank 3 [500], or deposited on a hydrophobic surface such as
Highly Ordered Pyrolytic Graphite1 (HOPG) [501].
1 Highly oriented pyrolytic graphite (HOPG) is a highly pure and ordered form of synthetic graphite. It is
characterised by a low mosaic spread angle, meaning that the individual graphite crystallites are well
aligned with each other. The best HOPG samples have mosaic spreads of less than 1 degree. The
method used to produce HOPG is based on the process used to make pyrolytic graphite, but with
additional tensile stress in the basal plane direction
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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In specific, G1 [100] consists of rotary pump 1 [101] and rotary pump 2 [102] connected
in series (Fig. 3.2). Gas and a liquid enter a mixer [103] through a capillary tube [104]
from a gas tank. Two check valves [105] ensure the flow is one directional. The MB-
containing liquid passes through pump 1 and then pump 2, then into G2 [200] pre
chamber [202] shown in Fig.3.3 Said generator contains a diaphragm assembly [203]
for compressing the MB-containing liquid and a fan [204] for stirring the MB-containing
liquid. In this step, the pre chamber [202] is pressurized at 30 to 40bar to ensure
permeability of the MB-containing liquid through the porous material [201]. Said porous
material can be rotated by switching on the DC motor [205], which in turn rotates the
two rollers [207], [208], a belt [206] and the porous material. The MNB are generated as
the liquid passes through the porous material and are collected in G2-Tank [400] shown
in detail in Fig. 3.4. A bypass system can be used to return the MNB-containing liquid
back to G1 [100].
Figure 3.1 Schematic representation of the main components of the Nanobubble Generating
Device, and their interconnection.
CHAPTER 3: EXPERIMENTAL PROCEDURE
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Figure 3.2 Schematic representation of G1, consisting of two rotary pumps connected in series
Figure 3.3 Pre-chamber, collecting liquid from pumps 1 and 2
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 40 - 2016
Figure 3.4 Generator's sample collection tank
3.3 POROUS PLUG CHARACTERIZATION
The above mentioned porous plug, part of the MNB generator, was characterized by
Scanning Electron Microscopy (SEM). The porous plug consists of sintered brass
particles.
3.3.1 PRINCIPLES OF SCANNING ELECTRON MICROSCOPY
A scanning electron microscope (SEM) is a type of electron microscope that produces
images of a sample by scanning it with a focused beam of electrons. The electrons
interact with atoms in the sample, producing various signals that contain information
about the sample's surface topography and composition. The electron beam is
generally scanned in a raster scan pattern, and the beam's position is combined with
the detected signal to produce an image. SEM can achieve resolution better than 1
nanometer. Specimens can be observed in high vacuum, in low vacuum, in wet
CHAPTER 3: EXPERIMENTAL PROCEDURE
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conditions (in environmental SEM), and at a wide range of cryogenic or elevated
temperatures.
The most common SEM mode is detection of secondary electrons emitted by atoms
excited by the electron beam. The number of secondary electrons that can be detected
depends, among other things, on specimen topography. By scanning the sample and
collecting the secondary electrons that are emitted using a special detector, an image
displaying the topography of the surface is created.
Figure 3.5 Schematic of a SEM
As a first step, the porous plug was rigorously cleaned according to the following
procedure:
1. The plug was placed in acetone and left in the sonicator for 15 minutes.
2. The plug was placed in Methanol and left in the sonicator for 15 minutes.
3. The plug was placed in isopropanol and left in the sonicator for 15 minutes.
After each step the plug was thoroughly cleaned with cotton swabs soaked in
acetone, methanol and isopropanol respectively. Finally, the plug was soaked in
acetone and left aside to dry completely.
SEM analyses were carried out on a Jeol JSM-6390LV instrument. Superficial
observations were performed on the specimens. For cross-section analyses, the sample
was cut.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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3.4 OPTICAL AND CONFOCAL MICROSCOPY
Produced nanobubbles were observed both by optical and by confocal microscopes.
3.4.1 CONFOCAL MICROSCOPE
3.4.1.1 CONFOCAL MICROSCOPY PRINCIPLES
Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM), is
an optical imaging technique for increasing optical resolution and contrast of a
micrograph by means of adding a spatial pinhole placed at the confocal plane of the
lens to eliminate out-of-focus light. The principle of confocal imaging was patented in
1957 by Marvin Minskyand aims to overcome some limitations of traditional wide-field
fluorescence microscopes. In a conventional (i.e., wide-field) fluorescence microscope,
the entire specimen is flooded evenly in light from a light source. All parts of the
specimen in the optical path are excited at the same time and the resulting fluorescence
is detected by the microscope's photodetector or camera including a large unfocused
background part. In contrast, a confocal microscope uses point illumination (see Point
Spread Function) and a pinhole in an optically conjugate plane in front of the detector to
eliminate out-of-focus signal - the name "confocal" stems from this configuration. As
only light produced by fluorescence very close to the focal plane can be detected, the
image's optical resolution, particularly in the sample depth direction, is much better than
that of wide-field microscopes. Figure 3.1is a schematic representation of a confocal
microscope’s principle.
