Acoustic Microfluidics Final

8/8/2019 Acoustic Microfluidics Final

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Acoustic MicrofluidicsSamuel R. Wilton

Acoustic wave applications within microfluidics have made an enormous impact on the state of the industry and continue to be imperative to the evolution of fully integrated lab-

on-a-chip devices. Innovations utilizing acoustic wave phenomena to manipulate particles and fluids have allowed scientists to perform functions faster, more easily, moreefficiently, with less power consumption, and in certain applications, better than anyother methods. Various applications of acoustics to manipulate particle flow and alignment, mix low Reynolds number microfluidic solutions, and propagate fluids in amicrofluidic channel are discussed thoroughly in this review paper.

I. INTRODUCTION

Piezoelectric transducers [1], devices capable of converting electrical signals into

acoustic pressure waves, allow a broad band of acoustical applications and innovations tofunnel into microfluidics technology. Particle manipulation and sorting, which mayrequire specific particle electrical or material properties for other methods, can begeneralized to encompass particles of almost any size, shape, density, or compressibility

by generating standing surface acoustic waves within the microfluid channel [2].Microfluidic mixing [3], which had once been a problem due to laminar flow andrelatively slow diffusion rates, has many solutions within the field of acoustics that utilizesurface acoustic waves to quickly and efficiently mix two fluids into a uniform solution[4]. Propagating fluids within micro-channels once required bulky syringes and/or control equipment to ensure proper flow rate, but can now be done with small

piezoelectric transducers [5]. With the arrival of acoustic microfluidics, scientists are

deriving new ways to push the industry forward and create next generation solutions tosolve key problems within the field of microfluidics.

II. LITERATURE REVIEW

1.0 Acoustic Particle Manipulation

Particle separation is often necessary for analyzing chemicals and cells that cannot easily be separated using standard mechanical processes. Using acoustic waves is an excellentway to separate, entrap, and direct particles to move in a desired direction, and can bedone with particles of almost any shape, size, or material type. Other popular methods of

particle separation require specific material properties, such as dielectrophoresis (DEP),which uses an applied electric field to separate particles based on their electrical

properties [6]. Standing surface acoustic wave (SSAW) particle separation worksextremely quickly and efficiently. It also does not require any specific material propertyfor the process to work. Non surface, bulk acoustic waves can also be used for particleseparation, but it requires larger transducers and requires that the microfluidic channelmaterial is capable of reflecting acoustic waves between the channel walls [2].



Particle manipulation in microfluidic channels allows for both the separation of impurities from a fluid and the exchange of a fluid medium. By utilizing different

particles with positive and negative φ values, otherwise known as the acoustic contrastfactor, engineers can create microfluidic devices that separate particles with differentdensity, compressibility, and size. In the biomedical field, one can harness this

technology to separate various biological particulates such as lipids and red blood cells[7]. Figure 1.1 shows an example acoustic contrast factor particle separation.

Another way the biomedical fieldcan harness this technology is byseparating the particles in asolution from their carrier medium, such as red blood cellsfrom plasma. This is especiallyimportant for cases where theoriginal carrier medium is

contaminated. This process caneasily be achieved by takingadvantage of laminar flow

properties and utilizing standing acoustic waves. Contaminated fluid can be pumped intoa micro-channel from side channels, and clean carrier fluid can be pumped into themicro-channel directly from the central channel. When a surface acoustic wave is

propagated through the medium in a way that allows a node to form at the center of themicro-channel, particles with positive φ values drift into the central part of the flow.Figure 1.2 displays the difference between particle flow with and without the influence of a standing acoustic wave [7].

An issue that arises during this process comesfrom diffusion between the two fluid

boundaries. With time exposure, a smallamount of the contaminated medium inevitablydiffuses into the clean medium. To minimizecontact time and diffusion rates, flow rate can

be increased. Furthermore, one can increase theflow rate of the central channel relative to theflow of the side channels to create a diffusion

buffer between the two boundaries of the centralflow [7].

