Micro-PIV measurements for hydrodynamic

characterizations of Microfluidic flows

By Sadiq

Rahim E S7

DOI,CUSAT

The application presented here is based on a non-invasive method for velocity measurements in micro channels.

A micro PIV measurement system is used to obtain velocity profile distributions in primary flow domain and in vortex zone.

Experimental data is compared to data from numerical simulations.

If both data are consistent to each other, it gives information about vortical structures.

AbbreviationsPIV : PARTICLE IMAGE VELOCIMETRYCCD : CHARGE COUPLED DEVICENd-YAG : NEODYMIUM-doped YITTRIUM ALUMINIUM GARNETRe : REYNOLDS NUMBERLCD : LIQUID CRYSTAL DISPLAYNA : NUMERICAL APERATURE

INTRODUCTION

Microfluidics is an emerging field with wide range of applications in tissue engineering, bioassays orchemical synthesis and single cell analysis drug Discovery.

used because in all the above applications characterization of microfluidic flow is important.

Microfluidic devices are used in applications ranging from biology to nanotechnology and manufacturing.

quantitative method that can be used to characterize the performance of such microfluidic systems with spatial resolutions better than one micron.

The resolution of this technique ranges from 10^−4 m to 10^ −7 m.

Flow-tracing particles, either artificially added or naturally occurring, are used to make the motion of the fluid observable.

Two or more images of the moving particles are captured and analysed using spatial correlation methods to infer the fluid’s velocity field from the particle motion.

This measurement technique borrows from the macroscopic particle image velocimetry (PIV) method.

the hydrodynamics in a micro bifurcation where vortices are developed is investigated.

Newtonian flows are studied in micro channels with aspect ratios AR = h/w=1

where h=viscous stress w=strain rate fabricated in cyclofelin copolymer (COC) – a transparent material

The study emphasizes the evolution of vortexes in the vicinity of the Y- bifurcation with the increase of flow rate.

Fig.1.1 Optical microscope image

DESCRIPTION

1. PARTICLE IMAGE VELOCIMETRY

• optical method of flow visualization.

• used to obtain instantaneous velocity and properties of fluids.

PIV apparatus consists of

• camera (digital camera with CCD chips).

• Strobe or laser (to limit the physical region of illumination).

• Synchronizer (external trigger for control of camera and laser).

The particles should be small enough so that response time of the particles to the motion of the fluid is reasonably short to accurately follow the flow, yet large enough to scatter a significant quantity of the incident laser light.

The particles are typically of a diameter in the order of 10 to 100 micro meters.

In a model where particles are modelled as spherical (microspheres) at a very low Reynolds number, the ability of the particles to follow the fluid's flow is inversely proportional to difference in density between the particles and the fluid, and also inversely proportional to the square of their diameter.

The scattered light from the particles is dominated by Mie scattering and so is also proportional to the square of the particles diameters.

Thus the particle size needs to be balanced to scatter enough light to accurately visualize all particles within the laser sheet plane, but small enough to accurately follow the flow.

2. MICRO CHANNELS

channels whose dimensions are less than 1 millimetre and greater than 1 micron.

Fabricated from glass, polymers, silicon, metals using processes including surface micromachining, bulk micromachining, moulding, embossing, and conventional machining with micro cutters.

used to transport biological materials such as proteins, DNA, cells, and embryos or to transport chemical samples and analytes.

sample is taken on-board the chip through a port and moved through the micro channels by pressure to various sites where it is mixed with analyte and moved to a different site where the output is read.

3.MICROFLUIDICS

Field intersecting engineering, physics, chemistry, biochemistry, nanotechnology, and biotechnology, with practical applications to the design of systems in which small volumes of fluids will be handled.

Micro means one of the following features:

• small volumes (µL, nL, pL, fL) • small size • Low energy consumption.

The behaviour of fluids at the micro scale can differ from 'microfluidic' behaviour in that factors such as surface tension, energy dissipation, and fluidic resistance start to dominate the system. Microfluidics studies how these behaviours change, and how they can be worked around, or exploited for new uses .

At small scales the Reynolds number can become very low. A key consequence of this is that fluids do not necessarily mix in the traditional sense, as flow becomes laminar rather than turbulent molecular transport between them must often be through diffusion.

These technologies are based on the manipulation of continuous liquid flow through micro fabricated channels.

Continuous-flow microfluidic operation is the mainstream approach because it is easy to implement and less sensitive to protein fouling problems.

Closed channel systems are inherently difficult to integrate and scale because the parameters that govern flow field vary along the flow path making the fluid flow at any one location dependent on the properties of the entire system.

3.REYNOLD’S NUMBER

The Reynolds number (Re) is a dimensionless quantity that is used to help predict similar flow patterns in different fluid flow situations.

The Reynolds number is defined as the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions.

