micro piv measurement

7/21/2019 micro piv measurement

http://slidepdf.com/reader/full/micro-piv-measurement 1/29

EIE DEPARTMENT 1 KMEA ENGINEERING COLLEGE

CHAPTER 1

INTRODUCTION

Microfluidics is an emerging field with a wide range of applications in tissue

engineering, single cell analysis drug discovery, bioassays or chemical synthesis. In

all these applications a reliable experimental characterization of the microfluidic

flows is an important issue. In this direction, the micro-PIV (Particle Image

Velocimetry) method emerges as a novel technique for the measurement of velocity

profiles in microfluidic devices.

Microfluidic devices are becoming increasingly common and are seen in

applications ranging from biology to nano technology and manufacturing. Flow

behavior in these small domains can often be counterintuitive because of the low

Reynolds number or the relative importance of surface forces. Micro particle image

velocimetry (μPIV) is a quantitative method that can be used to characterize the

performance of such microfluidic systems with spatial resolutions better than one

micron.

Micron-resolution particle image velocimetry (μPIV, also known as micro-

PIV) refers to the measurement technique wherein fluid motion is measured in a

spatially resolved manner with resolved length scales ranging 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 analyzed 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 present work investigates the hydrodynamics in a microbifurcation

where remarkable vortices are developed. Newtonian flows are studied in

microchannels with aspect ratios (AR = h/w) equal to one, fabricated in cycloofelin

copolymer (COC) – a transparent material (see Fig. 1.).




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

Fig 1.2 Scheme diagram

.




CHAPTER 2DESCRIPTION

2.1 PARTICLE IMAGE VELOCIMETRY

The Particle image velocimetry (PIV) is an optical method of flow

visualization used in education and research. It is used to obtain instantaneous

velocity measurements and related properties in fluids. Typical PIV apparatus

consists of a camera (normally a digital camera with a CCD chip in modern systems),

a strobe or laser with an optical arrangement to limit the physical region illuminated

(normally a cylindrical lens to convert a light beam to a line), a synchronizer to act

as an external trigger for control of the camera and laser.

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

meters. As for sizing, 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. For

some experiments involving combustion, seeding particle size may be smaller, in the

order of 1 micro meter, to avoid the quenching effect that the inert particles may have

on flames. Due to the small size of the particles, the particles motion is dominated by

stokes drag and settling or rising affects. In a model where particles are modelled asspherical (microspheres) at a very low Reynolds number , the ability of the particles

to follow the fluid's flow is inversely proportional to the 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.




2.2 MICRO CHANNELS

Micro channels can be defined as channels whose dimensions are less than

1 millimetre and greater than 1 micron. Micro channels can be fabricated in many

materials — glass, polymers, silicon, metals — using various processes including

surface micromachining, bulk micromachining, moulding, embossing, and

conventional machining with micro cutters. Micro channels offer advantages due to

their high surface to-volume ratio and their small volumes. The large surface-to-

volume ratio leads to high rate of heat and mass transfer, making micro devices

excellent tools for compact heat exchangers. A fibre optic cable or liquid light guide

may connect the laser to the lens setup.

Micro channels are used to transport biological materials such as (in order

of size) proteins, DNA, cells, and embryos or to transport chemical samples and

analytes. Typical of such devices is the i-STAT blood sample analysis cartridge. The

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. Flows in biological devices and chemicalanalysis micro devices are usually much slower than those in heat transfer and

chemical reactor micro devices.

Liquids are generally considered incompressible. Consequently, the density

of a liquid in micro channel flow remains very nearly constant as a function of

distance along the channel, despite the very large pressure gradients that characterize

micro scale flow. This behaviour greatly simplifies the analysis of liquid flows

relative to gas flows, wherein the large pressure drop in a channel leads to large

expansion and large heat capacity.

The large heat capacity of liquids relative to gases implies that the effects of

internal heating due to viscous dissipation are much less significant in liquid flows.

The pressure drop in micro channel flow can be very large, and since all of the work

of the pressure difference against the mean flow ultimately goes into viscous

dissipation, effects due to internal heating by viscous dissipation may be significant.

However they will be substantially lower in liquids than in gases, and they can often




be ignored allowing one to treat the liquid as a constant density, constant property

fluid.

