Acoustofluidics

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• Multiphysics of Microfluidics [ Acoustofluidics | Optofluidics ]
• Multiphysics of Active Nanostructures [ Nanomanufacturing | Nanophotonics ]

Acoustofluidics deals with the study and application of mechanical waves in the micro-/nano- meter scale fluidic environments. Enabled by MEMS/NEMS technologies, it offers a noninvasive solution for many applications such as “lab-on-a-chip”. Along this line, the Penn State Bio-NEMS laboratory aims to (1) develop effective techniques to filter, guide and confine acoustic energies at micro/nano meter scales, and (2) manipulate (e.g., focus, trap, and pattern) micro/nano objects (e.g., fluorescent beads, carbon nanotubes, DNAs, and cells) in microfluidic environments using acoustic waves.

Specifically, our research can be categorized into three topics: Acoustic Metamaterials, Acoustic Tweezers, and Acoustically-Driven Microbubbles.

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Go to Top | Acoustic Tweezers | Acoustically-Driven Microbubbles

J Shi et al., Applied Physics Letters, Vol. 92, pp. 111901, 2008.

We numerically investigated the collimation phenomena in phononic crystal
(PC) composites, a sequenced series of PCs with the same period but different filling ratios. The plane wave expansion
PWE method was used to obtain the band diagrams and the equal frequency surfaces of both single PCs and PC composites. The finite difference time domain
FDTD method was then utilized to simulate the propagation of acoustic waves inside a PC composite. The results from both PWE calculations and FDTD simulations show that in comparison to a single PC, a PC composite can significantly enlarge the collimation region and realize wide-band acoustic collimation.

Figure: An acoustic collimation lens composed of two PCs
steel cylinders in water with different filling ratios was shown to enlarge the collimation region over a normalized frequency range by a factor of 185%–220%, thereby realizing wide-band acoustic collimation. The methodology described in this letter will prove useful in applications that require confined acoustic energy flow over long operation distances, such as acoustic imaging, drug delivery, cell sonoporation, and nondestructive evaluation.

SCS Lin et al., Physical Review B, Vol. 79, pp. 094302, 2009.

We designed and characterized a two-dimensional, gradient-index phononic crystal
GRIN PC to control the propagation of acoustic waves. The GRIN PC was composed of solid cylinders arranged in a square lattice and immersed in an epoxy. The refractive index along the direction transverse to the phononic propagation was designated as a hyperbolic secant gradient distribution. This distribution was modulated by means of the density and elastic moduli of the cylinders. The effective refractive indices in each row of the GRIN PC were determined from band diagrams obtained via a plane-wave expansion method. The acoustic wave propagation was numerically investigated by a finite-difference time-domain method, and the results were compared to the analytical beam trajectories derived from the hyperbolic secant profile. These results show that the GRIN PC allows acoustic focusing over a wide range of working frequencies, making it suitable for applications such as flat acoustic lenses and couplers.

Figure: (a) Principle of a gradient-index medium. A hyperbolic secant refractive index profile along the direction transverse to propagation (y-axis) enables redirection of incident beams
arrows inside the medium. A GRIN PC can be realized by (b) adjusting radii of cylinders or (C) changing elastic properties of cylinders along the transverse direction to achieve a hyperbolic secant refractive index profile. (d) The simulated acoustic focusing in a domain filled with epoxy and a 20-layer-thick GRIN PC lens at a reduced frequency of 0.10.

SCS Lin et al., Journal of Physics D: Applied Physics, Vol. 42, pp. 185502, 2009.

We report a novel approach to effectively couple acoustic energy into a two-dimensional phononic-crystal waveguide by an acoustic beamwidth compressor using the concept of a gradient-index phononic crystal (GRIN PC). The GRIN PC-based beamwidth compressor is composed of a square array of solid scatterers embedded in epoxy. By gradually modulating the density and elastic modulus of the scatterers along the direction transverse to the phononic propagation, the beamwidth compressor can efficiently compress the wide acoustic beam to the scale of the phononic-crystal waveguide. This acoustic beamwidth compressor is investigated through a finite-difference time-domain method. A beam-size conversion ratio of 6.5 : 1 and a transmission efficiency of up to 90% is obtained over the working frequency range of the phononic-crystal waveguide. Potential applications for this device include acoustic biosensors and signal processors.

Figure: FDTD simulated acoustic wave propagation in the acoustic coupler at 13.66 KHz. Displacement fields were normalized to the maximum displacement of the guided beam.

SCS Lin et al., Journal of Applied Physics, Vol. 106, pp. 053529, 2009.