Petersson et al. in 2005 measured efficiencies up to 98% in tests separating red bloodcells from contaminated plasma to clean solution. They also found, similar to other experiments, that acoustic forces were harmless to the cells. If doctors are able to harnessthis microfluidic technology, blood that is currently disposed of can be recycled to

patients [7].

Figure 1.1: [7]Particles with + φ values will align at the node while particles with

– φ values will align at the anti-node of a pressure wave.

Figure 1.2: [7]When the acoustic signal is turned on, particlesdrift into the central fluid due to the acousticforces pushing particles toward the standing

wave node



Standing surface acoustic waves (SSAWs) can be used to separate and align particles in amicrofluidic channel based on various geometric and material properties such as volume,density, and compressibility. A SSAW can be created by using two inter-digitaltransducers (IDT) on opposite sides of a microfluidic channel. The IDTs are placed on a

piezoelectric substrate to convert an alternating electric field to surface acoustic waves.

When the acoustic pressure frequencies from each transducer are aligned, a standingwave can be generated and used to capture particles at a node or anti-node of the standing pressure wave [2].

The same approach used to align particles in a single one dimensional array can betranslated to align particles in multiple one dimensional arrays by using multiple nodes totrap a series of particles. The line separation can be modeled as a function of frequency,whereas, higher frequency standing waves cause the distance between parallel particlelines to decrease [8].

One important factor for determining how fast

particles align to the node of a SSAW is the size of the particles. Larger particles tend to move towardthe node more quickly. This happens due to the factthat acoustic wave forces are proportional tovolume, while the viscous forces from the fluid are

proportional to the radius of the particle. This proportion in force size allows for an interestingsolution to the problem of sorting different sizes of

particles into different channels. Engineers can takeadvantage of the particle alignment speed to quicklyalign larger particles into a central channel whilesmaller particles, which were not able to alignquickly enough, diverge into branching channels onthe sides of the microfluidic channel [2].

Figure 1.3 shows a graph of the average amount of time required for various particle sizes to flow intothe SSAW node, and figure 1.4 shows a schematicof small particles diverging into branch channels inthe microfluidic device while large particles deviateto the central branch. Using thedata from the graph of time vs. particle diameter,one can engineer a microfluidic channel and adjust

flow-rates to specify which sizes of particles they would like to flow into the centralchannel. By adjusting the amplitudes and wavelengths of the standing surface acousticwaves, one is capable further tuning the tolerance of particle separation [2].

According Jinjie Shi et al. at the Pennsylvania State University in 2009, separationefficiency has been noted at 90% for small particles and 80% for large particles. This isan exceptionally good efficiency for particle separation, and it makes this type of

Figure 1.3: [2]The time required for particles to align tothe acoustic wave nodes vs. diameter.

Figure 1.4: [2]When the SSAW is produced, larger

particles align to the node faster thansmaller particles, allowing smaller

particles to diverge into branchingchannels.



microfluidic particle separation extraordinarily attractive to chemists and biologists whodesire to separate one type of particle or cell from another in a quick and easy way [2].

Surface acoustic waves can also be used to directly guide particles to a certain directionrather than simply aligning them to a node. Momentum from the acoustic waves can be

transferred to the particles in solution, allowing them to move in the direction of wave propagation. Microfluidic channels can be designed so that, at a junction, one branch hasa higher flow resistance than the other branch. Similar to electrons in a circuit, particlesin a microfluid desire to go along the path of least resistance. All of the particles comingtoward a junction designed in this way will flow along the path of least resistance unlessan external force is applied which causes them to change paths. With an externallygenerated surface acoustic wave created by an IDT, the particles can be pushed towardthe branch of naturally higher resistance and flow along that route [9].

Figure 1.5 shows a diagram of an IDT creating asurface acoustic wave to guide particles into a

branch of naturally higher flow resistance. Usingthis technology, coupled with electrical sensors, hasthe potential to create some interesting designsolutions regarding the sorting of particles.Sensors, such as light sensors that can detect color and luminescence, can cause the IDT to switch onor off depending on the particle properties. In thisway, particles can be sensed and easily separatedinto different channels using directed droplet flow[9].