The character of flow in depends on four variables: • fluid density• fluid viscosity • Pipe diameter• average velocity of flow

Osborne Reynolds was the first to demonstrate that laminar or turbulent flow can be predicted if the magnitude of a dimensionless number, now called the Reynolds number, is known.

The Reynolds number is defined as the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions.

Where v - mean velocity of the object relative to the fluid L - characteristic linear dimension µ-the kinematic viscosity -is the density of the fluidꭍ

4.TYPES OF FLOWS

• Turbulence flow• Laminar flow

Turbulence flow:

characterized by recirculation, eddies, and apparent randomness.

Turbulent flows are unsteady by definition. A turbulent flow can, however, be statistically stationary.

In turbulent flow, unsteady vortices appear on many scales and interact with each other

Laminar flow

Flow in which turbulence is not exhibited is called laminar.

occurs when a fluid flows in parallel layers, with no disruption between the layers.

The fluid tends to flow without lateral mixing, and adjacent layers slide past one another.

Steady-state flow refers to the condition where the fluid properties at a point in the system do not change over time. Otherwise, flow is called unsteady.

5. MICROFLUIDIC DEVICES

Microfluidic devices were composed of silicon or glass and were fabricated using microma- chining techniques borrowed from the semiconductor industry.

Glass devices are commonly used in microfluidics due to the straightforward and fabrication techniques, as well as the beneficial optical properties, surface stability, and solvent compatibility of glass.

microfluidic device

APPLICATIONS

1.TISSUE ENGINEERING

Tissue engineering is the use of a combination of cells, engineering and materials methods, and suitable biochemical and physico-chemical factors to improve or replace biological functions.

tissue engineering is closely associated with applications that repair or replace portions of or whole tissues (i.e., bone, cartilage, blood vessels, bladder, skin, muscle etc.).

Tissue engineering utilizes living cells as engineering materials. Example is using living fibroblasts in skin replacement and cartilage repaired with living chondrocytes.

2.SINGLE CELL ANALYSIS

Single cell analysis helps us to:

Unlock the mystery in gene expression profiles between individual cells.

Avoid the mistake of taking averages of entire cell populations.

Discover previously undetected subpopulations and unveil new regulatory path.

3.DRUG DISCOVERY

Drug discovery is the process by which new candidate medications are discovered.

Modern drug discovery involves the identification of screening hits, medicinal chemistry and optimization of those hits to increase the affinity, selectivity (to reduce the potential of side effects), potency, metabolic stability and oral bioavailability. Once a compound that fulfils all of these requirements has been identified, it will begin the process of drug development prior to clinical trials.

4.BIOASSAYS

BIOASSAYS is a type of scientific experiment. A bioassay involves the use of live animal or plant (in vivo) or tissue or cell (in vitro) to determine the biological activity of a substance, such as a hormone or drug.

conducted to measure the effects of a substance on a living organism and are essential in the development of new drugs and in monitoring environmental pollutants.

Procedure by which the potency or the nature of a substance is estimated by studying its effects on living matter.

A bioassay can also be used to determine the concentration of a particular constitution of a mixture that may cause harmful effects on organisms or the environment.

Two types• qualitative • Quantitative

Qualitative bioassays are used for assessing the physical effects of a substance that may not be quantified, such as seeds fail to germinate or develop abnormally deformity.

Quantitative bioassays involve estimation of the concentration or potency of a substance by measurement of the biological response that it produces.

5.CHEMICAL SYNTHESIS

Chemical synthesis is a purposeful execution of chemical reactions to obtain a product, or several products.

This happens by physical and chemical manipulations usually involving one or more reactions.

It is the construction of complex chemical compounds from simpler ones.

A chemical synthesis begins by selection of compounds that are known as reactants. Various reaction types can be applied to these to synthesize the product, or an intermediate product. This requires mixing the compounds in a reaction vessel such as a chemical reactor or a simple round bottom flask. Many reactions require some form of work-up procedure before the final product is isolated. The amount of product in a chemical synthesis is the reaction yield.

Chemists synthesize chemical compounds that occur in nature in order to gain a better understanding of their structures.

Synthesis also enables chemists to produce compounds that do not form naturally for research purposes.

EXPERIMENTAL METHODOLOGY

The shape and the aspect ratio of the geometry were chosen to highlight the role on vortex formation and its manifestation.

We choose the Y-bifurcation with a close branch and aspect ratio one a flow configuration with potential to create vortices even at low Re.

Y-bifurcation

A micro-PIV measurement system is used to obtain velocity profile distributions in the main flow domains and in the vortex area.

The system can also be used to identify the vortex centre, stagnation points, and the separation line between the main flow and the secondary one.

The experimental data is compared with numerical simulations performed with commercial code FLUENT in 3D representation of the flow domains.

Quantitative representation of the flow field obtained with the micro-PIV measuring system.