The dynamic viscosity µ of a liquid is larger than that of a gas by a factor of about

100.

This implies much higher resistance to flow through the channels. The kinematic

viscosity of a liquid is typically much less than the kinematic viscosity of a gas, owing

to the much higher density of liquids qualitatively to the thermal conductivity and the

thermal diffusivity.

Liquids in contact with solids or gases have surface tension in the interface. At the

micro scale, the surface tension force becomes one of the most important forces, far

exceeding body forces such as gravity and electrostatic fields.

2.3 MICROFLUIDICS

Microfluidics is a multidisciplinary 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.

Typically fluids are moved, mixed, separated or otherwise processed.

Numerous applications employ passive fluid control techniques like capillary forces.

In some applications external actuation means are additionally used for a directed

transport of the media. Examples are rotary drives applying centrifugal forces for the




fluid transport on the passive chips. Active microfluidics refers to the defined

manipulation of the working fluid by active (micro) components as micro pumps or

micro valves. Micro pumps supply fluids in a continuous manner or are used for

dosing. Micro valves determine the flow direction or the mode of movement of

pumped liquids. Often processes which are normally carried out in a lab are

miniaturized on a single chip in order to enhance efficiency and mobility as well as

reducing sample and reagent volumes.

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 (channel diameters of around 100 nanometres to several

hundred micro meters) some interesting and sometimes unintuitive properties appear.

In particular, the Reynolds number (which compares the effect of momentum of a

fluid to the effect of viscosity) can become very low. A key consequence of this is

that fluids, when side-by-side, do not necessarily mix in the traditional sense, as flow

becomes laminar rather than turbulent; molecular transport between them must often

be through diffusion.

High specificity of chemical and physical properties (concentration, pH,

temperature, shear force, etc.) can also be ensured resulting in more uniform reaction

conditions and higher grade products in single and multi-step reactions.

These technologies are based on the manipulation of continuous liquid flow

through microfabricated channels. Actuation of liquid flow is implemented either by

external pressure sources, external mechanical pumps, integrated mechanical

micropumps, or by combinations of capillary forces. Continuous-flow microfluidic

operation is the mainstream approach because it is easy to implement and less

sensitive to protein fouling problems. Continuous-flow devices are adequate for many

well-defined and simple biochemical applications, and for certain tasks such as

chemical separation, but they are less suitable for tasks requiring a high degree of

flexibility or ineffect fluid manipulations. These closedchannel 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. Permanently etched microstructures also lead

to limited reconfigurability and poor fault tolerance capability.

2.4 TYPES OF FLOWS

Turbulence is flow characterized by recirculation, eddies, and apparent

randomness. Flow in which turbulence is not exhibited is called laminar . It should be

noted, however, that the presence of eddies or recirculation alone does not necessarily

indicate turbulent flow — these phenomena may be present in laminar flow as well.

Mathematically, turbulent flow is often represented via a Reynolds decomposition, in

which the flow is broken down into the sum of an average component and a perturbation component.

A secondary flow is a relatively minor flow superimposed on the primary

flow. The primary flow usually matches very closely the flow pattern predicted using

simple analytical techniques and assuming the fluid is inviscid. An inviscid fluid is a

theoretical fluid having zero viscosity.

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

transient). Whether a particular flow is steady or unsteady, can depend on the chosen

frame of reference. For instance, laminar flow over a sphere is steady in the frame of

reference that is stationary with respect to the sphere. In a frame of reference that is

stationary with respect to a background flow, the flow is unsteady.

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

statistically stationary.

2.5 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.

They are also used to characterize different flow regimes within a similar fluid, such

as laminar or turbulent flow:

• Laminar flow occurs at low Reynolds numbers, where viscous forces are

dominant, and is characterized by smooth, constant fluid motion;

• Turbulent flow occurs at high Reynolds numbers and is dominated by

inertial forces, which tend to produce chaotic eddies, vortices and other

flow instabilities.

eq.2.7.1

where:

• is the mean velocity of the object relative to the fluid (SI units: m/s)

• is a characteristic linear dimension, (travelled length of the fluid;

hydraulic diameter when dealing with river systems) (m)

µ is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s))

is the kinematic viscosity ( ) (m²/s)

is the density of the fluid (kg/m³).