Figure 3.6 Principle of confocal microscopy
CHAPTER 3: EXPERIMENTAL PROCEDURE
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3.4.1.2 EXPERIMENTAL PROCEDURE
A sample of nanobubbles produced with the cavitation generator after 10mins of
operation was colored with Fluorescein isothiocyanate (FITC) and examined with the
confocal microscope.
3.5 DYNAMIC LIGHT SCATTERING MEASUREMENTS
Dynamic Light Scattering (DLS) measurements were conducted in order to determine
the size distribution of the nanobubbles produced with the aforementioned generators.
3.5.1 PRINCIPLES OF DYNAMIC LIGHT SCATTERING
Dynamic light scattering (DLS) is a technique in physics that can be used to determine
the size distribution profile of small particles in suspension or polymers in solution. The
basic principle is simple: In a typical DLS experiment, a solution/suspension of analyte
is irradiated with monochromatic laser light and fluctuations in the intensity of the
diffracted light are measured as a function of time. Intensity data is then collected using
an autocorrelator to determine the size distribution of particles or molecules in a sample.
Figure 3.2 depicts a typical Dynamic Light Scattering setup.
In general, when a sample of particles with diameter much smaller than the wavelength
of light is irradiated with light, each particle will diffract the incident light in all directions.
This is called Rayleigh scattering. In practice, particle samples are typically not
stationary because they are suspended in a solution and as a result they are moving
randomly due to collisions with solvent molecules. This type of motion is called
Brownian motion. Brownian motion is defined as:
“The random movement of particles in a liquid due to the bombardment by the
molecules that surround them”.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 44 - 2016
Figure 3.7 Brownian motion, relation of particle size to speed of movement
The particles in a liquid move about randomly and their speed of movement is used to
determine the size of the particle. Brownian motion is vital for DLS analysis because it
allows the use of the Stokes-Einstein equation (Eq. 3.1.) to relate the velocity of a
particle in solution to its hydrodynamic radius:
6
ktD
Eq. 3.1
In the Stokes-Einstein equation, D is the diffusion velocity of the particle, k is the
Boltzmann constant, T is the temperature, η is the viscosity of the solution and a is the
hydrodynamic radius of the particle. The diffusion velocity (D) in the Stokes-Einstein
relation is inversely proportional to the radius of the particle (a) and this shows that for a
system undergoing Brownian motion, small particles should diffuse faster than large
ones.
CHAPTER 3: EXPERIMENTAL PROCEDURE
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Figure 3.8 Typical Dynamic Light Scattering setup
3.5.2 DYNAMIC LIGHT SCATTERING EXPERIMENTAL PROCEDURE
A Malvern Zetasizer Nano ZS instrument was used to determine the NB size distribution
is a series of samples that were prepared (Table 3.1). The Zetasizer Nano system
measures the rate of the intensity fluctuation and then uses this to calculate the size of
the particles.
Table 3.1 Samples for DLS measurements
Porous Plug Vibration
P10 10min
P20 20min
P30 30min
P40 40min
V10 10min
V20 20min
V30 30min
V40 40min
Cavitation Counter Flow
C10 10min
C20 20min
C30 30min
C40 40min
CF10 10min
CF20 20min
CF30 30min
CF40 40min
The first measurement for the samples from the porous plug generator (P) and the
vibration generator (V) were conducted 24 hours after their production, while for
Cavitation (C) and Counter Flow (CF) generators the first measurements were
conducted immediately after production.
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Deionized water is used as a reference for P and V samples while tap water is used as
a reference for C and CF samples.
Samples P40 and V40 were measured every 24 hours for a period of 7 days and then
every few days for a month.
3.6 ZETA POTENTIAL
Zeta potential is a scientific term for electrokinetic potential[1] in colloidal dispersions. In
the colloidal chemistry literature, it is usually denoted using the Greek letter zeta (ζ),
hence ζ-potential. From a theoretical viewpoint, the zeta potential is the electric potential
in the interfacial double layer (DL) at the location of the slipping plane relative to a point
in the bulk fluid away from the interface. In other words, zeta potential is the potential
difference between the dispersion medium and the stationary layer of fluid attached to
the dispersed particle.
The zeta potential is caused by the net electrical charge contained within the region
bounded by the slipping plane, and also depends on the location of that plane. Thus it is
widely used for quantification of the magnitude of the charge.
The zeta potential is a key indicator of the stability of colloidal dispersions. The
magnitude of the zeta potential indicates the degree of electrostatic repulsion between
adjacent, similarly charged particles in a dispersion. For molecules and particles that
are small enough, a high zeta potential will confer stability, i.e., the solution or
dispersion will resist aggregation. When the potential is small, attractive forces may
exceed this repulsion and the dispersion may break and flocculate. So, colloids with
high zeta potential (negative or positive) are electrically stabilized while colloids with low
zeta potentials tend to coagulate.
Zeta potential is not measurable directly but it can be calculated using theoretical
models and an experimentally-determined electrophoretic mobility or dynamic
electrophoretic mobility.
