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Ultrafast microfluidics using surface acoustic waves
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Figures

Image of FIG. 1.

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FIG. 1.

(a) Schematic depiction of a SAW propagating on the surface of a piezoelectric substrate. Note the localization of the Rayleigh wave on the surface, which decays exponentially into the substrate such that the motion is negligible at a depth of 4–5 SAW wavelengths into the substrate. (b) Image of the SAW device consisting of the piezoelectric substrate on which the SAW propagates, (c) the portable power supply used to generate the SAW, and, (d) the interdigitated transducers (IDTs) patterned onto the piezoelectric substrate.

Image of FIG. 2.

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FIG. 2.

(a) Schematic depiction of a liquid drop placed on the substrate in the propagation pathway of the SAW. (b) Due to the different sound velocities in the substrate and in the liquid phases, the SAW energy leaks into the drop at a specific angle, the Rayleigh angle , as it comes into contact with the drop. This gives rise to bulk liquid recirculation (acoustic streaming) within the drop and a body force on the drop itself in the SAW propagation direction. (c) The body force on the drop causes the drop to deform into an axisymmetrical conical shape whose trailing edge leans at the Rayleigh angle and subsequently to translate across the substrate.

Image of FIG. 3.

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FIG. 3.

Typical microfluidic manipulations that arise due to the fluid-structural interaction between the SAW and a liquid drop placed on the SAW substrate. In the order of increasing rf power applied to the IDTs, the drop vibrates, translates (the left image shows the translation of a drop on the bare lithium niobate substrate, which is hydrophilic, and the right image shows the same drop translating on a substrate which has been coated with a hydrophobic substance), forms a long slender jet, and, eventually atomizes.

Image of FIG. 4.

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FIG. 4.

Interfacial colloidal patterns as a function of the initial drop diameter and input power induced by low power SAW drop vibration (Ref. 7). The scale bars in the image indicate a length scale of .

Image of FIG. 5.

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FIG. 5.

The number of colloidal islands formed depends on the drop size which decreases due to evaporation in time. The bottom schematic shows the successive decrease in the number of intersection points between the nodal lines and the circular nodal ring as the drop and hence the nodal ring shrinks (Ref. 7).

Image of FIG. 6.

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FIG. 6.

Metastable transient state in which the system cycles randomly between colloidal island formation when there is no streaming and colloidal island erasure when streaming commences (Ref. 7).

Image of FIG. 7.

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FIG. 7.

Rapid and efficient collection and concentration of microparticles on a substrate by sweeping carrier drops across the surface using the SAW (Ref. 10). The top image shows the collection of melamine (hydrophilic) particles whereas the bottom image shows the collection of polystyrene (hydrophobic) particles. The inset shows a magnification of the polystyrene particles left behind on the track, which assemble into concentric ring assemblies.

Image of FIG. 8.

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FIG. 8.

Comparison between static (gravity perfusion) and SAW-driven cell seeding (Ref. 15). The images in the columns on the left show successive cross-sectional slices of the scaffold after the seeding, in this case using fluorescent particles, has occurred. The slices a–f correspond to that in the schematic shown in the inset of the right image; the arrow shows the scaffold face along which the drop containing the cell suspension first enters. The first column in the left image shows the results of the SAW seeding whereas the second column in the left image shows that from the static seeding. The right image shows the normalized pixel intensity of the fluorescent particles in each scaffold slice for both seeding methods. Both sets of results indicate deeper and more uniform penetration with the SAW method.

Image of FIG. 9.

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FIG. 9.

(a) SEM images of yeast cells seeded into a poly(caprolactone) scaffold using SAW. The morphology of the cells do not appear to be compromised by the SAW radiation. (b) Proliferation rate of the yeast cells after irradiation with the SAW. Cells are observed to continue proliferating during the subsequent , which further confirms the viability of the yeast cells (Ref. 16). (c) Viability of primary murine osteoblast cells treated under the SAW irradiation at different rf powers. (d) Average cell proliferation of SAW-treated and untreated cells as function of the fluorescence intensity from the Alamar Blue uptake; the power of the SAW applied is . In both (c) and (d), the cells were treated by the SAW at for , and the cell density and suspension volume is and , respectively.

Image of FIG. 10.

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FIG. 10.

SAW microchannel pumping configurations. (a) A PDMS microchannel is mounted on the SAW substrate (Ref. 20). (b) A microchannel is laser ablated into the SAW substrate (Refs. 18 and 19).

Image of FIG. 11.

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FIG. 11.

Flow configuration as a function of channel width . As , the uniform through-flow observed in image (a) is replaced by an oscillatory vortical flow, as seen in images (b)–(d) (Refs. 18 and 19).

Image of FIG. 12.

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FIG. 12.

Comparison between (a) the computed first order pressure field in the fluid and (b) the number of linear colloidal particle assemblies that arise in the quiescent fluid region at the bottom of the wide microchannel. It can be seen in (c) that the particles appear to collect along the nodes of the pressure field (Refs. 18 and 19).