2.6 MICROFLUIDIC DEVICES

Microfluidic devices were composed of silicon or glass and were fabricated

using micromachining techniques borrowed from the semiconductor industry.

Microma- chining of silicon and glass involves the use of wet and dry etching, photo-

lithography, electron beam lithography, and a variety of other techniques, all of which

require the use of clean- room facilities and equipment.

Glass devices are commonly




used in microfluidics due to the straightforward and well-understood fabrication

techniques, as well as the beneficial optical properties, surface stability, and solvent

compatibility of glass, and it is likely that glass devices will continue to be utilized in

many applications. The high cost involved in processing glass and of the material

itself, however, will likely limit their usage as disposable devices.

Fig.2.1 microfluidic device




CHAPTER 3APPLICATIONS

3.1 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.

Most definitions of tissue engineering cover a broad range of applications,

in practice the term 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. Examples

include using living fibroblasts in skin replacement or repair, cartilage repaired with

living chondrocytes, or other types of cells used in other ways.




Fig.3.1 tissue engineering

3.2 SINGLE CELL ANALYSIS

Single cell analysis can enable 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.

3.3 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), efficacy/ potency, metabolic

stability (to increase the half ), 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. One or more of these steps may, but not

necessarily, involve computer-aided drug design.

Discovering drugs that may be a commercial success, or a public health

success, involves a complex interaction between investors, industry, academia,

patent laws, regulatory exclusivity, marketing and the need to balance secrecy with

communication.

3.4 BIOASSAYS

Bioassay (commonly used shorthand for biological assay or assessment), or

biological standardization 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. Bioassays are typically

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. Both are procedures 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.

Bioassays may be qualitative or 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.

3.5 CHEMICAL SYNTHESIS

In chemistry, 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. In modern




laboratory usage, this tends to imply that the process is reproducible, reliable, and

established to work in multiple laboratories.

A chemical synthesis begins by selection of compounds that are known as

reagents or 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 roundbottom 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.

Typically, chemical yields are expressed as a weight in grams or as a percentage of

the total theoretical quantity of product that could be produced. A side reaction is

an unwanted chemical reaction taking place that diminishes the yield of the desired

product.

Chemical synthesis, the construction of complex chemical compounds from

simpler ones. It is the process by which many substances important to daily life are

obtained. It is applied to all types of chemical compounds, but most syntheses are

of organic molecules.

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. In industry,

synthesis is used to make products in large quantity.

Fig.3.2 chemical synthesis




CHAPTER 4

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 as benchmark geometry

the Y-bifurcation with a close branch and aspect ratio one a flow configuration with

potential to create vortices even at low Re.

Fig. 4.1.a Comparison between numerical prediction




4.1.b Quantitative representation of the flow

field obtained with the micro-PIV measuring system.

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.

A well-known method, used in the present paper, consists in direct

comparison of local flow field characteristics with numerical predictions as in the

above fig.

The experimental methods used are based on (i) microscopic flow

visualization – for a quantitative representation of the secondary flows and (ii)

micro-PIV measurements – for quantitative measurements of the velocity profiles

in a primary flow domain, and for vortex identification.




Fig. 4.2 Schematic representation of the micro-PIV measuring system.

A double-pulsed laser is used to generate monochromatic Nd:YAG laser

light. These types of laser systems are specifically designed for PIV application, and

consists of two Nd:YAG laser cavities, beam-combining optics, and frequency-

doubling crystals. The laser emits two light pulses at a wave length of λ = 532 nm.

The pulse duration is on order of 10 ns, and the time delay between light pulses

varies for the presented measurements, from 40 nanoseconds to 1 second. The

illumination light is delivered to an inverted microscope. The optical connection

between the microscope and the laser is obtained via an optical fibre cord.

A dichroic mirror is employed to perform the separation of illumination light

from the emitted fluorescent light of the fluorescent micro-particles.

A CCD camera with an adjustable exposure time (depending on flow

conditions) was used to acquire the images. The camera is coupled to the lens of the

inverted microscope by a Cmount connection. In the presented experiments a 10X

magnification with a numerical aperture of NA = 0.25 was used. For all cases, a

minimum of 100 image pairs were recorded and divided into interrogation areas of

32×32 pixels.