CHAPTER 3: EXPERIMENTAL PROCEDURE
Elisavet D. Michailidi - 47 - 2016
Figure 3.9 Figure depicting the EDL on a negatively charged particle.
Figure 3.3 shows the EDL on a negatively charged particle. Immediately on top of the
particle surface there is a strongly adhered layer (Stern layer) comprising of ions of
opposite charge i.e. positive ions in this case. Beyond Stern layer a diffuse layer
develops consisting of both negative and positive charges. During electrophoresis the
particle moves towards the electrodes with the slipping plane becoming the interface
between the mobile particles and dispersant. The ζ potential is the electrokinetic
potential at this slipping plane.
Electrophoretic light scattering is commonly used to determine the zeta potential.
3.6.1 ELECTROPHORETIC LIGHT SCATTERING INSTRUMENTATION
The electrophoretic mobility (μe) of the particles is first calculated as (Eq.3.2.):
e
V
E Eq. 3.2
Where V = particle velocity (μm/s), E = electric field strength (Volt/cm) – both known
quantities. The ZP is then calculated from the obtained μe by the Henry's equation
(Eq.3.3.):
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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02 ( )
3
re
e e f K
Eq. 3.3
Whereεr = relative permittivity/dielectric constant, ε0 = permittivity of vacuum, ζ = ZP,
f(Ka) = Henry's function and η = viscosity at experimental temperature.
The mobile particles during electrophoresis scatter an incident laser. As the particles
are mobile the scattered light has different frequency than the original laser and the
frequency shift is proportional to the speed of the particles (Doppler shift). The
instrumentation used for this technique is shown in Fig. 3.4.
The laser beam is split into two and while one beam is directed towards the sample the
other one is used as reference beam. The scattered light from the sample is combined
or optically mixed with the reference beam to determine the Doppler shift. The
magnitude of particle velocity (V) is deduced from the Doppler shift and then the ZP is
measured through the series of mathematical equations enlisted as Eqs. 3.2 and 3.3.
This technique is often used in conjunction with DLS.
Figure 3.10 Schematic showing the instrumentation of ZP measurement by electrophoretic light
scattering
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3.6.2 Ζ POTENTIAL EXPERIMENTAL PROCEDURE
A Malvern Zetasizer Nano ZS instrument was used to determine the NB size distribution
is a series of samples that were prepared (Table 3.2).
Table 3.2 Samples for ζ potential measurements
Porous Plug Vibration
P10 10min
P20 20min
P30 30min
P40 40min
V10 10min
V20 20min
V30 30min
V40 40min
Cavitation Counter Flow
C10 10min
C20 20min
C30 30min
C40 40min
CF10 10min
CF20 20min
CF30 30min
CF40 40min
The first measurement for the samples from the porous plug generator (P) and the
vibration generator (V) were conducted 24 hours after their production, while for
Cavitation (C) and Counter Flow (CF) generators the first measurements were
conducted immediately after production.
Deionized water is used as a reference for P and V samples while tap water is used as
a reference for C and CF samples.
3.7 VAPOR PRESSURE
Vapor pressure (VP) is defined as the pressure exerted by a vapor in thermodynamic
equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed
system.
The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to
the tendency of particles to escape from the liquid (or a solid). A substance with a high
vapor pressure at normal temperatures is often referred to as volatile. The pressure
exhibited by vapor present above a liquid surface is known as vapor pressure. As the
temperature of a liquid increases, the kinetic energy of its molecules also increases. As
the kinetic energy of the molecules increases, the number of molecules transitioning
into a vapor also increases, thereby increasing the vapor pressure.
3.7.1 EXPERIMENTAL PROCEDURE
Experimental measurement of vapor pressure is a simple procedure for common
pressures between 1 and 200 kPa.[1] Procedure consists of isolating the sample in a
container, evacuating the gas, then measuring the equilibrium pressure of the gaseous
phase of the sample in the container at different temperatures. Better accuracy is
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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achieved when care is taken to ensure that the entire substance and its vapor are at
the prescribed temperature. This is done by submerging the containment area in a liquid
bath.
For the measurement of VP, a self-made experimental configuration was used (Figure.
The configuration consists of a vacuum pump, a liquid container, a pressure transmitter
and a water bath. The samples were measured at 20 oC, 30 oC and 40 oC.
A Wika D-10-P pressure transmitter is used in conjunction with a computer, running the
EasyCom 2011 V2.1.2 software, to record the measurements. In order to keep a
constant temperature a Julabo F25 water bath is used.
Figure 3.11 Vapor Pressure experimental configuration
CHAPTER 4: RESULTS AND DISCUSSION
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4. CHAPTER 4
RESULTS AND DISCUSSION
4.1 INTRODUCTION
As stated in Chapter 1, this study aimed at giving evidence for the existence of
nanobubbles as well as studying their effect on the properties of water. As part of the
research, a number of experiments were conducted; the experimental procedures are
extensively described at Chapter 3. The purpose of this chapter is to summarize the
collected data, present them in a comprehensive way. Finally, the results are
extensively discussed and associated the extracted results with the theory.