Further advancements in utilizing two dimensional SSAWs has lead to advancements in atechnology coined as “acoustic tweezers” to create two dimensional arrays and patternsof particles within a solution using parallel or perpendicular SSAWs. Acoustic tweezerscan use up to 5 orders of magnitude less power than optical tweezers, which utilize afocused laser beam to precisely move particles. There are many other techniques for

particle manipulation such as optoelectronic tweezers [10], electrophoresis (electricalfield based tweezers) [6], and magnetic tweezers [11], but all of these have somedisadvantages when compared to acoustic tweezers regarding either the scope of particlesthat can be aligned, the speed at which they can be aligned, and the power consumptionrequired [12].

SSAW based tweezers are low power, easy to miniaturize, fast, and effective. Parallelinter-digital transducers are capable of creating parallel lines of particles along a channel,whereby the parallel lines of particles settle on the acoustic wave node or anti-node lines.Likewise, perpendicular inter-digital transducers are capable of forming a twodimensional pattern of particles, whereby the particles align on an acoustic wave node or anti-node point within the solution. Figure 1.6 shows an example of acoustic tweezers set

Figure 1.5: [9]The difference between natural particleflow with the IDT turned off and SAWinduced directed droplet flow with the IDTturned on.



up in a parallel and perpendicular fashion. Once particles are immersed in solution andthe IDT’s are turned on, particles quickly align to the nodes, and the user can alter thesize of the particle groupings just by adjusting the power of the acoustic waves. Higher

power causes the amplitude of the pressure waves to be more extreme, thus, causing the particles to form more closely together within the node or anti-node [12].

If cells are used as the particles for the 2D patterning,one worry stems from the acoustic forces damaging thecells. Research has shown that this isn’t the case. Oncecells reach their resting position in the acoustic standingwave, they experience almost zero net force acting onthem. The effects of acoustic waves on various sizes of

particles can be calculated by accounting for the possible forces acting upon them. Particles generallyfeel forces from four different sources: acousticradiation, viscosity, buoyancy, and gravity. Viscous and

acoustic forces account for the rate of particle grouping,while gravitational and buoyant effects on particles aregenerally negligible due to their relatively equal andopposite magnitude. Particles with a diameter greater than 1 micrometer experience acoustic forces muchmore strongly than viscous forces. Furthermore, asapplied acoustic wavelength decreases, acoustic forcesare able to overtake viscous forces [12].

The high precision, low power consumption, high tunability, speed, minimalism, andgentle mechanical nature make acoustic tweezers a useful and innovative tool for manyindustries within chemistry and biology [12]. In fact, a number of biological applicationshave already been tested in the laboratory to see the effects that acoustic waves have onliving cells. Studies have shown that acoustic trapping does virtually no harm to the

biological cells [13]. The low stress environment of acoustic waves can allow live cellsto live and reproduce freely while trapped in solution, and it allows the user to have

precise control over his or her microenvironment [14].

Three experiments in particular were done at Uppsala University in Sweden by MichaelEvander in 2007 to see how acoustic wave trapping would affect rat spleen cells, yeastcells, and neural stem. In the first experiment, rat spleen cells were dispersed through afluid, and the standing acoustic wave was emitted to see if the cells could be captured.This experiment performed flawlessly, and the rat spleen cells were trapped almostinstantly. In the second experiment, yeast cells were trapped in solution for six hours tosee how the cells would react to the environment. The yeast cells grew and reproduced

just as expected, and the acoustic wave environment had no visible negative short or longterm effects on the observed cells. In addition, it was concluded from this experimentthat acoustic trapping applied to cells in different mediums of solution would be a

phenomenal way to study growth rates and activity in a highly controlledmicroenvironment for further studies. The third experiment, done on neural stem cells,

Figure 1.6: [12](a) Parallel (line) tweezers(b) Perpendicular (point) tweezers



allowed a fluorescent dye known as “acridine orange” to be filtered into the channelwhile they were acoustically trapped. If the cells were still alive and well, they would beable to absorb the fluorescent dye. After 15 minutes, no perceptible harm was done to thecells and they remained alive in solution. Moreover, it was concluded that the thermalenvironment of an acoustic wave trap is mild enough to create a sustainable living

environment for cells for an indefinite period of time [14].

