Proceedings of the 3rd Joint US-European Fluids Engineering Summer Meeting and 8th International Conferenceon Nanochannels, Microchannels and Minichannels

FEDSM2010-ICNMM2010August 1-5, 2010, Montreal, Canada

FEDSM-ICNMM2010-30181

FAST INERTIAL MICROFLUIDIC ACTUATION AND MANIPULATION USINGSURFACE ACOUSTIC WAVES

Leslie Y. Yeo!

James R. FriendMicro/Nanophysics Research Laboratory

Department of Mechanical and Aerospace EngineeringMonash University

Clayton, VIC 3800, AustraliaEmail: leslie.yeo@eng.monash.edu.au

ABSTRACTThough uncommon in most microfluidic systems due to the

dominance of viscous and capillary stresses, it is possible to drivemicroscale fluid flows with considerable inertia using surfaceacoustic waves (SAWs), which are nanometer order amplitudeelectro-elastic waves that can be generated on a piezoelectricsubstrate. Due to the confinement of the acoustic energy to athin localized region along the substrate surface and its subse-quent leakage into the body of liquid with which the substratecomes into contact, SAWs are an extremely efficient mechanismfor driving fast microfluidics. We demonstrate that it is possi-ble to generate a variety of efficient microfluidic flows using theSAW. For example, the SAWs can be exploited to pump liquidsin microchannels or to translate free droplets typically one ortwo orders of magnitude faster than conventional electroosmoticor electrowetting technology. In addition, it is possible to drivestrong microcentrifugation for micromixing and bioparticle con-centration or separation. In the latter, rich and complex colloidalpattern formation dynamics have also been observed. At largeinput powers, the SAW is a powerful means for the generationof jets and atomized aerosol droplets through rapid destabiliza-tion of the parent drop interface. In the former, slender liquidjets that persist up to centimeters in length can be generatedwithout requiring nozzles or orifices. In the latter, a monodis-

!Address all correspondence to this author.

persed distribution of 1–10 micron diameter aerosol droplets isobtained, which can be exploited for drug delivery and encapsu-lation, nanoparticle synthesis, and template-free polymer arraypatterning.

INTRODUCTIONMicrofluidics inherently involves low Reynolds number (Re

< 1) flows due to its small length scales that endow the systemwith large surface-area-to-volume ratios. As a consequence, vis-cous and capillary stresses are typically dominant over inertialstresses in such systems, thus rendering it considerably diffi-cult to actuate fluids or manipulate particles in microscale de-vices without requiring large and cumbersome ancillary equip-ment such as capillary pumps. Further, the laminarity of lowReynolds number flows means that carrying out mixing in thesedevices is also difficult in the absence of turbulent or chaoticflows [1]. Whilst considerable advances have been made over thepast decade in developing novel actuation mechanisms to carryout microscale fluid actuation and particle manipulation for a va-riety of microfluidic applications such as genomic and proteomicanalysis, point-of-care diagnostics, drug delivery, high through-put drug screening and pathogen detection, with some technolo-gies such as electrokinetic actuation [2] showing considerablepromise, there has yet to be a reliable and robust technology thatcan drive microfluidics on a portable chip-based platform includ-

1 Copyright c" 2010 by ASME

Proceedings of the ASME 2010 3rd Joint US-European Fluids Engineering Summer Meeting and 8th International Conference on Nanochannels, Microchannels, and Minichannels

FEDSM-ICNMM2010 August 1-5, 2010, Montreal, Canada

FEDSM-ICNMM2010-30181

ing an integrated miniature power supply. Here, we demonstratea novel microfluidic actuation platform based on surface acous-tic waves (SAWs) that has the potential to carry out such. Due tothe large acoustic intensities that can generate even at low powerand the rather efficient energy transfer from the actuator surfaceto the fluid, as will be discussed below, Reynolds numbers onthe order of 10–100 are not uncommon with these technologies,and hence microscale fluid and particle motion can be inducedat considerably high speeds due to the appreciable inertia thatcan be generated. This can also be exploited to introduce suffi-cient chaotic oscillation of the flow in the system to drive effec-tive mixing. Besides fluid pumping and mixing, we also showthat it is possible to use the SAW to generate a whole host ofmicrofluidic manipulation, such as particle synthesis, concentra-tion, sorting and patterning as well as free droplet translation,jetting, atomisation.