4.1 INVERTED MICROSCOPE

Fig.4.3 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 (e.g., a tissue culture flask).




4.2 DICHROIC MIRROR

Fig.4.4 dichroic mirror

A dichroic mirror is a mirror with significantly different reflection or

transmission properties at two different wavelengths. Dichroic mirrors are required

e.g. for separating or combining laser beams with different wavelengths, e.g. to

introduce the pump beam into a laser resonator.

Dichroic mirrors are usually fabricated as dielectric mirrors. In many cases,

the design involves a compromise between the obtained optical properties, the

required number of layers, and the required growth precision.

As a dichroic mirror has to be transparent for at least one wavelength of

interest, the quality (e.g. transmission losses) 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.

Used before a light source, a dichroic filter produces light that is perceived

by humans to be highly saturated (intense) in colour. Although costly, such filtersare popular in architectural and theatrical a pplications.

Used behind a light source, dichroic reflectors commonly reflect visible light

forward while allowing the invisible infrared light(radiated heat) to pass out of the

rear of the fixture, resulting in a beam of light that is literally cooler(of lower thermal

temperature).Such an arrangement allows a given light to dramatically increase its

forward intensity while allowing the heat generated by the backward-facing part of

the fixture to escape.

http://en.wikipedia.org/wiki/Perception

http://en.wikipedia.org/wiki/Saturation_(color_theory)

http://en.wikipedia.org/wiki/Architecture

http://en.wikipedia.org/wiki/Stage_lighting

http://en.wikipedia.org/wiki/Infrared

http://en.wikipedia.org/wiki/Heat




















Many quartz halogen bulbs have an integrated dichroic reflector f or this

purpose, being originally designed for use in slide projectors to avoid melting the

slides, but now widely used for interior home and commercial lighting. This

improves whiteness by removing excess red however, it poses a serious fire hazardif used in recessed or enclosed luminaires by allowing infrared r adiation into those

luminaires. For these applications non cool beam (ALU or Silverback) lamps must

be used.

In fluorescence microscopy, 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.

Some LCD projectors use dichroic filters instead of prisms to split the white

light from the lamp into the three colours before passing it through the three LCD

units.

They are used as Laser Harmonic Separators. They separate the various

harmonic components of frequency doubled laser systems by selective spectral

reflection and transmission.

Fig.4.5 wavelength vs transmittance graph

http://en.wikipedia.org/wiki/Incandescent_light_bulb

http://en.wikipedia.org/wiki/MR16

http://en.wikipedia.org/wiki/Luminaire




http://en.wikipedia.org/wiki/Aluminium

http://en.wikipedia.org/wiki/Silver

http://en.wikipedia.org/wiki/Fluorescence_microscopy

http://en.wikipedia.org/wiki/Beam_splitter

http://en.wikipedia.org/wiki/LCD_projector






























4.3 CHARGE COUPLED DEVICE (CCD)

Fig.4.6 A specially developed CCD used for ultra violet imaging

Charge-coupled device (CCD) is a device for the movement of electricalcharge, 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. CCD image sensors are

widely used in professional, medical, and scientific applications where high-quality

image data is required. In applications with less exacting quality demands, such as

consumer and professional digital cameras, active pixel sensors (CMOS) are

generally used.

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 (the CCD,

properly speaking).

An image is projected through a lens onto the capacitor array (the

photoactive region), causing each capacitor to accumulate an electric charge

proportional to the light intensity at that location. A one-dimensional array, used in

line-scan cameras, captures a single slice of the image, while a two-dimensional

array, used in video and still cameras, captures a two-dimensional picture

corresponding to the scene projected onto the focal plane of the sensor. Once the

http://en.wikipedia.org/wiki/Epitaxy

http://en.wikipedia.org/wiki/Shift_register

http://en.wikipedia.org/wiki/Lens_(optics)

http://en.wikipedia.org/wiki/Light

















array has been exposed to the image, a control circuit causes each capacitor to

transfer its contents to its neighbour (operating as a shift register). The last capacitor

in the array dumps its charge into a charge amplifier, which converts the charge into

a voltage.