As well to these three experiments, at Penn State University in 2009, JinJie Shi et al. performed an experiment using a trapped, two dimensional arrays of e-coli cells for 12hours. A control sample was heated to 70 degrees Celsius to compare cells which hadobvious visible membrane damage to undamaged cells trapped in the acoustic tweezers.Results showed no visible signs of membrane damage for the acoustically trapped e-colicells, further verifying that using acoustic trapping methods on biological structuresleaves them virtually unharmed [12].

In addition to research based bio-applications of acoustic microfluidics, forensic

applications of acoustic microfluidics show great potential to help solve criminalinvestigations. For certain criminal cases, it is necessary for forensic investigators toseparate genetic material and DNA from cells. The current process to analyze samples isthrough using a method called differential extraction. The method of differentialextraction is notoriously time consuming, inefficient, and difficult. Utilizing microfluidictechnology and acoustic wave trapping with differential extraction has lead to newdevelopments in a combined form of analysis called acoustic differential extraction.Acoustic differential extraction can utilize a large range of sample sizes, ranging frommicroliters to milliliters and is capable of separating DNA very quickly. The differencein size between cells and DNA allows scientists to tune the acoustic waves to hold thesperm cells in place while allowing DNA molecules to flow freely without becomingtrapped. Using acoustic differential extraction, the time required to analyze a same wascut down to only 14 minutes, an amount of time that could completely revolutionizedifferential extraction methods in forensics and allow microfluidics and lab-on-a-chipapplications to aid criminal investigations [15].

2.0 Acoustic Mixing

Microfluidic mixing is one of the most important processes required to create true lab ona chip devices. In a microfluidic device, turbulent fluid mixing does not naturally occur due to highly laminar flow. Diffusion does occur between fluids, but is too kineticallyslow to be very useful for most mixing purposes [16]. A variety of IDT setups can beused to generate surface acoustic waves to agitate fluid streams and mix them much faster than diffusion can occur. Single or multiple transducers at the same power output yieldexcellent mixing results [4] [16]. Transducers can also be used at varying power outputs,taking advantage of wave interference, to create a mixing effect known as chaoticadvection [17].



Transducers can be used to generate acoustic wavescapable of thoroughly mixing a series of laminar fluidstreams. By placing a transducer underneath fluidflow, the generated acoustic waves have enoughenergy to disturb the laminar flow and create violent

uniform mixing across the channel. However, if fluidflow rate is too high, the transducer may not be ableto mix the fluid well enough to create a homogeneoussolution. To alleviate this problem, grid-likestructures of transducers can be placed underneath thechannel to more thoroughly mix multiple streams of laminar fluids. Figure 2.1 depicts various transducer

patterns that can be used to mix fluid [16].

Surface acoustic waves can be produced to agitate and mix fluids by using an IDT toexcite a piezoelectric substrate. Previous discussions of IDTs in this paper were used to

create standing waves to align particles, but they can also be used to agitate fluids with proper orientation and power output. SAWs applied to fluid droplets create an effectcalled acoustic streaming. In acoustic streaming, the acoustic pressure wave can pushdroplets along a surface in the direction of wave propagation [18]. Figure 2.2 depictsacoustic streaming for a droplet. Internal streaming within a droplet can mix the fluid as

the droplet moves along the surface. If a highfrequency SAW is generated by an IDT placeddirectly underneath multiple fluid streams,

perpendicularly to the flow direction, canefficiently agitate and mix the fluid streams [4-5][16]. In addition, SAWs oriented parallel to flowdirection can aid in pumping the fluid along thechannel, similar to the acoustic streaming effectthat can be seen with droplets [19-20].

Chaotic advection [21] can be used to agitate and mix multiple fluids together to allowmixing of fluids with incredibly low Reynolds numbers [22], less than .1, and high Pecletnumbers [23], greater than 4x10 5. The Peclet number is the product of the Reynoldsnumber and the Prandtl number [24]. It is defined as the heat transport by convectiondivided by the heat transport by conduction. The Prandtl number is a number describingthe convection of a fluid, and is defined as the quotient between the kinematic viscosityand the thermal diffusivity. Chaotic advection can be induced using a surface acousticwave to create streaming patterns. When two IDTs are operate at separate, variable, andout of phase powers, they can be used to effectively mix two laminar flowing fluids.