SURFACE ACOUSTIC WAVE MICROFLUIDICSSAWs are electro-elastic Rayleigh waves with amplitudes

on the order of 10 nm that skim the surface of a piezoelectricsubstrate. The SAW devices used in our work predominantlyconsist of a pair of interdigitated transducers (IDTs) each withseveral aluminum electrode finger pairs sputter-deposited ontoa 127.68# yx-cut lithium niobate (LiNbO3) piezoelectric singlecrystal substrate. Application of an oscillating electrical signalto the IDT using a RF signal generator and power amplifier leadsto the generation of the SAW with a wavelength ! that correlateswith the IDT finger width and spacing; more specifically, thefinger width and spacing are both !/4. The corresponding SAWresonant frequency is f = cs/!, where cs $ 3965 m/s is the speedof the SAW in the substrate.

The intricate fluid-structural coupling that arises when theSAW comes into contact with the fluid is the fundamental mech-anism that gives rise to SAW microfluidics. At the leading edgeof the drop (Fig. 1(A)), the SAW diffracts into the drop due tothe difference between the sound velocities in the substrate csand the fluid, cl ($ 1485 m/s for water). The diffraction of theacoustic waves into the fluid occurs at a specific angle given bythe ratio of the sound velocities, known as the Rayleigh angle"R = sin%1 (cl/cs) $ 22# (Fig. 1(B)), and provides a mechanismfor energy transfer into the drop. As this occurs, a longitudinalpressure wave front is generated, which drives bulk fluid recir-culation within the drop, or acoustic streaming. The horizontalcomponent of the energy transfer into the drop at the Rayleighangle also results in a body force that acts on the entire drop in theSAW propagation direction, causing the drop to deform into anasymmetrical conical shape along an axis defined the Rayleighangle (Fig. 1(C)). Eventually, the drop is driven to translate inthe direction of the SAW propagation once the contact line pin-ning forces are exceeded.

Figure 2 shows the response of the drop to the SAW in the

(A)

(B)

(C)

R

Figure 1. (A) FLUID-STRUCTURAL INTERACTION BETWEEN THESAW AND A DROP IN ITS PROPAGATION PATHWAY CAUSES (B) THEACOUSTIC ENERGY TO LEAK INTO THE DROP AT THE RAYLEIGHANGLE "R, GIVING RISE TO ACOUSTIC STREAMING IN THE DROPAND A BODY FORCE ACTING ON THE DROP, WHICH, IN TURN, (C)CAUSES THE DROP TO DEFORM INTO AN AXISYMMETRICAL CON-ICAL SHAPE AND SUBSEQUENTLY TO TRANSLATE ACROSS THESUBSTRATE.

order of increasing substrate vibration amplitude with increas-ing input power to the IDT [3]. At low powers, the drop de-forms into the axisymmetrical conical structure shown, whichalso vibrates—the low amplitude capillary wave vibration herecan be exploited for particle assembly and sorting on the freesurface. As the input power is increased, the body force act-ing on the drop overcomes the contact line pinning force causingit to translate, thus allowing the SAW to be exploited for mov-ing and manipulating drops in open microfluidic systems. If theSAW energy is concentrated underneath the drop, a peculiar jet-ting phenomenon occurs. Alternativelu, if the vibration energy issufficiently large to overwhelm the capillary stresses holding thedrop together, drop atomization occurs. Below, we discuss eachof these SAW microfluidic phenomena, as well as other manip-ulations that can be induced, with particular emphasis on theirpotential for application in practical microfluidic systems.

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Figure 2. TYPICAL MICROSCALE FLUID MANIPULATIONS THATCAN BE GENERATED USING THE SURFACE ACOUSTIC WAVE. INTHE ORDER OF INCREASING RF POWER APPLIED TO THE IDT:THE DROP VIBRATES, TRANSLATES (THE LEFT SHOWS A DROPTRANSLATING ON A BARE HYDROPHILIC LITHIUM NIOBATE SUB-STRATE AND THE RIGHT SHOWS THE DROP TRANSLATING ON ASUBSTRATE COATED WITH A HYDROPHOBIC LAYER), FORMS ALONG SLENDER JET, AND, EVENTUALLY ATOMIZES.

