Surface Acoustic Wave Radiation Research Articles

Full-wave modeling of micro-acoustofluidic devices driven by standing surface acoustic waves for microparticle acoustophoresis

Surface acoustic wave (SAW)-based acoustofluidic systems are emerging as an important tool for acoustophoresis. In this paper, we present a full cross-sectional model of standing SAW acoustofluidic devices for obtaining full-wave results. Our model involves a piezoelectric substrate with interdigitated electrodes and a rectangular water channel enclosed in a finite soft elastic solid. This model accounts for piezoelectric SAWs with electromechanical coupling, simultaneous transverse and longitudinal wave fields in the elastic solid from SAW radiation, and acoustic and streaming fields in the enclosed water channel in an integrated system by solving the elastodynamic and Navier–Stokes field equations. Accordingly, the acoustic radiation force and streaming-induced Stokes drag force are obtained to analyze the acoustophoretic motion of microparticles of different sizes. Using the full-wave results, we reveal the influences of the channel wall displacements and acoustic and flow fields in the water domain. The full-wave field also allows us to determine the effects of the channel dimensions and its location in the finite elastic solid on the force strengths. We demonstrate that the critical diameter of the microparticles can be reduced by an order of magnitude by changing the channel location, while maintaining the same acoustic frequency. We note that the results, mechanisms, and method presented in this study can be usefully applied to the rational design of standing SAW acoustofluidic devices and for developing innovative acoustophoretic systems involving complex structure–fluid interactions.

Observational and theoretical validation of longitudinal type surface acoustic wave devices

The authors have proposed longitudinal type surface acoustic wave (SAW) devices comprising a LiNbO3 thin plate, acoustic mirror, handle substrate and interdigital transducers (IDT). This paper describes the observational and theoretical validation of the proposed devices by using a laser probe system and finite element analysis (FEA). It is shown that the laser probe system based on the Sagnac interferometer is effective for direct observation of the devices operated at even 2 GHz range through the observation of SAW field distribution of the existence and lack of acoustic mirror, their comparison with FEA results and changes of the transverse mode pattern. In addition, a phenomenon of SAW radiation normal to the busbars from IDT is found both observationally and theoretically, and a possible radiation mechanism explaining the results is also discussed.

A new microvalve controlled by surface acoustic wave

ABSTRACTA new microvalve controlled by surface acoustic wave was presented. A shape memory alloy (SMA) with plastic sheet was integrated into a printed circuit board by way of small copper pipe. A 128°-YX LiNbO3 piezoelectric substrate was under the SMA wire with small gap. A polydimethylsiloxane (PDMS) microchannel was squeezed by the extension of the heated SMA wire due to surface acoustic wave radiation. The microvalve state was then changed. Experimental results show that the switching-off time of the microvalve is 39.533 s at 28.5 dBm electric signal power. The variation of SMA size during expansion is theoretically calculated.

The attenuation of poly(dimethylsiloxane) film to surface acoustic wave on a piezoelectric substrate

ABSTRACTThe attenuation characteristics of poly(dimethylsiloxane) (PDMS) film to surface acoustic wave was studied. An interdigital transducer with 27.7 MHz center frequency was fabricated on a 1280 yx-LiNbO3 substrate using microelectric technology. PDMS film with different thickness was coated on the piezoelectric substrate. Paraffin oil was pipetted on the PDMS film and heated by surface acoustic wave radiation. The temperature of the paraffin oil was measured by a temperature measuring meters. Then, the energy radiated into the paraffin oil per unit time was calculated. Comparison to SAW without the attenuation of PDMS film, the attenuation of SAW is 12.6%, 31.6% and 38% respectively, when the thickness of the PDMS film is 50μm, 100μm or 150μm.