4.2 TYNDALL EFFECT
The first experimental evidence of the existence of micro-nanobubbles in water was the
observation of the Tyndall effect in MNB-enriched water.
The sample was hit with monochromatic green light beam, and the Tyndall effect was
observed as it can be seen in Figure 4.1 and Figure 4.2.
Figure 4.1 Presence of Tyndall scattering in a sample containing micro-nanobubbles
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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Figure 4.2 Tyndall scattering is observed in the first two samples which contain nanobubbles. The
phenomenon cannot be observed to the last sample (right) which is simple water
The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a
colloid or else particles in a very fine suspension (Figure 4.3). In this case, the Tyndall
effect indicates the presence of gaseous phase in the form of nanobubbles in the water.
Due to the particles (in this case bubbles), which absorb light energy and then emit it
the beam can be seen in the sample.
Figure 4.3 Tyndall effect. in colloidal solution light beam is visible. This is due to the particles (in
this case bubbles) absorb light energy and then emit it
CHAPTER 4: RESULTS AND DISCUSSION
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4.3 SIZE DISTRIBUTION
The size distribution of O2nanobubbles in water was determined by Dynamic Light
Scattering. The results were extracted using the ZetaSizer software and processed
using Microsoft Excel.
4.3.1 SIZE OF MNB AS A FUNCTION OF TIME
The distribution of size of micro-nanobubbles, is varied as a function of time. The
samples were measured for 7 weeks after their production, and the results are
presented in the following diagrams.
Figure 4.4 Size - Time Diagram for Porous Plug 10 min Sample
Obviously, during the first day after production, microbubbles still occur in the water and
the average size of the MNB is 1071 nm. In the second day after production, the mean
size drops to 649 nm. This fact indicates that bigger bubbles had burst while smaller
size bubbles still occur. However, three days after production the mean size rises again
up to 865 nm and reaches 991 nm at day 8. This result, indicates that some of the
bubbles a coalescing and form bigger bubbles, following the Ostwald rippening
phenomenon.
1071
649,9
865,1 859
974,3 991,7
0
200
400
600
800
1000
1200
Size
(n
m)
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Figure 4.5 Size - Time Diagram for Porous Plug 20 min Sample
The same phenomenon was also observed for the 20 min porous plug sample. Here,
the decrease of size is much more steep, dropping from 2370 nm the first day, to 769
nm two days after production and then remaining quite stable.
Figure 4.6 Size - Time Diagram for Porous Plug 30 min Sample
However, as the production time is increased to 30 mins, the size, one day after
production is significantly lower (796.8 nm). This means that the generator produces
less microbubbles.
2370,00
769,60 756,90 794,70 776,30 777,90
0,00
500,00
1000,00
1500,00
2000,00
2500,00
1st day 2nd day 3rd day 4th day 5th day 8th day
Size
(n
m)
796,8
847,9 860,9
764,7
722,4
650
700
750
800
850
900
1st day 2nd day 3rd day 4th day 8th day
Size
(n
m)
CHAPTER 4: RESULTS AND DISCUSSION
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Figure 4.7 Size - Time Diagram for Porous Plug 40 min Sample
The 40 min sample seems to have a great stability over time. It can be observed that for
a period of 7 weeks, the mean size of bubbles dropped from 578 nm the first day after
production to 516 nm the last day. Due to the long production time, no micro-bubbles
occur.
Figure 4.8 Size- Time Diagram for Vibrating Generator Samples
The samples generated from the vibrating device were measured every 24 hours for
seven days. Here, it is also observed that the NB size tends to increase over time, while
the production time has an important effect on the size. The 40 min sample, appears to
578,8 545,4 534,3
610,6 597,8
546,7 563,1
588,5 554,3
588,8
435,2
516
0
100
200
300
400
500
600
700
1st day 3rd day 4th day 5th day 8th day 9th day 10th day 11th day 15th day 17th day 24th day 7 weeks
Size
(n
m)
0
200
400
600
800
1000
1200
1400
1600
1800
1st day 7th day
10 min
20 min
30 min
40 min
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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not only have smaller sized bubbles but also is more stable. As the time passes, the
water is re-circulates into the tank, so bigger bubbles burst driving to the formation of
smaller size bubbles.
The 40 min sample was measured again after seven weeks. The average size is at this
time 200 nm. The hypothesis is again that bigger bubbles had burst but the
concentration is much lower.
Figure 4.6 depicts the size - intensity distribution for porous plug generator and vibration
generator for the 40 min sample. Also, the Auto Correlation Function diagram is cited.