The experimental methods used are based on

(i) microscopic flow visualization – for a quantitative representation of the secondary flows

(ii) micro-PIV measurements – for quantitative measurements of the velocity profiles in a primary flow domain, and for vortex identification.

Schematic representation of the micro-PIV measuring system

The important components are

1) INVERTED MICROSCOPE

2) DICHROIC MIRROR

3) CHARGE COUPLED DEVICE (CCD)

4) C-MOUNT

5) Nd: YAG LASER

6) STREAK IMAGING TECHNIQUE

7) EXPERIMENTAL INFERENCE

INVERTED MICROSCOPE

An inverted microscope is a microscope with its light source and condenser on the top, above the stage pointing down, while the objectives and turret are below the stage pointing up.

The stage of an inverted microscope is usually fixed, and focus is adjusted by moving the objective lens along a vertical axis to bring it closer to or further from the specimen

The focus mechanism typically has a dual concentric knob for coarse and fine adjustment.

Depending on the size of the microscope, four to six objective lenses of different magnifications may be fitted to a rotating turret known as a nosepiece.

Inverted microscopes are useful for observing living cells or organisms at the bottom of a large container.

DICHROIC MIRROR

A dichroic mirror is a mirror with significantly different reflection or transmission properties at two different wavelengths.

Dichroic mirrors are required for separating or combining laser beams with different wavelengths.

As a dichroic mirror has to be transparent for at least one wavelength of interest, the quality of the substrate material and possible reflections from the back side need to be considered.

An antireflection coating on the backside can help to reduce such a reflection, and a slight wedge form of the substrate can often eliminate the effects of residual reflection.

Dichroic filters are used as beam splitters to direct illumination of an excitation frequency toward the sample and then at an analyser to reject that same excitation frequency but pass a particular emission frequency.

CHARGE COUPLED DEVICE (CCD)

Charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value.

This is achieved by "shifting" the signals between stages within the device one at a time. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins.

The CCD is a major piece of technology in digital imaging. In a CCD image sensor, pixels are represented by p-doped MOS capacitors.

In a CCD for capturing images, there is a photoactive region (an epitaxial layer of silicon), and a transmission region made out of a shift register .

C-MOUNT

C-mount is a type of lens mount commonly found on 16mm movie cameras, closed-circuit television cameras, machine vision cameras and microscope phototubes. The vast majority of C-mount lenses produce an image circle too small to effectively cover the entire (micro-) four-thirds sensor.

Nd: YAG LASER

Nd: YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) is a crystal that is used as a lasing medium for solid-state lasers.

Nd:YAG lasers are optically pumped using a flashtube or laser diodes. These are one of the most common types of laser, and are used for many different applications.

STREAK IMAGING TECHNIQUE

A schematic dig. Of streak imaging technique

The particle images are recorded by CCD cameras with coplanar continuous and pulsed laser light sheets to reproduce a single 3D particle streak image instead of conventional particle point images recorded at two instants.

This new approach has several advantages: • Cameras do not require optional double exposure

mode.• Continuous laser source can be used and focusing of

particle image is not so severely required for successful velocity recovery.

Moreover, the ghost particles are less generated and if any, can be filtered out more easily.

EXPERIMENTAL INFERENCE

The quantitative characterization of the flow in the micro bifurcation area was made using the micro-PIV measuring system. To assess the velocity and vortex evolution in the bifurcation area, both experimental and numerical results were obtained by plotting the velocity vectors and making qualitative representations of them.

Inertia effect on the streamlines obtained in a 3D numerical

simulation.

At first site, the experimental and the numerical prediction may suggest a closed recirculation area formed in the branch. But the self intersected path lines are only illusive. In fact, the 3D numerical predictions show the real effect of an open spiral that develops in the closed branch.

Another effect is regarding the velocity profiles, which tend to lose the parabolic symmetrical shape, as the Reynolds number increases. This manifestation appears due to the inertial effect amplifications caused by a deviation of 60o of the main flow path before entering in the bifurcation area.

velocity distribution obtained with the micro-PIV system,

compared with the numerical prediction

CONCLUSION

In this study the manifestation of primary flows and the developing of vortical formation were investigated. • The micro-PIV technique allowed performing

accurate measurements of velocity profiles in the main flow as well as in the secondary one.

• The developing of an asymmetric velocity profiles in the main flow has been measured, numerically predicted and physically explained. .

• The capability of the 3D numerical prediction to accurately capture both the dynamics and the kinematics observed experimentally, proves to have a relative rapid impact when we are thinking to exploit the geometrical parameters that are governing the flow in a microfluidic device.

REFERENCE

• S.T. Wereley and C.D. Meinhart, “Recent advances in Micro-Particle Image Velocimetry”.

• S. Dittrich, A. Manz, “Lab-on-a-chip: microfluidics in drug discovery”

• . Karnik, et al, “Microfluidic platform for controlled synthesis of polymeric nanoparticles”

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