Analysis of two dimensional composite surface grating structures with applications to low loss microacoustic resonators

2D composite grating building blocks with hexagonal symmetry are proposed and analyzed as efficient reflectors and transducers in the design of low loss surface acoustic wave (SAW) resonators. Eigen-frequency and frequency response finite element analyses are used to study the propagation and excitation characteristics of SAWs in the 2D composite structures. The proposed structures have the advantage of being compatible with the planar SAW technology, while introducing design principles reminiscent to the semiconductor devices. More specifically, the frequency bandgap characteristics of the various resonator building blocks can be adjusted so that losses related to the SAW diffraction, impedance mismatch, and parasitic SAW radiation from the electrical contacts can be suppressed. Three distinct types SAW resonant topologies employing 2D gratings are deduced from the analytical results.

Surface Acoustic Wave Enhancing Drop-Drop Microextraction with an Original Position Loop for Stabilizing Drops

A surface acoustic wave based drop-drop microextraction, in which an original position loop is used for stabilizing drops, is proposed. An interdigital transducer with 27.5 MHz center frequency was fabricated on a 128° yx-LiNbO3 piezoelectric substrate for exciting surface acoustic wave. Mass transfer of extractive matter between two liquid phases was enhanced due to acoustic streaming caused by radiation of surface acoustic wave. An original position loop, which was used to prevent extraction liquid from movement along the surface of piezoelectric substrate, was fixed on the substrate with 1 mm gap. An ionic liquid and an organic dye (acid green-25) solution were used for extraction experiments. Results show that the extraction time is greatly decreased due to surface acoustic wave radiation, and extraction process can be finished within two minutes at 30.0 dBm RF signal power.

Rapid drop-to-drop liquid–liquid microextraction by help of surface acoustic wave

A novel drop-to-drop liquid–liquid microextraction method is presented, by which a liquid–liquid microextraction is greatly accelerated. A pair of interdigital transducers (IDTs) with 27.5MHz center frequency is fabricated on a 128° yx-LiNbO3 substrate. Once a RF signal is applied to an IDT, surface acoustic wave is generated and then radiated into drops to be extracted on the acoustic path. The surface acoustic wave radiation, leading to rapid movement of droplets in original position, enhances the microextraction. An ionic liquid and an organic dye aqueous solution are used for extraction experiments and extraction kinetics is studied using an image-based concentration measurement technique. Extraction experiments show that a drop-to-drop liquid–liquid microextraction process can be finished within 100s when the RF signal power applied to the IDT is 26.9dBm, and the extraction time is decreased with RF signal power applied to the IDT.

A Microreactor With Surface Acoustic Wave Micro-Heating System

A new microreactor with surface acoustic wave (SAW) heating system was presented. The microreactor is comprised of surface acoustic wave heating source, a PDMS micro-vessel and a metal micro-vessel. An interdigital transducer (IDT) and a reflector were fabricated on a 1280 yx-LiNbO3 piezoelectric substrate by microelectric technology. A RF signal with 27.5 MHz frequency was applied to the IDT to generate surface acoustic wave. When hitting a glycerin microfluid in the PDMS micro-vessel on the piezoelectric substrate, SAW radiated into the glycerin microfluid and heated it. Then, the temperature of the reactants in the metal micro-vessel was raised by heating transfer. The experiments of melting solid paraffin and color development reaction have been examined using the microreactor. The results show that the temperature of the reactants in the metal micro-vessel can be raised at least 42°C, which is enough for lots of biochemical reactions. The results also show that the microreactor can greatly improve the reaction velocity of color development reaction due to SAW radiation. This work is valuable for biochemical analysis on microfluidic device.

Acousto-optic interaction with leaky surface acoustic waves in Y-cut LiTaO3 crystals

The acousto-optic interaction with leaky surface acoustic wave radiation into the bulk of YX-cut LiTaO3 crystals has been investigated. The light incidence and diffraction angles corresponding to the strongest acousto-optic interaction were calculated and measured as functions of the acoustic wave frequency. The dependencies of the diffracted light intensity on the amplitude of radio-frequency voltage applied to the interdigital transducer (IDT) were studied. Our acousto-optic measurements revealed generation, by the IDTs, of slow shear bulk acoustic waves propagating at different angles depending on their frequency. A secondary acousto-optic interaction from the bulk waves radiated by the receiving IDT has been studied.