DROP VIBRATION & PARTICLE ASSEMBLYA sessile drop containing a suspension of 500 nm particles

subject to low amplitude SAW vibration gives rise to a uniqueparticle assembly phenomenon [4]. Figure 3 shows the differ-ent particle assembly patterns that arise on the drop free surface.Initially, the colloidal particles assemble into linear concentricrings coinciding with the nodal lines of the low amplitude 20MHz standing wave vibrations induced on the free surface, asshown by the fingerprint-like patterns in Regime A. As the SAWvibration is increased, the colloidal particles in the linear ringlikeassemblies are observed to cluster to form pointwise colloidal is-lands as depicted by the patterns in Regime B. This regime is as-sociated with large amplitude 1 kHz order vibrations associatedwith the capillary-viscous resonance of the drop. The colloidalislands therefore appear to form at the intersection between thenodal lines of the low amplitude 20 MHz standing wave vibrationand the circular nodal ring of the large amplitude 1 kHz capillary-

Figure 3. FREE SURFACE COLLOIDAL PATTERNS AS A FUNCTIONOF THE INITIAL DROP DIAMETER AND INPUT POWER [4]. THESCALE BARS CORRESPOND TO 200 MICRON LENGTH SCALES.

viscous vibration of the drop. Further increases in the SAW vi-bration amplitude leads to the onset of significant fluid streamingwithin the drop which disperses the particles and hence erasesthe colloidal island assemblies (Regime C). The streaming, how-ever, only lasts for a short transient before ceasing, at which pointthe colloidal islands are observed to reform until the streamingrecommences and erases them again. This cyclic phenomenonoccurs aperiodically and the direction of the streaming (clock-wise and anticlockwise) is noted to be reasonably random, sug-gesting that this regime is a transient metastable state and thatthe commencement and cessation of the streaming is triggered bya peculiar instability arising from the highly nonlinear couplingbetween the acoustic, hydrodynamic and capillary forces. Withfurther increases in the SAW vibration, however, the streamingbecomes stronger and more consistent, leading to permanent dis-persion of the particles (Regime D).

DROP TRANSLATION & MICROCHANNEL PUMPINGIf the SAW vibration amplitude is sufficiently large such

that the body force imparted by the leakage of the SAW irra-diation on a sessile drop sitting on the substrate overcomes thesurface forces that pin the contact line of the drop, the drop canbe linearly translated at speeds typically around 1–10 cm/s [5,6],which are one to two orders of magnitude larger than the speedsthat can be achieved through other actuation schemes, for ex-ample, electrowetting [7]. This drop translation capability hasbeen employed for various biological applications, e.g., transportof DNA in discrete droplets for polymerase chain reaction [8],rapid bioparticle collection and concentration for biosensing ap-

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Figure 4. FLOW STRUCTURE AS A FUNCTION OF THE MI-CROCHANNEL WIDTH W FOR A FIXED OPERATING FREQUENCY.AS W IS INCREASED, IT CAN BE SEEN THAT THE UNIFORMTHROUGH FLOW FOR FLUID PUMPING IN (A) IS PROGRESSIVELYREPLACED BY AN OSCILLATORY CHAOTIC FLOW AS SEEN IN (B)–(D), WHICH CAN BE EXPLOITED FOR CONCURRENT FLUID MIXINGIN THE SAME DEVICE [13].

plications [6] and efficient seeding of cells into bioscaffolds fortissue and orthopedic engineering [9, 10]. Due to the high fre-quency associated with the SAW, the cells are not observed to bedenatured; their differentiation and proliferation capabilites alsoappear to remain intact [11].

In addition, the SAW can be used to pump liquid through mi-crochannels [12,13]. As with the SAW-driven drop transport, themicropumping velocities achieved are extremely high, on the or-der 1 cm/s, which is one to two decades larger than that possiblewith electrokinetic micropumps [14]. Due to the transmission ofsound waves into the fluid through the undulating channel wallsas the SAW traverses, it is also possible to switch between uni-form through flow for fluid delivery, as shown in Fig. 4(a), andchaotic oscillatory flow for micromixing, as shown in Figs. 4(b)–4(d) [13]. This can be achieved either by altering the microchan-nel width keeping the SAW excitation frequency constant, asshown in Fig. 4, or by changing the SAW frequency for a fixedwidth microchannel.

If, on the other hand, the fluid contained a suspension ofcolloidal particles and the input power is below the threshold re-quired for fluid flow, the particles are observed to collect in a lin-ear particle assembly along the nodes of the pressure field corre-sponding with the first order fluid motion arising due to the com-pressibility of the sound waves that are radiated into the fluid.The number of nodal lines and the number of aligned particleassemblies can be seen to correlate closely, as seen in Fig. 5.Such particle manipulation behavior can then be exploited formicrochannel particle focusing [15].