Comparing the results of DLS for both methods (i.e. porous plug and nozzle) it can be
seen that the size of NB from porous plug generator is smaller than those produced by
the nozzle (824 nm), fact also shown from auto-correlation function (ACF) decay time,
as it can be seen that the curve for the porous plug is much steeper. However, the
nozzle performs more uniform distribution compared to the porous plug; where two
peaks are observed at 580 nm and 120 nm. Figure 4.6 shows the correlation curve of
the typical size of particles. Since the Brownian motion of large particles is slow and the
fluctuation of scattering light intensity changes slowly, the correlation will persist for a
long period of time. Moreover, since the Brownian motion of small particles is fast and
the fluctuation of scattering light intensity changes quickly, the correlation will reduce for
a short period of time. The diffusion coefficient of particles
CHAPTER 4: RESULTS AND DISCUSSION
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Figure 4.9 Size distribution of NB produced by porous head (blue) and nozzle (green) generators.
At the left: The auto-correlation coefficient (ACF) diagram in the same colors.
Figure 4.10 Size-Production Time Diagram for Vibrating and Porous Plug Generator
y = -9,2173x + 979,14
y = -16,813x + 1427,4
300
500
700
900
1100
1300
1500
0 5 10 15 20 25 30 35 40 45
Size
(n
m)
Production Time (min)
Porous Generator Vibrating Generator Linear (Vibrating Generator)
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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Figure 4.7, presents the effect of production time to the average size of the MNB for
both generators. It is observed that for both samples the tendency is negative, and the
average size decreases as a function of time. This can be explained by the fact that as
the process continues, the O2 saturation of the water increases as it re-circulates into
the tank, leading to the formation of smaller bubbles. In any case, the porous plug
generator produces smaller sized bubbles than the vibrating generator.
4.3.2 SIZE OF NB AS A FUNCTION OF TEMPERATURE
Temperature is an important factor affecting the size distribution of nanobubbles. The
samples were measured at different temperatures and the results are shown below.
Table 4.1 NB size as a function of temperature
Day 1
p40 size (nm)
37 469,1
25 529,9
5 640
Day 1
v40 Size (nm)
37 538,9
25 404,2
5 664,5
Figure 4.11 Nanobubble size as a function of temperature for the porous plug (P40) and nozzle
(V40) generators after 40 minuntes of operations
The samples were measured at 37 oC, 25 oC and 5 oC. Interestingly, the mean size
slightly decreases as the temperature is increased. This could be caused by the fact
0
100
200
300
400
500
600
700
800
900
1000
37 25 5
P40
V40
CHAPTER 4: RESULTS AND DISCUSSION
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that in lower temperatures the hydrogen bonds are stronger, then preventing the gas
escaping the bubble and expanding.
4.4 ZETA POTENTIAL
As mentioned earlier, zeta potential is indicative of the stability of nanobubbles. The
measurements were run along with DLS measurements and the results are shown
below.
Figure 4.12 Zeta potential as a function of time for the porous plug generator samples
The absolute value of zeta potential increases to a maximum of -18.3 mV for the 20 min
sample and then constantly decreases. However, seven days after the production, zeta
potential increases as the production time increases. In conjunction with the DLS
results, absolute zeta potential value increases as the bubble size decreases. This is
explained by the fact that smaller bubbles offer more surface for ions to be absorbed
onto the surface, thus repelling the bubbles.
The high value of ζ-potential can be related to the stability of bubbles, explained by the
repulsion forces generated by the electrically charged surfaces of bubbles, which avoid
the bubble coalescence.
The negative value is explained by Kelsall et al. (1996) as attributed to the
predominance of hydroxide ions in the first molecular layers of water at the gas-liquid
interface. It is also described by Najafi et al. (2007) that the negative charge on the
bubble surface is believed to be due to preferential adsorption of hydroxyl ions (OH- ). It
is also described that as the enthalpy of hydration of hydrogen ion (H+ ) and OH- is -
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
1st day 7th day
10 min
20 min
30 min
40 min
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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1104 and -446.8 kJ·mol-1, respectively, H+ preferentially remain in the bulk aqueous
phase, leaving space at the gas-water interface for OH- . Similar understanding is that
an increase in OH concentration near the bubble surface suppresses the dissolution of
gas from bubbles into water and serves as “shells” for the bubbles, thus improving
stability (Takahashi, 2005). Apart from the zeta potential, an explanation for the NBs
stability is reported as the interface of NBs consists of hard hydrogen bonds that are
similar to the hydrogen bonds found in ice and gas hydrates (Ohgaki et al., 2010).
At a high absolute zeta potential, the electrical charged particles tend to repel each
other, avoiding aggregation of particles in a colloidal dispersion. In the case of NB
dispersion, the high absolute values of zeta potential could create repulsion forces that
would avoid the coalescence of NBs and contribute to the stabilization of the NBs.
In conclusion, the higher initial concentration of dissolved gas in water could explain the
extension of the NB stability because a higher dissolved gas concentration is expected
to suppress the dissolution of gas from NB into water.
However, for the vibrating generator, the results are quite different.
Figure 4.13 Zeta potential as a function of time for the vibrating generator samples
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
1st day 7th day
10 min
20 min
30 min
40 min
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The combined results for the average zeta potential for both generators, is presented
below.