Figure 2.2: [4]Droplet pushed by acoustic streaming

Figure 2.1: [16]Multiple variations of transducer arrays are shown. These transducersare placed below the microfluidicchannel to create acoustic waveswhich mix the fluid above.



Figure 2.3 shows an experiment using SAWs to produce chaotic advection mixing. The processused in these photographs was done using twotapered IDTs to create interfering SAWs. The

power of one IDT was held constant, while the

power of the other IDT was modulated over variousfrequencies to create acoustic pressure disturbancewithin the fluid, causing mixing to occur. Each

picture represents a different frequency of power oscillation for the second IDT. Power modulationat a frequency of .17Hz is the only frequency thatcould produce a uniform distribution in theexperiment [17].

Sectored Fresnel ring transducers allow acoustic waves to be

focused toward the tip of the transducer section. Although theyare generally used to pump or eject fluids, they can also bearranged in circular or crossing patterns to create an acousticmixing effect. Further applications of Fresnel ring transducerswill be discussed in detail later in the paper. Figure 2.4 shows acircular pattern of sectored Fresnel ring mixers [25].

3.0 Acoustic Fluid Pumping / Ejection

Microfluidic systems are often difficult to operate without proper control mechanisms,which are often large relative to the small microfluidic system. Improving controlmechanisms to change flow rate and pressure is essential to the evolution of microfluidics

because reliance on externally applied pressures from relatively bulky equipment resistsdevelopments of completely integrated lab-on-a-chip devices [26]. New and innovativeways of propagating fluid through a microfluidic channel using acoustic waves will help

promote microfluidics in mainstream applications and allow engineers to develop self-contained systems.

In recent research done at Harvard University, Langelier et al. devised a way to control pressure of a microfluidic system using acoustic wave signals. Their device can control pressures between 0 and 200 Pascal’s, have a control sensitivity of 10 Pascal’s, and can potentially reduce the need for external pressure control systems. Figures 3.1 and 3.2show a drawing of these different sized acoustic resonator cavities and a possible seriesof musical notes can be played to induce resonant frequencies in the resonance cavities tocause fluid motion in the device [26].

Figure 2.3: [17]Two tapered IDTs were used to c reate mixing

between fluids. One IDT was held at constant power, while another IDT had an oscillating power at 0Hz, .042Hz, .083Hz, .17Hz, .34Hz,and .68Hz for ictures a b c d e and f.

Figure 2.4 : [25]Sectored Fresnel rings

focus acoustic waves toswirl and mix fluid.



The operation of the device is quite unique. Musicalnotes cause fluidic motion using resonance cavities toconvert the oscillating pressure into a net directionalflow. These resonance cavities react to acoustic wavematching their individual resonant frequencies.

Shorter cavities have higher resonant frequencies,while longer cavities have lower resonant frequencies.Resonance within the cavities occurs due to thecreation of standing waves that arise when theacoustic reflections inside the cavity create perfectlyconstructive interference. This constructive

interference occurs when the length of the cavity is ¼ the wavelength of theacoustic signal and on everyconsecutive odd integer harmonic.Groups of these acoustic resonators

can be clustered together and used asmultiple pumps for a microfluidicsystem. By generating a code of musical notes, information can be sentfrom a speaker to the resonators andcause the fluids to move according tothe notes played by the speaker [26].

Rectifiers are structures that cause the oscillating resonance to impart a unidirectionalfluid motion instead of allowing the fluid to oscillate back and forth with the acousticwave. It works by using a flap valve which only opens during one direction of fluid inputand closes if the fluid tries to move in the opposite direction. The flap valve isengineered so that oscillating pressure is strong enough to open the flap valve only if resonance is achieved within the cavity. Using a computer to control the frequency,duration, and amplitude of the acoustic signal allows for precise control over the velocityand distance traveled by the fluid [26].