MICROCENTRIFUGATIONIt is possible to generate microcentrifugation by symmetry

breaking of the SAW through a number of ways, as shown inFig. 6 [16]. The azimuthal fluid recirculation can be further in-tensified by pointwise focusing the SAW beneath the drop withthe use of the curved electric-width-controlled single-phase uni-directional transducer (SPUDT) electrodes shown in Fig. 7(A)

Figure 5. COMPARISON BETWEEN (A) THE COMPUTED FIRST OR-DER PRESSURE FIELD IN THE FLUID AND (B) THE NUMBER OFLINEAR 500 NANOMETER COLLOIDAL PARTICLE ASSEMBLIES THATARISE IN THE QUIESCENT FLUID REGION. IT CAN BE SEEN IN (C)THAT THE PARTICLES APPEAR TO COLLECT ALONG THE NODALLINES OF THE PRESSURE FIELD [13].

in place of the conventional IDT design [17, 18]. Unlike theconventional bidirectional IDT design shown schematically inFig. 1(A), internally tuned reflectors within the IDT fingers inthe SPUDT electrode design generates a unidirectional SAW thatpropagates from a single side of the IDT. The correspondingSAW pattern, which can be visualized through laser Doppler vi-brometry (LDV) or simply through the patterns arising from thedeposition of smoke particles, as shown in Figs. 7(B) and 7(C),respectively. Recirculatory flows in the azimuthal direction withlinear velocities of around 1 mm/s with the conventional IDTsand as high as 20 mm/s with the focusing SPUDTs are observed,which can be exploited to drive intense micromixing. Shiltonet al. [18] have shown that the chaoticlike inertial flow gener-ated provides a means for driving effective turbulent-like mix-ing with an enhancement ratio Deff/D0 that scales roughly as thesquare of the input power, where Deff is the effective diffusiv-ity and D0 is the diffusivity in the absence of flow. In [18], asmall amount of food dye is shown to be completely mixed ina glycerine-water drop in under 1 sec. We have shown this in-tense mixing to be extremely useful for microscale chemical andbiochemical synthesis—yields comparable or even significantlyhigher than that obtained using ultrasound or microwave chem-istry have been demonstrated with significantly shorter reactiontimes [19]. In addition, rapid particle concentration and separa-tion can also be achieved if the drop is suspended with micro-or nano-particles, as shown in Fig. 8, in which the particles areseen to concentrate in under 1 s, thus providing a convenient andeffective means for pathogen preconcentration for fast and sen-sitive detection in biosensors or the separation of red blood cellsfrom plasma in miniaturized point-of-care diagnostic kits.

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Figure 6. MICROCENTRIFUGATION WITHIN A SESSILE DROP CANBE GENERATED THROUGH SYMMETRY BREAKING OF THE SAWRADIATION ACROSS THE SUBSTRATE EITHER BY PLACING THEDROP OFF-CENTER (TOP-LEFT), THROUGH ASYMMETRIC REFLEC-TION OF THE SAW BY INTRODUCING A DIAGONAL CUT AT THEEDGE OF THE SUBSTRATE (TOP-RIGHT) OR THROUGH THE USE OFABSORPTION GEL TO SUPPRESS THE REFLECTION AT ONE-HALFOF THE IDT (BOTTOM) [16].

JETTING & ATOMIZATIONFinally, if the input power and hence the SAW vibration in-

tensity is sufficiently large such that the capillary stresses main-taining the shape of the parent drop is overcome, resulting insignificant interfacial deformation. At these high powers, thesubstrate displacement velocity as the SAW traverses along thesubstrate surface is typically 1 m/s irrespective of the excitationfrequency. Given the SAW substrate vibration amplitude of ap-proximately 10 nm, large surface accelerations, on the order of10 million g’s can typically be generated, which in turn, drivesthe interfacial destabilization. If the SAW energy is concentratedto a point underneath the drop with the use of two elliptical fo-cusing SPUDTs at two opposite ends, the transmission of radi-ation at the Rayleigh angle into the liquid from both the frontand rear of the drop causes it to deform into a coherent elongatedliquid column, as shown in Fig. 9 [20]. It is therefore possi-ble to generate a fluid jet that persists over centimeters in lengthfrom a millimeter diameter drop without requiring fluid confine-ment mechanisms such as nozzles or orifices to accelerate thefluid to adequately sufficient velocities necessary to produce anelongated jet. Not only does this demonstrate the ability for theSAW to drive strong inertial forcing on a liquid, the ability to in-duce such jets could have potential applications in inkjet or softbiological printing, fiber synthesis, and drug delivery, amongstothers. Figure 10 shows the rich dynamics of the jetting phe-nomenon, in which we observe the jet length and the ability togenerate single or multiple droplet ejection to depend on the jetWeber number We j & #U2