We report the design of a two-dimensional gradient-index phononic crystal (GRIN PC) structure, which effectively demonstrates the “acoustic mirage” effect on the wavelength scale. Using the GRIN PC, the propagating direction of acoustic waves can be continuously bent along an arc-shaped trajectory by gradually tuning the filling ratio of PCs. We investigate the acoustic mirage effect through both plane wave expansion and finite-difference time-domain methods. By controlling the incident angle or operating frequency, the arc-shaped trajectory of acoustic wave propagation can be dynamically adjusted. The GRIN PC structure is composed of steel cylinders, positioned in a square lattice, and immersed in an epoxy. It can be fabricated through a simple process and seamlessly integrated with existing acoustic devices. In addition, we establish that such an acoustic effect can be used in the design of tunable acoustic waveguides, which could find applications in acoustic switching, filtering, and biosensing.

Figure: (a) A smooth redirection of the acoustic wave propagation can be achieved by a GRIN PC where each layer can be considered an independent PC of different filling ratio. (b) “Acoustic mirage” inside a GRIN PC illuminated by a SV-mode acoustic beam with a width of 4a, an incident angle of 10°, and an operating frequency of 0.79. The light and dark regions correspond to the strong and weak amplitudes of the displacement field, respectively.

[SCS Lin et al., Physical Review B, Vol. 83, pp. 174303, 2011.]

We present a theoretical study on the tunability of phononic band gaps in two-dimensional phononic crystals consisting of various anisotropic cylinders in an isotropic host. A two-dimensional plane wave expansion method was used to analyze the band diagrams of the phononic crystals; the anisotropic materials used in this work include cubic, hexagonal, trigonal, and tetragonal crystal systems. By reorienting the anisotropic cylinders, we show that phononic band gaps for bulk acoustic waves propagating in the phononic crystal can be opened, modulated, and closed. The methodology presented here enables enhanced control over acoustic metamaterials which have applications in ultrasonic imaging, acoustic therapy, and nondestructive evaluation.

Figure: (a) the top view of an infinite two-dimensional square-lattice phononic crystal. (b) Slowness surfaces of bulk acoustic waves propagating in the XZ plane of GaAs. (C) Dispersion relations for bulk modes of a GaAs (XZ-plane)/Epoxy square-lattice phononic crystal with a filling fraction of 0.65 and a cylinder rotation angle of 45 degrees. (d) the angular dependence of relative bandwidth of the phononic band gaps.

1. Sz-Chin Steven Lin and Tony Jun Huang, Tunable Phononic Crystals with Anisotropic Inclusions, Physical Review B, Vol. 83, pp. 174303, 2011. [PDF]
2. Tsung-Tsong Wu, Yan-Ting Chen, Jia-Hong Sun, Sz-Chin Steven Lin, and Tony Jun Huang, Focusing of Lamb Wave in a Gradient-Index Phonoinc Crystal Plate, Applied Physics Letters, Vol. 98, pp. 171911, 2011. [PDF]
3. Sz-Chin Steven Lin, Bernhard R. Tittmann, Jia-Hong Sun, Tsung-Tsong Wu, and Tony Jun Huang, Acoustic Beamwidth Compressor Using Gradient-Index Phononic Crystals, Journal of Physics D: Applied Physics, Vol. 42, pp. 185502, 2009. (featured as front cover image) [PDF]
4. Sz-Chin Steven Lin and Tony Jun Huang, Acoustic Mirage in Two-Dimensional Gradient-Index Phononic Crystals, Journal of Applied Physics, Vol. 106, pp. 053529, 2009. [PDF]
5. Sz-Chin Steven Lin, Tony Jun Huang, Jia-Hong Sun and Tsung-Tsong Wu, Gradient-Index Phononic Crystals, Physical Review B, Vol. 79, pp. 094302, 2009. [PDF]
6. Jinjie Shi, Sz-Chin Steven Lin, Tony Jun Huang, Wide-Band Acoustic Collimating by Phononic Crystal Composites, Applied Physics Letters, Vol. 92, pp. 111901, 2008. (featured as front cover image) [PDF]

Go to Top | Acoustic Metamaterials | Acoustically-Driven Microbubbles

J Shi et al., Lab on a Chip, Vol. 8, pp. 221-223, 2008.