It is possible to further scale down acoustic fluid propagation by using piezoelectrictransducers of the vibrations from a membrane. If this small scale system can bedeveloped, lab on a chip devices can be simplified and have more widespreadapplications due to the ease of control and lack of large external equipment [26].

Modern day ink-jet printers use nozzle based printing techniques in which the ink ejection direction is fixed perpendicular to the nozzle surface. This scheme has draw

backs due to the fact that the printing head must be moved if the user desires to use morethan one type of ink. Utilizing focused acoustic waves with a nozzleless ejector designallows atomized ink droplets to be precisely ejected in any desired direction. Without theacoustic waves to guide the liquid, the nozzleless system will spray ink in randomdirections. In addition, using this technique allows for multiple inks to be used withoutrequiring the printer head to move [27].

Figure 3.1: [26]Resonance cavities resonate at differentfre uencies de endin on their len th.

Figure 3.2: [26]Signals generated from musical notes can be played into theresonance cavities to cause fluid motion within the



Both acoustic lenses and constructive interference can be used to focus acoustic waves.Constructive interference, also known as self-focusing, can be produced by using fresnelring electrodes on a piezoelectric surface to generate acoustic pressure orthogonal or angled to the surface of the substrate. Piezoelectric substances, along with the sectioned

annular electrodes, allow the nozzleless printer to direct the ink droplet. Acoustic wavesgenerated by the piezoelectric material create a constructive pressure interference waveorthogonal to the ink surface and allow a small particle of ink to be ejected from theliquid due to the accumulated pressure [27].

As shown in figure 3.3, dueto the ring design, ink droplets can be ejected atan angle from the surface of the ejector. Since theelectrodes create larger

acoustic waves than the pieslice that has no electrodes,the droplet direction ejectstoward the pie slice shape[27]

Changing the angle of the open slice allows for ink droplets to be ejected at manydifferent angles. The smaller the slice angle, the closer the ejection angle comes to 90degrees perpendicular to the ink surface. For ZnO piezoelectric material, as the sliceangle increases to 90 degrees the ink ejection angle can change all the way to 62.5degrees from the surface. Using this process, ink drops of 10 micrometers or larger indiameter can be ejected using a nozzleless, piezoelectric, Fresnel ring electrodestimulated device [27].

It is also possible to create a nozzleless device with multiple ejection angles based on asingle concentric annular ring design. One can change the acoustic vibrations by

blocking one of the electrode slices. With an electrode design using 8 slices, 8 differentejection directions can be used for the ink. By turning off a slice of an electrode, the ink drop becomes influenced to eject in the direction of the non-powered electrode due to alack of acoustic pressure in that area. Figure 3.4 shows a simple diagram explaining this

process of ink ejection in the direction of the un-powered electrode ring slice [27].

Using these sectioned electrodes, multi-directional ink ejection has beenachieved. If a large array of these

piezoelectric ink ejectors was made, a printer could be designed to print withultra high quality and resolution.

Figure 3.4: [26]Fresnel rings are sectioned to allow ink droplets to ejectat 8 different angles surrounding the surface of thesubstrate simply by adjusting which section is turned off.

Figure 3.3: [26]Openings in the electrode rings on top of the piezoelectric substrate causean non-symmetric distribution of acoustic waves, causing liquid particles toe ect at an an le from the surface.



The ejection rate of the ink droplets is governed by the radio frequency pulse repetition,and ink droplets of sizes ranging from a few micrometers to tens of micrometers have

been ejected with radio pulses ranging from a few microseconds to a few hundredmicroseconds. Moreover, liquid temperature during the acoustic ejection process remainsconstant, that is, the acoustic waves do not affect the temperature of the ejected liquid.

This benefit makes self-focusing acoustic transducer ejectors a good tool for handlingtemperature sensitive liquids which need to be printed [26]. Acoustic based liquid jettinghas already found commercial applications to collect precisely measured nano-liter fluidvolumes [28].

As previously discussed, SAW streaming effects applied to droplets can push them alonga surface. When SAWs are directed along the flow of fluid in a microchannel, they arecapable of pushing entire streams of fluid. At higher powers, SAW induced fluid

propagation can create jetting [5] [18] or even atomization of liquid [5] [29-30].Moreover, SAWs can push liquid through microchannels at an extraordinarily fast rate,up to speeds ranging from1cm/s and 10cm/s [5].