j R j/$, where # is the fluid density, Ujthe jet velocity, R j the jet radius and $ the interfacial tension.

(A)

(B)

(C)

Figure 7. (A) FOCUSING ELLIPTICAL SPUDT. THE CORRESPOND-ING SAW PATTERNS THAT ARE GENERATED, OBTAINED USING (B)LDV, AND, (C) SMOKE PARTICLE DEPOSITION, INDICATE THE INTEN-SIFICATION OF THE SAW TOWARDS A FOCAL POINT [17,18].

If the power is not focused to a point underneath the dropor if the input power to the IDTs is further increased, the entiredrop is completely destabilized and atomizes to produce aerosoldroplets with a monodisperse distribution around a mean di-ameter 5 µm [21]. These monodispersed micron-sized aerosoldroplets are particularly useful as drug-carrying vehicles for pul-monary drug delivery in which 2–5 µm droplet sizes are typi-cally required for optimum dose efficiency in order to deliver themaximum amount of drug to the lower respiratory airways fordirect local administration to target organs [22].The power re-quired, typically 1 W or less, is at least a factor of ten smallerthan that of ultrasonic atomizers which use Langevin transducersand single lead zirconium titanate element thickness-mode pis-ton atomizers operating between 10 kHz and 1 MHz. At theselower frequencies, drug molecules, DNA or proteins are suscep-tible to denaturing as cavitation becomes prevalent. Indeed, wedemonstrate that the post-atomization biomolecular viability andfunction is preserved [23]. Another advantage of the SAW at-omization technique, as with the jetting phenomenon discussedabove, is the ability to do away with nozzles and orifices whichare prone to clogging, thus simplifying considerably the deviceand hence reducing its costs and increasing its reliability.

By atomizing a polymer solution, it is also possible to pro-duce 150–200 nm spherical clusters of polymeric nanoparticlescomprising sub-50 nm particulates, as shown in the schematic inFig. 11 [24] . This one-step process for nanoparticle synthesis is

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t=0

t=1st=0.5s

t=0.25s

Figure 8. CONCENTRATION OF 500 NANOMETER FLUORESCENTPARTICLES IN A 0.5 MICROLITER WATER DROP DUE TO THE INER-TIAL BULK FLUID RECIRCULATION THAT DRIVES STRONG MICRO-CENTRIFUGATION WITHIN THE DROP [18].

Figure 9. ELONGATED JET GENERATED BY CONCENTRATING THESAW INTO THE DROP [20].

therefore a straightforward, fast and attractive alternative to themultistep conventional methods for nanoparticle production suchas spray drying, nanoprecipitation, emulsion photocross-linking,etc., which are slow and cumbersome. In addition, we havealso demonstrated the possibility of generating 100 nm order di-mension protein (insulin) nanoparticles as well as 3 µm aerosoldroplets for inhalation therapy [25]. Further, it is possible toload protein and other therapeutic molecules into the biodegrad-able polymer nanoparticle shells produced for controlled releasedrug delivery [26]. The encapsulation of the drug within thebiodegradable polymer essentially shields the drug from rapidhydrolysis and degradation, permitting sustained release overtime, and thus prolonging the effect of the drug over longer pe-riods whilst preventing dangerous dose spikes. Our recent inves-

Time (ms)

Wej

1 mm

0 1 2 3 4 5

1 mm

0 2 4 8 10 12

0 2 4 6 8 10 12 14 16

0 2 4 6 8 10 12 14 16

0 5 10 11 12 13 14 15 16 17

1 mm

1 mm

1 mm(a)

(b)

(c)

(d)

(e)

0.5

0.1

1.0

Figure 10. SAW JETTING AS A FUNCTION OF TIME, SHOWING THEDEPENDENCE OF ITS LENGTH AND BREAK-UP ON THE JET WEBERNUMBER We j [20].

tigations have indicated the viability of primary mamalian os-teoblast stem cells after SAW atomization thus suggesting thatthe technique can be used for drug loading without altering thenature of the biomolecule [11].