In a digital device, these voltages are then sampled, digitized, and usually

stored in memory; in an analog device (such as an analog video camera), they are

processed into a continuous analog signal (e.g. by feeding the output of the charge

amplifier into a low-pass filter) which is then processed and fed out to other circuits

for transmission, recording, or other processing.

4.4 C-MOUNT

Fig.4.7 A 12mm f/1.2 C-mount lens with a C-mount to CS-mount adapter

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.

Merely to say that a lens is "C-mount" says very little about the lens'

intended use. C-mount lenses have been made for many different formats, the

largest being 4 times as large as the smallest. C-mount lenses are built for the

8mmand 16mm film f ormats and the 1/3, 1/2, 2/3, and 1-inch video formats, which

corresponds to a range of image circles from 5 to 17 mm diameter, approximately.

Some manufacturers have recently introduced lenses for the 4/3 inch / 1.3 inch

format but these remain very expensive.

This is no trivial difference. For example, for the 4/3 format, a 12-mm lens

is a wide-angle lens and will have a retrofocus design. For the 2/3 inch format, a

http://en.wikipedia.org/wiki/Charge_amplifier

http://en.wikipedia.org/wiki/Voltage

http://en.wikipedia.org/wiki/8_mm_film



















12mm lens is "normal" and can have a simple and fast double Gauss layout. For the

1/3 inch format, a 12-mm lens is long and can have a telephoto design.

Some TV lenses lack provision to focus or vary the aperture, so may not

operate properly with film cameras. Also, some TV lenses may have bits that

protrude behind the mount far enough to interfere with the shutter or reflex finder

mechanisms of a film camera.

4.5 Nd: YAG LASER

Fig.4.8 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. The dopant, triply

ionized neodymium, Nd(III), typically replaces a small fraction (1%) of the yttrium

ions in the host crystal structure of the yttrium aluminium garnet (YAG), since the

two ions are of similar size. It is the neodymium ion which provides the lasing

activity in the crystal, in the same fashion as red chromium ion in ruby lasers.

Nd:YAG lasers ar e o ptically pumped using a flashtube or laser diodes. These

are one of the most common types of laser, and are used for many different

applications. Nd:YAG lasers typically emit light with a wavelength of 1064 nm, in

the infrared. However, there are also transitions near 940, 1120, 1320, and 1440 nm.

Nd:YAG lasers operate in both pulsed and continuous mode. Pulsed Nd:YAG lasers

are typically operated in the so-called Q-switching mode: An optical switch is

inserted in the laser cavity waiting for a maximum population inversion in the

neodymium ions before it opens. Then the light wave can run through the cavity,

depopulating the excited laser medium at maximum population inversion.

http://en.wikipedia.org/wiki/Dopant

http://en.wikipedia.org/wiki/Neodymium

http://en.wikipedia.org/wiki/Yttrium_aluminium_garnet


http://en.wikipedia.org/wiki/Laser_pumping


http://en.wikipedia.org/wiki/Flashtube

http://en.wikipedia.org/wiki/Laser_diode


http://en.wikipedia.org/wiki/Wavelength

http://en.wikipedia.org/wiki/Nanometer


http://en.wikipedia.org/wiki/Q-switching

http://en.wikipedia.org/wiki/Population_inversion






































Nd:YAG absorbs mostly in the bands between 730 – 760 nm and 790 –

820 nm. At low current densities krypton f lash lamps have higher output in those

bands than do the more common xenon lamps, which produce more light at around

900 nm. The former are therefore more efficient for pumping Nd:YAG lasers.

4.6 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. Compared to

conventional tomographic PIV, 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.

One major issue that has to be solved in particle path based image

velocimetry is the directional ambiguity of the recovered velocity vectors. Therehas been proposed a lot of counter measures suitable to the current technology of

image recording and processing since early days of 2D particle path velocimetry as

well as in later days of 2D auto correlation based PIV. The continuous laser provides

the base illumination for the particles while the pulse laser is only fired for a short

interval less than the camera exposure time so that the start of the particle streak is

brighter than its end. As a matter of fact, one can compare the mean voxel intensity

between two ends to determine the start and end of the streak.

http://en.wikipedia.org/wiki/Current_density

http://en.wikipedia.org/wiki/Krypton

http://en.wikipedia.org/wiki/Xenon












Fig.4.9 A schematic dig. Of streak imaging technique

4.7 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 (Fig.4.10b) in the centre plane (z = 0; see Fig. 1.a for the system coordinate

used).