Surface Acoustic Wave Driven Microfluidics – A Review

This paper presents a systematic overview on the recent works on surface acoustic wave (SAW) driven microfluidics. SAW microfluidics is based on acoustic streaming induced by leaky SAW radiation into a liquid. The development of this field attracts attention from microfluidic research community due to its rapid actuation, programmable capability, simple and yet efficient operation. In our paper, SAW microfluidic applications are categorized into droplet-based applications and continuous-flow applications. Droplet is actuated into unique behaviours depending on the applied SAW power. A wide range of droplet based applications have been employed utilizing these behaviours. In continuous-flow system, applications are further categorized based on the interaction of travelling SAW and standing SAW with the bulk liquid. Finally, future perspectives of SAW driven microfluidics are discussed

Particle concentration via acoustically driven microcentrifugation: microPIV flow visualization and numerical modelling studies

Through confocal-like microparticle image velocimetry experiments, we reconstruct, for the first time, the three-dimensional flow field structure of the azimuthal fluid recirculation in a sessile drop induced by asymmetric surface acoustic wave radiation, which, in previous two-dimensional planar studies, has been shown to be a powerful mechanism for driving inertial microcentrifugation for micromixing and particle concentration. Supported through finite element simulations, these insights into the three-dimensional flow field provide valuable information on the mechanisms by which particles suspended in the flow collect in a stack at a central position on the substrate at the bottom of the drop once they are convected by the fluid to the bottom region via a helical spiral-like trajectory around the drop periphery. Once close to the substrate, the inward radial velocity then forces the particles into this central stagnation point where they are trapped by sedimentary forces, provided the convective force is insufficient to redisperse them along with the fluid up a central column and into the bulk of the drop.

Microfluidic Colloidal Island Formation and Erasure Induced by Surface Acoustic Wave Radiation

Spatiotemporal patterns form in many nonlinear physicochemical or biological systems. Although unusual, microfluidic systems are no exception. We observe such patterns to form by colloids along the free surface of a drop beneath which surface acoustic waves propagate, and propose fundamental mechanisms to elucidate their formation. With increasing excitation amplitude, the colloids first assemble into concentric rings and then cluster into islands due to a combination of capillarity and surface acceleration. As the excitation is further increased, fluid streaming commences within the drop, inducing a transient metastable state in which the system alternates between colloidal island formation on the quiescent drop surface and subsequent erasure due to local vortex generation.

Surface acoustic wave concentration of particle and bioparticle suspensions

A rapid particle concentration method in a sessile droplet has been developed using asymmetric surface acoustic wave (SAW) propagation on a substrate upon which the droplet is placed. Due to the asymmetry in the SAW propagation, azimuthal bulk liquid recirculation (acoustic streaming) is generated. Once the local particle concentration is sufficiently high within a particular streamline of the acoustic streaming convective flow, shear-induced migration gives rise to an inward radial force that concentrates the particles at the centre of the droplet. In this paper, a SAW device consists of a 0.75-mm thick, 127.68 degrees Y-X-axis-rotated cut, X-propagating LiNbO3 for a substrate and an interdigital transducer electrode (IDT) with 25 straight finger pairs in a simple repeating pattern, 12 mm aperture, and a wavelength of lambda=440 microm was patterned on the substrate. The IDT was then driven with a sinusoidal signal at the resonance frequency f0 of 8.611 MHz. To investigate the effect of particle type and size on the concentration process, three types of particles were used in this study, including fluorescent particles (1 microm), polystyrene microspheres (3, 6, 20, 45 microm), and living yeast cells (10-20 microm). Different RF powers were applied ranging from 120 to 510 mW. The concentration processes occurs within 2 to 20 s, depending on the particle size, type and input radio frequency (RF) power, much faster than currently available particle concentration mechanisms due to the large convective velocities achieved using the SAW device. Moreover, this concentration method is efficient, concentrating the particles into an aggregate one-tenth the size of the original droplet. Most importantly, bioparticles can also be concentrated by this method; we have verified that yeast cells are not lysed by the SAW radiation during concentration. By using the rapid concentration process described in this work, the breadth of applications and measurement sensitivity of SAW biosensor systems should be greatly enhanced.