We introduce a novel on-chip microparticle focusing technique using standing surface acoustic waves (SSAW) to enable fast and effective microparticle focusing inside a microfluidic channel. In comparison to other particle focusing techniques, including hydrodynamic, electrokinetic and DEP focusing, this method is simple, fast, dilution-free, and can be used to focus virtually any microparticles. Moreover, the transparency of the focusing device makes it compatible with most optical characterization tools used in biology and medicine. In contrast to the BAW-based microparticle manipulation method, the SSAW-based technique localizes most of the acoustic energy on the surface of the substrate and has little loss along the propagation line, thus lowering the power consumption and improving the uniformity of the standing waves. The technique is compatible with standard soft lithography techniques. We expect that it can be used in a wide variety of on-chip biological/biochemical applications.

Figure: (Left) Photograph of the bonded SSAW focusing device consisting of a LiNbO3 substrate with two parallel IDTs and a PDMS channel. Inset: zoomed-in photograph of IDTs. (Right) the recorded fluorescent images at four sites (I–IV) indicated in the lower left schematic, respectively.

J Shi et al., Lab on a Chip, Vol. 9, pp. 2890-2895, 2009.

In this work we present an active patterning technique named “acoustic tweezers” that utilizes standing surface acoustic wave (SSAW) to manipulate and pattern cells and microparticles. This technique is capable of patterning cells and microparticles regardless of shape, size, charge or electrical/magnetic/optical properties. We verified this versatility by patterning polystyrene beads, bRBC, and E. coli cells. The required power intensity of acoustic tweezers is approximately 500000 times lower than that of optical tweezers, compares favorably with those of other active patterning methods. Such a low power intensity also contributes to the technique's non-invasive nature, as confirmed by our cell viability studies (a flow cytometry study). The aforementioned advantages, along with this technique's simple design and ability to be miniaturized, render the “acoustic tweezers” technique a promising tool for various applications in biology, chemistry, engineering, and materials science.

Figure: (a) Optical images of the “acoustic tweezers” device used in 2D patterning experiments. (b) Distribution of the microbeads after the 2D patterning process. The SAW wavelength was 200 um. (C) Patterning of bRBC. The wavelength of the applied SAW was 100 um.

J Shi et al., Lab on a Chip, Vol. 9, pp. 3354-3359, 2009.

This work introduces a method of continuous particle separation through standing surface acoustic wave (SSAW)-induced acoustophoresis in a microfluidic channel. Using this SSAW-based method, particles in a continous laminar flow can be separated based on their volume, density and compressibility. In this work, a mixture of particles of equal density but dissimilar volumes was injected into a microchannel through two side inlets, sandwiching a deonized water sheath flow injected through a central inlet. A one-dimensional SSAW generated by two parallel interdigital transducers (IDTs) was established across the channel, with the channel spanning a single SSAW pressure node located at the channel center. Application of the SSAW induced larger axial acoustic forces on the particles of larger volume, repositioning them closer to the wave pressure node at the center of the channel. Thus particles were laterally moved to different regions of the channel cross-section based on particle volume. The particle separation method presented here is simple and versatile, capable of separating virtually all kinds of particles (regardless of charge/polarization or optical properties) with high separation efficiency and low power consumption.

Figure: (a) Schematic of the separation mechanism showing particles beginning to translate from the sidewall to the center of the channel due to axial acoustic forces applied to the particles when they enter the working region of the SSAW (site 1). The differing acoustic forces cause differing displacements, repositioning larger particles closer to the channel center and smaller particles farther from the center (site 2). (b) Comparison of forces (normally in pN range) acting on particles at site 1 and site 2, espectively.

YJ Liu et al., Advanced Materials, Vol. 23, pp. 1656-1659, 2011.

In this work, we demonstrate a SAW-driven PDLC light shutter based on the acoustic streaming-induced realignment of LC molecules as well as absorption-related thermal diffusion. The working mechanism was analyzed theoretically and the acousto-optical properties of the PDLC sample were characterized experimentally. This device shows excellent performance in terms of energy consumption and optical contrast, which is important for applications such as displays and smart windows. In addition, the IDTs fabricated by standard hotolithography are highly compatible for future system integration. Our future work includes developing SAW-driven PDLC systems with faster response and lower energy consumption—this could be achieved by optimizing the mechanical and dielectric properties of PDLC (e.g., using less viscous ferroelectric LCs with higher birefringence, controlling the LC droplets size, and adding surfactants). We expect that with further developments, the SAWbased driving scheme could have significant impact on future PDLC-based nanophotonic and plasmonic devices.

Figure: The device structure and working principle for the SAW-driven PDLC light shutter. The magnifi ed part shows a reversible switching process between two different LC droplet configurations.