At first site, the experimental and the numerical prediction presented in Fig. 4, may

suggest a closed recirculation area formed in the branch. But the selfintersected 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 (see Fig. 4.10.a).

Another effect surprised in Fig.4.10.b. 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.




Because of this deviation and with the increase of flow rate another type of

secondary flows will appear.

Fig.4.10 (a) Inertia effect on the streamlines obtained in a 3D numerical

simulation.

Fig.4.10.b.velocity distribution obtained with the micro-PIV system,

compared with the numerical prediction




These are the Dean vertices that are manifesting in elbows, with a direct impact on

the velocity profiles shape.




CHAPTER 5

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.

Even if the flow at first glance may appear to be quasi-2D, we found that

actually this has a well- defined 3D topology of an open spiralling in the closed

branch.

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

[1] S.T. Wereley and C.D. Meinhart, “Recent advances in Micro-Particle Image

Velocimetry”, Annu. Rev. Fluid Mech., 42, pp. 557– 576, 2010.

[2]

C. Iliescu, H. Taylor, M. Avram, J. Miao, S. Franssila, “A practical guide

for the fabrication of microfluidic devices using glass and silicon,”

Biomicrofluidics, 6(1), 016505, 2012.

[3] D. Choudhury, et al, “Exploitation of chemical and physical constraints for

3D microtissue construction”, Biomicr ofluidics, 5(2), 022203, 2011. [4] T. Sun,

H. Morgan, “Single-cell microfluidic impedance cytometry: a review”,

Microfluidics and nanoflluidics, 8(4), pp. 423 – 443, 2010.

[5] P.S. Dittrich, A. Manz, “Lab-on-a-chip: microfluidics in drug discovery”,

Nature Reviews Drug Discovery, 5, pp. 210 – 218, 2006.

[6]

D. Choudhury, et al, “Fish and Chips: A microfluidic perfusion platform for

monitoring the development of early stage zebrafish embryos”, Lab on a Chip

12(5), pp. 892 – 900, 2012.

[7] E. Jang, S. Kim, W.G. Koh, Microfluidic bioassay system based on

microarrays of hydrogel sensing elements, Biosensors and Bioelectronics,

31(1), pp. 529 – 536, 2012.

[8] S. Marre, K.F. Jensen, “Synthesis of micro and nanostructures in microfluidic

systems”, Chem. Soc. Rev., 39, pp. 1183– 1202, 2010.

[9] R. Karnik, et al, “Microfluidic platform for controlled synthesis of polymeric

nanoparticles”, Nano Lett., 8(9), pp. 2906– 2912, 2008.



[10] C. Cierpka and C.J. Kähler, “Particle imaging techniques for volumetric three-

component (3D3C) velocity measurements in microfluidics”, J. Vis., DOI

10.1007/s12650-011-0107-9.

[11]

C.M. Bălan, D. Broboană, C. Bălan, “Investigation of vortex formation in

microbifurcations” J. Microfluid Nanofluid, doi 10.1007/s10404-012-1005-8.

[12] J.G. Santiago, S.T. Wereley, C.D. Meinhart, D.J. Beebe and R.J. Adrian,

“Particle image velocimetry systems for microfludics”, Exp. Fluids, 25, pp. 316–

319, 1998.

[13]

C. Bălan, “Lecţii de mecanica fluidelor”, Editura Tehnică, Bucureşti, 2003.

[14] M.S.N. Oliveira, L.E. Rodd, G.H. McKinley and M.A. Alves, “Simulations of

extensional flow in microrheometric devices”, J. Microfluid Nanofluid, 5, pp.

809 – 826, 2008.

[15]

M.S.N Oliveira, M.A. Alves, F.T. Pinho, G.H. McKinley, “Viscous flow

through microfabricated hyperbolic contractions”. J. Exp Fluids, 43, pp. 437 – 451, 2007.

[16] M. Gad-el-Hak, “The MEMS handbook”. CRC Press, Boca Raton, 2002.

Documents

micro piv measurement