SAW atomization also offers the potential for the long-range, regular spatial-ordering of polymer spot patterns on asubstrate without physical or chemical templating or other sur-face treatment procedures [27]. As shown in Figs. 12(a) and12(b), the translation of a drop containing the polymer solutionleaves behind a thin trailing film, which simultaneously desta-bilizes, thus thinning the film at the antinodes of the standingSAW vibration and resulting in its depletion of the film there.The breakup of the film across the entire substrate surface atthese positions and the subsequent evaporation of the solventthen yields the evenly spaced solidified polymer droplet patternsshown in Figs. 12(c) and 13(a). The pattern periodicity as well as

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A

B

Figure 11. SYNTHESIS OF POLYMER NANOPARTICLES AROUND150–200 NANOMETER IN DIAMETER WITH SURFACE ACOUSTICWAVE ATOMIZATION. (A) TRANSMISSION ELECTRON MICROSCOPY(TEM) IMAGE OF A NANOPARTICLE. (B) MAGNIFICATION OF THEIMAGE SHOWING THE AGGREGATION OF SUB-50 NANOMETERPARTICULATES TO FORM A 200 NANOMETER CLUSTER, THESCHEMATIC SHOWING A RECONSTRUCTION OF THE PARTICLE AG-GREGATE [24].

(a)

(b)

(c)

Figure 12. LONG-RANGE REGULAR POLYMER SPOT PATTERN-ING ARISING FROM THE ATOMIZATION OF A THIN FILM OF POLY-MER SOLUTION DISPENSED FROM THE NEEDLE ABOVE THE SUB-STRATE (A,B). THE INITIAL DROP THAT IS DISPENSED TRANS-LATES IN THE DIRECTION OF THE SAW PROPAGATION, LEAVINGBEHIND A THIN TRAILING FILM, WHICH SUBSEQUENTLY DESTA-BILIZES AND ATOMIZES, THUS LEAVING BEHIND (C) SOLIDIFIEDPOLYMER SPOTS AT THE ANTINODAL POSITIONS [27].

(b)

(

Figure 13. (A) POLYMER SPOT PATTERNING. (B) SPOT DIAMETER,LONGITUDINAL PITCH SPACING A AND TRANSVERSE PITCH SPAC-ING B AS A FUNCTION OF THE SAW FREQUENCY AND WAVE-LENGTH [27].

the polymer spot size is observed to strongly correlate with theSAW frequency or wavelength, as shown in Fig. 13(b), thereforepresenting the ability for control and fine tuning, which is a keyadvantage over other conventional patterning methods [27].

ACKNOWLEDGMENTThis work would not have been possible without close col-

laboration with J. Friend and the talented students and postdocswho have shared in our journey on SAW microfluidics and whosework has led to the discoveries described above. They are, inalpabetical order, M. Alvarez, D. Arifin, K. Kulkarni, H. Li,A. Qi, R. Shilton, R. Raghavan and M. K. Tan. In addition, wewould also like to thank our collaborators A. Dasvarma (Aus-tralian Stem Cell Centre), M. P. McIntosh (Monash University),A. Mechler (LaTrobe University), D. A. Morton (Monash Uni-versity), P. Perlmutter (Monash University), L. Spiccia (MonashUniversity) and K. Traianedes (Australian Stem Cell Centre).We also acknowledge funding from the Australian ResearchCouncil (Discovery Projects DP098525 & DP1092955; Link-age, Infrastructure, Equipment & Facilities LP0668435), theNational Health and Medical Research Council (DevelopmentGrant 1000513), Nanotechnology Victoria, and the ResearchSupport for Counter-Terrorism programme administered by the

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Department of Prime Minister & Cabinet’s Office of NationalSecurity.

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[25] Alvarez, M., Friend, J. R., and Yeo, L. Y., 2008. “RapidGeneration of Protein Aerosols and Nanoparticles Via Sur-face Acoustic Wave Atomization”. Nanotechnology, 19,455103.

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[27] Alvarez, M., Friend, J. R., and Yeo, L. Y., “Surface Vibra-tion Induced Spatial Ordering of Periodic Polymer Patternson a Substrate”. Langmuir, 24, pp. 10629–10632.

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