Figure 4.14 Effect of production time on zeta potential for the porous plug (blue) and nozzle
generator (red)
4.5 OPTICAL & CONFOCAL MICROSCOPY
Images of nanobubbles were recorded using laser scanning confocal microscope. The
sample was dyed with fluorescein. Fluorescein is a synthetic organic compound
available as a dark orange/red powder slightly soluble in water and alcohol. It is widely
used as a fluorescent tracer. In low concentrations, the color in aqueous solutions is
green. The images are a strong evidence of the existence of nanobubbles in aqueous
solutions. The sample used was produced from the low-pressure cavitation nanobubble
generator. It is observed that the size of bubbles is approximately 1000 nm.
For the optical microscopy images, the sample used was produced from the nozzle (left)
and porous plug generator (right), after 40 minutes of operation. It can be optically
observed that the porous plug generator, gives a larger concentration of nanobubbles
compared to the nozzle generator.
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
0 5 10 15 20 25 30 35 40 45
zeta
po
ten
tian
(m
V)
Production time (min) Porous Plug Vibrating
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More specifically, it was calculated that the NB concentration for the porous plug
generator is approximately 750×103 NB/cm2 or 750×106 NB/ml. On the other hand, the
nozzle generator only produces 125×103 NB/cm2 or 125×106 NB/ml.
Figure 4.15 Optical microscopy images for the nozzle (left) and porous plug generators (right).
Both of the samples were taken after 40 mins of operation.
Furthermore, it can be seen that in the nozzle sample the bubbles are aggregated. The
aggregation happens due to the fact the size of the bubbles is bigger compared to the
porous plug generator. As a result, the bubbles are less stable and this leads to form
aggregates. According to Hunter, this happens due to the fact that bigger bubbles have
low potential and fluctuate[30].
Figure 4.16, is a photograph taken with confocal microscope. The nanobubbles, colored
with fluorescein can be clearly seen as bright green spots. Indicatively, some of them
are marked with red arrows. The mean size is 1000 nm.
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Figure 4.16 Confocal Microscopy Image of a Nanobubble Sample, with fluoresceine
4.6 POROUS PLUG CHARACTERIZATION
The porous plug was characterized using a Scanning Electron Microscope, and the
following images were captured.
The mean size of the sinterened metal spheres is about 150 μm. If it is assumed that
the angle formed between three tangents of the circles is 60o then the radius of the
inscribed circle is:
r=75(1-cos30o)/cos30o=11.5μm.
The porous plug structure plays a critical role to the size of the produced nanobubbles
as the mixture of water and gas is forced through the plug under great pressure and
nanobubbles are formed during this procedure. Thus, the smaller size of the metal
spheres leads to smaller bubbles. Moreover, the NB size distribution is depended on the
size distribution of the spheres; a more uniform sintered spheres distribution leads to
more uniform NB size distribution. Hence, the porous plug is an expedient to control the
properties of the produced samples.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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Figure 4.17 Scanning Electron Microscope images from the sintered porous plug at X140 (up) and
X40 (down)
4.7 VAPOR PRESSURE MEASUREMENTS
The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to
the tendency of particles to escape from the liquid (or a solid). A substance with a high
vapor pressure at normal temperatures is often referred to as volatile. The pressure
exhibited by vapor present above a liquid surface is known as vapor pressure. As the
temperature of a liquid increases, the kinetic energy of its molecules also increases. As
CHAPTER 4: RESULTS AND DISCUSSION
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the kinetic energy of the molecules increases, the number of molecules transitioning
into a vapor also increases, thereby increasing the vapor pressure.
Figure 4.18 Vapour Pressure of NB samples, produced from the porous plug generator at different
temperatures; 20 oC (blue), 30
oC (red) and 40
oC (green)
It is observed that VP value for the NB45 sample shows an increase of approx.116%. It
is well known that increased vapor pressure indicates weaker intermolecular forces.
This is related to the surface tension of water, changes with the introduction of
nanobubbles according to many researchers. Ohgaki et al.[36] suggested that this is
strongly related to hydrogen bonding at water–gas interface. They reported that the
surface of the nanobubble contains hard hydrogen bonds. More recently, Wang, Liu,
and Dong[37] (2013) reported that the surface of a nanobubble is kinetically stable and
the water–gas interface is gas impermeable. A reduction in surface tension is observed.
Tolman and others predict a decrease of the surface tension for large curvature on
small scales[38-41]. Specifically, Tolman calculated theoretically that the surface tension
in drops should decrease significantly at small sizes.
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4.8 CONDUCTIVITY MEASUREMENTS
Figure 4.19 Electrical Conductivity as a function of production time
Figure 4.16 presents the effect of production time on the electrical conductivity of the
samples as a function of time. It is evident that electrical conductivity highly increases
with the introduction of NB in water and is strongly depended on production time.
The results are in agreement with the theory, which suggests the existence of –OH and
–H ions. . It is described by Najafi et al[34]. (2007) that the negative charge on the bubble
surface is believed to be due to preferential adsorption of hydroxyl ions (OH- ). It is also
described that as the enthalpy of hydration of hydrogen ion (H+ ) and OH- is -1104 and
-446.8 kJ·mol-1, respectively, H+ preferentially remain in the bulk aqueous phase,
leaving space at the gas-water interface for OH-. Therefore, hydrogen ions (H+) are
remaining in the bulk and their presence increases the electrical conductivity.