1. Yan Jun Liu, Xiaoyun Ding, Sz-Chin Steven Lin, Jinjie Shi, I-Kao Chiang, and Tony Jun Huang, Surface Acoustic Wave Driven Light Shutters Using Polymer-Dispersed Liquid Crystals, Advanced Materials, Vol. 23, pp. 1656-1659, 2011. [PDF]
2. Jinjie Shi, Hua Huang, Zak Stratton, Aitan Lawit, Yiping Huang and Tony Jun Huang, Continuous Particle Separation in a Microfluidic Channel via Standing Surface Acoustic Waves (SSAW), Lab on a Chip, Vol. 9, pp. 3354-3359, 2009. (featured as back cover image) [PDF]
3. Jinjie Shi, Daniel Ahmed, Xiaole Mao, Sz-Chin Steven Lin, and Tony Jun Huang, Acoustic Tweezers: Patterning Cells and Microparticles Using Standing Surface Acoustic Waves (SSAW), Lab on a Chip, Vol. 9, pp. 2890-2895, 2009. (featured as front cover image) [PDF]
4. Jinjie Shi, Xiaole Mao, Daniel Ahmed, Ashley Colletti, Tony Jun Huang, Focusing Microparticles in a Microfluidic Channel with Standing Surface Acoustic Waves (SSAW), Lab on a Chip, Vol. 8, pp. 221-223, 2008. [PDF]

Go to Top | Acoustic Metamaterials | Acoustic Tweezers

D Ahmed et al., Lab on a Chip, Vol. 9, pp. 2738-2741, 2009.

In this work, we present ultra-fast homogeneous mixing inside a microfluidic channel via single-bubble-based acoustic streaming. The device operates by trapping an air bubble within a “horse-shoe” structure located between two laminar flows inside a microchannel. Acoustic waves excite the trapped air bubble at its resonance frequency, resulting in acoustic streaming, which disrupts the laminar flows and triggers the two fluids to mix. Due to this technique’s simple design, excellent mixing performance, and fast mixing speed (through quantitative analysis, we have proven that our mixer can achieve excellent homogenized mixing across the entire width of the channel with a mixing time of ~7 ms), our single-bubble-based acoustic micromixer may prove useful for many biochemical studies and applications.

Figure: (Left) Schematic of experimental setup. The piezo transducer is placed adjacent to the microfluidic device. Inset: illustration of a bubble trapped inside the horse-shoe structure and streaming pattern around the bubble membrane in the presence of acoustic waves. (Right) Characterization of the acoustic streaming pattern around a single bubble. (Off) An air bubble trapped in the horse-shoe structure and stationary polystyrene particle solution. (On) Recirculating flow pattern around the air bubble when the bubble membrane oscillates at its resonance frequency.

D Ahmed et al., Microfluidics and Nanofluidics, Vol. 7, pp. 727-731, 2009.

Due to the low Reynolds number associated with microscale fluid flow, it is difficult to rapidly and homogenously mix two fluids. In this letter, we report a fast and homogenized mixing device through the use of a bubble-based microfluidic structure. This micromixing device worked by trapping air bubbles within the predesigned grooves on the sidewalls of the channel. When acoustically driven, the membranes (liquid/air interfaces) of these trapped bubbles started to oscillate. The bubble oscillation resulted in a microstreaming phenomenon—strong pressure and velocity fluctuations in the bulk liquid, thus giving rise to fast and homogenized mixing of two side-by-side flowing fluids. The performance of the mixer was characterized by mixing deionized water and ink at different flow rates. The mixing time was measured to be as small as 120 ms.

Figure: (a) Schematic of the experimental setup. The microfluidic channel and the piezo transducer was bonded onto a Petri dish and placed adjacent to each other. The polyethylene tubings from the channel were connected to syringe pumps (marked as white arrows). The acoustic transducer was driven by a function generator and the whole setup was mounted on an optical microscope stage. (b) Deionized water was injected into the channel, thus trapping air bubbles on the sidewalls of the channel. (C) Magnified image of a single bubble trapped in the sidewall groove. (d) Schematic of the microstreaming phenomenon around an acoustically activated microbubble

1. Daniel Ahmed, Xiaole Mao, Bala Krishna Juluri and Tony Jun Huang, A Fast Microfluidic Mixer Based on Acoustically Driven Sidewall-Trapped Microbubbles, Microfluidics and Nanofluidics, Vol. 7, pp. 727-731, 2009. [PDF]
2. Daniel Ahmed, Xiaole Mao, Jinjie Shi, Bala Krishna Juluri and Tony Jun Huang, A Millisecond Micromixer via Single-Bubble-Based Acoustic Streaming, Lab on a Chip, Vol. 9, pp. 2738-2741, 2009. [PDF]