Image of FIG. 13.

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FIG. 13.

Azimuthal acoustic streaming within a drop placed on the substrate can be generated by symmetry breaking of the SAW propagation across the substrate either by placing the drop off-center (top-left image) or through asymmetric reflection of the SAW by introducing a diagonal cut at the edge of the substrate (top-right image) or by using absorption -gel to suppress the reflection at one-half of the IDT (bottom image) (Ref. 28).

Image of FIG. 14.

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FIG. 14.

(a) Focusing elliptical SPUDT electrode. The corresponding SAW patterns that emerge from the SPUDT, obtained using (b) laser Doppler vibrometry and (c) smoke particle deposition indicate the intensification of the SAW energy towards a focal point (Refs. 29 and 30).

Image of FIG. 15.

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FIG. 15.

Rapid concentration of fluorescent particles in a water drop due to the strong inertial bulk fluid recirculation induced by the SAWs generated using elliptical focusing SPUDTs (Ref. 30).

Image of FIG. 16.

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FIG. 16.

Initial stages of the jet formation as a consequence of the concentration of acoustic radiation into the drop arising as a consequence of the convergence of two SAWs produced by elliptical focusing SPUDTs placed on both sides of the drop (Ref. 31).

Image of FIG. 17.

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FIG. 17.

Time sequence of the SAW jets produced, showing the dependence of its length and breakup as a function of the Weber number (Ref. 31).

Image of FIG. 18.

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FIG. 18.

polymer nanoparticles synthesized using the SAW atomization technique. (a) Transmission electron microscopy image of a nanoparticle. (b) Magnification of the image showing the sub- particulates that aggregate to form the cluster; the schematic shows a three-dimensional reconstruction of the particle aggregate that resembles a grape bunch (Ref. 33).

Image of FIG. 19.

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FIG. 19.

Self-organization of regular polymer spot patterns. A drop of polymer solution dispensed from the needle above the SAW substrate [(a), (b)] translates under the SAW leaving behind a thin trailing film, which subsequently destabilizes leading towards atomization and evaporation of the solvent, thus leaving behind (c) solified polymer spot patterns (Ref. 36).

Image of FIG. 20.

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FIG. 20.

(a) The two-dimensional array of polymer spots produced by the SAW translation and atomization process is extremely regular and organized. (b) The spot diameter, longitudinal pitch spacing and transverse pitch spacing [see image (a)] are strongly correlated to the SAW frequency, and hence, wavelength (Ref. 36).

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/content/aip/journal/bmf/3/1/10.1063/1.3056040
2009-01-02
2014-04-25

Abstract

We demonstrate that surface acoustic waves(SAWs), nanometer amplitude Rayleigh waves driven at megahertz order frequencies propagating on the surface of a piezoelectric substrate, offer a powerful method for driving a host of extremely fast microfluidic actuation and micro/bioparticle manipulation schemes. We show that sessile drops can be translated rapidly on planar substrates or fluid can be pumped through microchannels at velocities, which are typically one to two orders quicker than that afforded by current microfluidic technologies. Through symmetry-breaking, azimuthal recirculation can be induced within the drop to drive strong inertial microcentrifugation for micromixing and particle concentration or separation. Similar micromixing strategies can be induced in the same microchannel in which fluid is pumped with the SAW by merely changing the SAW frequency to rapidly switch the uniform through-flow into a chaotic oscillatory flow by exploiting superpositioning of the irradiated sound waves from the sidewalls of the microchannel. If the flow is sufficiently quiescent, the nodes of the transverse standing wave that arises across the microchannel also allow for particle aggregation, and hence, sorting on nodal lines. In addition, the SAW also facilitates other microfluidic capabilities. For example, capillary waves excited at the free surface of a sessile drop by the SAW underneath it can be exploited for micro/nanoparticle collection and sorting at nodal points or lines at low powers. At higher powers, the large accelerations off the substrate surface as the SAW propagates across drives rapid destabilization of the dropfree surface giving rise to inertial liquid jets that persist over in length or atomization of the entire drop to produce monodispersed aerosol droplets, which can be exploited for ink-jet printing, mass spectrometry interfacing, or pulmonary drug delivery. The atomization of polymer/protein solutions can also be used for the rapid synthesis of polymer/protein particles or biodegradable polymeric shells in which proteins, peptides, and other therapeutic molecules are encapsulated within for controlled release drug delivery. The atomization of thin films behind a translating drop containing polymer solutions also gives rise to long-range spatial ordering of regular polymer spots whose size and spacing are dependent on the SAW frequency, thus offering a simple and powerful method for polymer patterning without requiring surface treatment or physical/chemical templating.

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Scitation: Ultrafast microfluidics using surface acoustic waves
http://aip.metastore.ingenta.com/content/aip/journal/bmf/3/1/10.1063/1.3056040
10.1063/1.3056040
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