In addition to using Fresnel rings to eject [27] andmix [25] fluids, they can also create an acousticstreaming effect used to propagate fluid through thechannel. Each Fresnel ring works by producing afocused surface acoustic wave which propagates inthe direction moving from the largest ring radius tothe smallest ring radius. An array of sectioned

electrode Fresnel rings on a piezoelectric substrate is capable of producing powerfulacoustic waves when patterned to create constructive interference. Figure 3.5 shows anexample of a sectioned Fresnel ring array that can be used to push liquid through amicrochannel [25].

Surface acoustic waves are capable of producinghuge stresses and accelerations on fluids whenactuators are used at high powers. As the power output approaches 1 watt, they SAWs are capable of accelerating fluid up to 10 7 to 10 8 m2/s, so fast thatthe liquid can be converted into atomized drops

between 1 and 10 micrometers in diameter [5] [29-30]. Using this technique to create nano-sized

particles can be widely used throughout a variety of industries such as printing, drug delivery, agriculturalspraying, and fuel injection. SAWs are also anexcellent way to generate polymeric nano-particles of 150 to 200 nanometers as shown in figure 3.6 [5].

Figure 3.6: [5]Polymeric nanoparticles generated fromsurface acoustic wave fluid atomization

Figure 3.5 : [25]Sectioned Fresnel rings can pump fluid in amicrochannel by focusing acoustic waves along



III. CONCLUSIONS

Acoustic microfluidic technology has steadily improved as new and creative applicationshave been developed and researched. Acoustics has proven itself to be one of the mostuseful ways of separating particles, mixing fluids, and propagating microfluidic solutions,

in addition to its capabilities of miniaturizing microfluidic control systems. Biology, biomedicine, chemistry, forensics, ink-jet printing, and nano-particle fabrication are just afew of the many potential avenues that acoustic microfluidics can assist. Ultimately,acoustics is just one solution of many to solve problems within the field of microfluidics,

but few other methods provide the generality, tunability, and development potential of utilizing acoustic waves.

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[23] Calculating the Peclet Number. (n.d.). Rice . Retrieved November 15, 2009, fromwww.owlnet.rice.edu/~chbe402/proj05/jigarb/peclet_number.htm

[24] Weisstein, E. W. (n.d.). Prandtl Number -- from Eric Weisstein's World of Physics. ScienceWorld . Retrieved November 15, 2009, from http://scienceworld.wolfram.com/physics/PrandtlNumber.html

[25] Yu, H., Kwon, J. W., & Kim, E. S. (2006). Microfluidic mixer and transporter based on PZT self-focusing acoustictransducers. Journal of Microelectromechanical Systems , 15(4), pp.1015-1024.

[26] Langelier, S. M., Chang, D. S., Zeitoun, R. I., & Burns, M. A. (2009). Acoustically driven programmable liquidmotion using resonance cavities. PNAS , 106 , 12617-12622; doi:10.1073/pnas.0900043106.

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[28] Olechno, J. (2007, December 13). Acoustic Droplet Ejection. SciVee | Making Science Visible . Retrieved November 15, 2009, from http://www.scivee.tv/node/4522

[29] Chono, K., Shimizu, N., Matsui, Y., Kondoh, J., & Shiokawa, S. (2004). Development of Novel AtomizationSystem Based on SAW Streaming. Japanese Journal of Applied Physics , 43(5B), pp.2987-2991.

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http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/adv/adv.rxml


http://www.owlnet.rice.edu/~chbe402/proj05/jigarb/Reynolds_number.htm

http://www.owlnet.rice.edu/~chbe402/proj05/jigarb/peclet_number.htm

http://scienceworld.wolfram.com/physics/PrandtlNumber.html


http://www.scivee.tv/node/4522



http://www.owlnet.rice.edu/~chbe402/proj05/jigarb/Reynolds_number.htm

http://www.owlnet.rice.edu/~chbe402/proj05/jigarb/peclet_number.htm



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