0
5
10
15
20
25
30
35
-10 10 30 50 70 90 110 130 150
Co
nd
uct
ivit
y (μ
S)
Production Time (min)
Conductivity-Production Time
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Figure 4.20 Distribution of ions at and near the gas-water interface in an aqueous solution of
electrolyte. The electrolyte ions are attracted to the interface and create the electrical double
layer.
4.9 EFFECT ON BIOLOGICAL MATTER; THE CASE OF PLANTS
The application of MNB technology in biological processes has been examined. Water
that contains MNBs has been reported to accelerate the growth of plants. Experiments
were conducted using oxygen and atmospheric air nanobubbles on soya and oat plants.
Micro-nanobubble enriched water was used on the aforementioned plants and their
growth rate was examined in comparison with normal water. All the plants were
exposed to the same environmental conditions and watered with the same volume.
In Figure 4.21, the results for oat plants are pictured after 8 days, while figure 4.22
shows the soya plants growth.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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Figure 4.21 Left: Oat seeds watered with oxygen nanobubbles; Middle: Oat seeds watered with
atmospheric air nanobubbles; Right: Oat seeds watered with normal water
Figure 4.22 Left: Soya seeds watered with oxygen nanobubbles; Middle: Soya seeds watered with
atmospheric air nanobubbles; Right: Soya seeds watered with normal water
More experiments were conducted on wheat; and total plant weight was measured.
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Figure 4.23 Wheat plant dry weight as a function of time.
These results suggested that NBs in water could influence its physical properties, which
provides an explanation for the effect of NB promotion on the physiological activity of
living organisms. Negatively charged NBs may influence the bioelectric field of plants,
which is strongly related to their elongation growth. Previous studies[81] have
demonstrated that hyperoxia promotes the growth of plants; air and oxygen-
nanobubbles may affect the growth of life by changing oxygen condition. Furthermore, it
is speculated that larger specific surface area of the microbubbles as well as negative
electronic charges on their surface may promote the growth of plants because
microbubbles can attract positively charged ions that are dissolved in the nutrient
solution.
It is suggested that hyperoxia may induce hypermetabolic state to maintain higher rate
of food digestion and absorption. These reports are in accordance with the results of our
study, suggesting that air and oxygen-nanobubble water solution may contribute to
elevated metabolism and promoted growth.
0
50
100
150
200
250
1 2 3 4 5
We
igh
t (g
r)
Days
NANOBUBUBBLES Ο2
Normal Water
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As we have not used other gas-nonobubble water, whether promoting effect on growth
is due to nanobubbles themselves or elevated oxygen concentration in the water is still
unresolved. Further examination using other gas nanobubbles is required to determine
the effect of nanobubbles themselves on growth of lives. Although there are several
limitations, air and oxygen-nanobubble water significantly promoted the growth of plants
After completely understanding NBs' ability to promote plant growth is achieved, the
manipulation of NBs will provide an efficient and cost-effective approach for the
cultivation of hydroponic vegetables and allow the development of a new technology in
agriculture applications.
CHAPTER 5: CONCLUSIONS
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5. CHAPTER 5
CONCLUSIONS
5.1 CONCLUSIONS
The results of this study are strong experimental evidences for the existence of
nanububbles in the bulk as well as their effect on important water properties such as
vapor pressure and conductivity.
As it is is mentioned, two types of nanobubble generators were designed and
manufactured; the “porous plug generator” and the “nozzle generator”. The nozzle
generator is based on the Venturi effect. In the Venturi-type generator system, both gas
and liquid are passed simultaneously via the Venturi tube to generate the bubble. When
pressurized fluid is introduced in the tubular part, the liquid flow velocity in the cylindrical
throat becomes higher whereas pressure becomes lower compared to the inlet section,
thus resulting in cavitation. According to the literature, similar generators already exist
and are studied by many researchers. However, the porous plug generator is an
innovative device which was designed in EMaTTech and is under EPO patent. Hence,
it was of vital importance to thoroughly examine both generators and compare their
performance.
The first experimental evidence of the existence of micro-nanobubbles in water was the
observation of the Tyndall effect in MNB-enriched water. In this case, the Tyndall effect
indicates the presence of gaseous phase in the form of nanobubbles in the water. Due
to bubbles, which absorb light energy and then emit it, the beam can be seen in the
sample.
As it derives from Dynamic Light Scattering and zeta potential measurements,
nanobubbles produced from the porous plug generator are smaller (≃580 nm) and more
stable ( -20 mV for 40 mins of operation) compared to those produced from the nozzle
generator, the mean size of which is ≃580 nm and their zeta potential is -6 mV. This
fact is also shown from auto-correlation function (ACF) decay time, as it can be seen
that the curve for the porous plug is much steeper. However, the nozzle performs more
uniform distribution compared to the porous plug; where two peaks are observed at 580
nm and 120 nm.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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The high value of ζ-potential can be related to the stability of bubbles, explained by the
repulsion forces generated by the electrically charged surfaces of bubbles, which avoid
the bubble coalescence. The negative value is explained by the predominance of
hydroxide ions in the first molecular layers of water at the gas-liquid interface.
The zeta-potential measurement shows that the nanobubbles are negatively charged
with an electric double layer, presumably due to adsorption of negative OH- ions at the
gas/water interface. It is this double layer that plays a critical dual role in the formation
of stable nanobubbles in aqueous solutions. It not only provides a repulsive force to
prevent interbubble aggregation and coalescence but also reduces the surface tension
at the gas/water interface to decrease the internal pressure inside each bubble.
Another important factor to examine was the concentration of nanobubbles in the liquid.
The concentration was calculated based on microscopy images. It seems that the
porous plug generator produces ≃750×103 NB/cm2 or 750×106 NB/ml. On the other
hand, the nozzle generator only produces ≃125×103 NB/cm2 or 125×106 NB/ml.
It was observed that production time is an important factor for both generators; large
production time leads to the formation of smaller and more stable bubbles. . It is
observed that for both samples the tendency is negative, and the average size
decreases as a function of time. This can be explained by the fact that as the process
continues, the O2 saturation of the water increases as it re-circulates into the tank,
leading to the formation of smaller bubbles.
All of the samples were examined for several weeks. According to Dynamic Light
Scattering Measurements, the average size of the bubbles tends to decrease a few
days after production due to the fact that larger bubbles burst and smaller ones remain.
However, as times passes, the mean size tends to increase again. This can be
explained by the phenomenon of Ostwald ripening.
Again, porous plug nanobubbles seem to have an excellent stability over time. After 8
weeks their size dropped from 578 nm to 516 nm.
Temperature also has an effect on size, which decreases with reduction of temperature.
Electrical conductivity and vapor pressure which are some of the most important
properties of water were studied. It turns out that both of them were affected from the
introduction of nanobubbles. Nanobubble-enriched water has a significantly higher
electrical conductivity than normal water, due to the excess of free ions in the bulk.
CHAPTER 5: CONCLUSIONS
Elisavet D. Michailidi - 73 - 2016
Moreover, the vapor pressure is higher due to the fact of weaker bonds between the
molecules.
It is observed that vapor pressure value shows an increase of approx.116%. It is well
known that increased vapor pressure indicates weaker intermolecular forces. This is
related to the surface tension of water, changes with the introduction of nanobubbles
according to many researchers. It is suggested that this is strongly related to hydrogen
bonding at water–gas interface. They reported that the surface of the nanobubble
contains hard hydrogen bonds.
The growing significance of nanotechnology as well as the special properties of
nanobubbles has drawn huge attention in many sectors due to their wide range of
applications, including mine industry, medical applications, food processing and
wastewater treatment.
The effect on biological matter has been studied; water that contains MNBs has been
reported to accelerate the growth of plants. Micro-nanobubble enriched water was used
on soya, oat and wheat plants and their growth rate was examined in comparison with
normal water. All the plants were exposed to the same environmental conditions and
watered with the same volume. These results suggested that NBs in water could
influence its physical properties, which provides an explanation for the effect of NB
promotion on the physiological activity of living organisms. Negatively charged NBs may
influence the bioelectric field of plants, which is strongly related to their elongation
growth. Hyperoxia promotes the growth of plants; air and oxygen-nanobubbles may
affect the growth of life by changing oxygen condition. Furthermore, it is speculated that
larger specific surface area of the microbubbles as well as negative electronic charges
on their surface may promote the growth of plants because microbubbles can attract
positively charged ions that are dissolved in the nutrient solution.
5.2 FURTHER RESEARCH
The purpose of the dissertation was to elucidate the effects of nanobubble suspensions,
produced with nanobubbles generators, and study the nanobubble formation, size
distribution, coalescence, stability and dynamic behavior. Consequently, gain insight
into the properties of nanobubbles. This study discussed the effects of bulk
nanobubbles on the physicochemical properties of water based on research results
from a variety of experiments.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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While their existence has been confirmed, there are many open questions related to
their formation and dissolution processes along with their structures and properties,
which are difficult to investigate experimentally.
It is considered important to examine nanobubbles under a wide range of pH in order to
gain understanding on their charging mechanism.
Moreover, nanobubbles consist of a condensed gaseous phase with a surface tension
smaller than that of an equivalent system under atmospheric conditions, and contact
angles larger than those in the equivalent nanodroplet case. We anticipate that further
study will provide useful insights into the physics of nanobubbles and will stimulate
further research in the field. For that reasons, surface tension and contact angle
measurements should be conducted.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 75 - 2016
6. ABBREVIATIONS – INITIALS
MNB Micro-Nano Bubbles
NB Nano-Bubbles
VP-LP Vapor Phase-Liquid Phase
BP Boiling Point
MP Melting Point
VP Vapor Pressure
DLS Dynamic Light Scattering
SEM Scanning Electron Microscope
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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