Surface Acoustic Wave Field Research Articles

Volumetric Measurement Of The Acoustically Induced Fluid Flow In A Micro Chamber With A Two-Dimensional Standing Surface Acoustic Wave Field

The non-invasive, contactless and label-free trapping and patterning of individual cells with acoustic tweezers is a key technology to reliably analyze single cells and cell-cell interactions in biological applications. By using two-dimensional standing surface acoustic waves (2DsSAW), the precise manipulation of the cells in a micro chamber is primarily governed by the acoustic radiation force, which acts directly on the cells due to scattering effects of the acoustic waves. However, a fluid flow of three-dimensional nature is concurrently induced acoustically, which additionally influences the cell pattern by Stokes' drag. In this study, we shed light on the complex acoustically induced fluid flow forming in an acoustic tweezer by 3D3C velocity measurements using the astigmatism particle tracking velocimetry (APTV). Vortex structures above the antinodes of the 2DsSAW field were revealed. The fluid flow converge and diverge in the vicinity of the chamber bottom and ceiling, respectively. Hence, the fluid flow influences the stable trapping locations of cells or particles in different ways depending on their height position, which might explain the recently reported formation of various particle patterns at different height levels. Moreover, the experimental findings are used to validate a three-dimensional numerical model. The numerical model provides deep insights into the underlying acoustic fields responsible for the formation of the fluid flow and acoustic radiation force. By combining the experimental and numerical results, a profound understanding of the physical mechanisms for cell or particle trapping and patterning is established.

Investigation on submicron particle separation and deflection using tilted-angle standing surface acoustic wave microfluidics

With the development of in vitro diagnostics, extracting submicron scale particles from mixed body fluids samples is crucial. In recent years, microfluidic separation has attracted much attention due to its high efficiency, label-free, and inexpensive nature. Among the microfluidic-based separation, the separation based on ultrasonic standing waves has gradually become a powerful tool. A microfluid environment containing a tilted-angle ultrasonic standing surface acoustic wave (taSSAW) field has been widely adapted and designed to separate submicron particles for biochemical applications. This paper investigated submicron particle defection in microfluidics using taSSAWs analytically. Particles with 0.1–1 μm diameters were analyzed under acoustic pressure, flow rate, tilted angle, and SSAW frequency. According to different acoustic radiation forces acting on the particles, the motion of large-diameter particles was more likely to deflect to the direction of the nodal lines. Decreasing the input flow rate or increasing acoustic pressure and acoustic wave frequency can improve particle deflection. The tilted angle can be optimized by analyzing the simulation results. Based on the simulation analysis, we experimentally showed the separation of polystyrene microspheres (100 nm) from the mixed particles and exosomes (30–150 nm) from human plasma. This research results can provide a certain reference for the practical design of bioparticle separation utilizing acoustofluidic devices.

On the behavior of prolate spheroids in a standing surface acoustic wave field

The active manipulation of particle and cell trajectories in fluids by high-frequency standing surface acoustic waves (sSAW) allows to separate particles and cells systematically depending on their size and acoustic contrast. However, process technologies and biomedical applications usually operate with non-spherical particles, for which the prediction of acoustic forces is highly challenging and remains a subject of ongoing research. In this study, the dynamical behavior of prolate spheroids exposed to a three-dimensional acoustic field with multiple pressure nodes along the channel width is examined. Optical measurements reveal an alignment of the particles orthogonal to the pressure nodes of the sSAW, which has not been reported in literature so far. The dynamical behavior of the particles is analyzed under controlled initial conditions for various motion patterns by imposing a phase shift on the sSAW. To gain detailed understanding of the particle dynamics, a three-dimensional numerical model is developed to predict the acoustic force and torque acting on a prolate spheroid. Considering the acoustically induced streaming around the particle, the numerical results are in excellent agreement with experimental findings. Using the proposed numerical model, a dependence of the acoustic force on the particle shape is found in relation to the acoustic impedance of the channel ceiling. Hence, the numerical model presented herein promises high progress for the design of separation devices utilizing sSAW, exploiting an additional separation criterion based on the particle shape.

Acoustofluidic Engineering of Functional Vessel-on-a-Chip.

Construction of in vitro vascular models is of great significance to various biomedical research, such as pharmacokinetics and hemodynamics, and thus is an important direction in the tissue engineering field. In this work, a standing surface acoustic wave field was constructed to spatially arrange suspended endothelial cells into a designated acoustofluidic pattern. The cell patterning was maintained after the acoustic field was withdrawn within the solidified hydrogel. Then, interstitial flow was provided to activate vessel tube formation. In this way, a functional vessel network with specific vessel geometry was engineered on-chip. Vascular function, including perfusability and vascular barrier function, was characterized by microbead loading and dextran diffusion, respectively. A computational atomistic simulation model was proposed to illustrate how solutes cross the vascular membrane lipid bilayer. The reported acoustofluidic methodology is capable of facile and reproducible fabrication of the functional vessel network with specific geometry and high resolution. It is promising to facilitate the development of both fundamental research and regenerative therapy.

A Numerical Study of Collective Cell Migration in a Microchannel Driven by Surface Acoustic Wave (SAW) Device

Collective cell migration is involved in a variety of biological contexts, including tissue morphogenesis, wound healing, and cancer invasion. Many studies have revealed that chemical, mechanical, and electrical stimulation all affect cell migration. Although an acoustic stimulus has been shown to influence cell migration in the past, the underlying mechanism is still unknown. A computational model that accounts for acoustic–structure interaction was constructed in this study to simulate the formation of a surface acoustic wave (SAW) field and the application of the acoustic pressure field on collective cell migration. A group of cells within a microchannel device and two ports of interdigitated transducers (IDTs) with different wavelengths were modeled. The stresses within cells were investigated as it was influenced by substrate displacement and pressure acoustic in the cell media generated by the SAW device. As a result, we observed the local stress within cells near the solid-fluid interfaces. For propagating SAW, the shorter wavelength of IDTs (600 μm) attributed to high stress at the cell’s top and bottom as compared to the SAW device with the longer wavelength (1000 μm). The standing SAW occurred underneath collective cells. The results of standing SAW on cell stress at the bottom confirm that the SAW device can be useful to regulate the abnormalities cellular activities associated with cell migration.

Acoustically Operated Excitonic Transistor Using Poly(3-hexylthiophene)

An organic semiconductor, regioregular poly(3-hexylthiophene) (rrP3HT), based excitonic transistor on a piezoelectric YZ lithium niobate substrate is reported for the first time. The propagating surface acoustic wave (SAW) field ionizes the excitons, stores the charge pair in the SAW modulated potential field in the semiconductor, and carries them forward. A long-range transport ∼4.7 mm at room temperature has been demonstrated. The electrical control of the exciton flux was achieved with SAW propagating through a dual metal–insulator–semiconductor (MIS) structure. The working of the device has been demonstrated using white and green light. A threshold voltage of −20.65 V was observed, and the working mechanism of the proposed device has been verified through numerical analysis using MATLAB. The potential use of the proposed device structure as an all optical device was verified electrically with additional electrode terminals at the output end.

Power-controlled acoustofluidic manipulation of microparticles

Recently, surface acoustic wave (SAW) based acoustofluidic separation of microparticles and cells has attracted increasing interest due to accuracy and biocompatibility. Precise control of the input power of acoustofluidic devices is essential for generating optimum acoustic radiation force to manipulate microparticles given their various parameters including size, density, compressibility, and moving velocity. In this work, an acoustophoretic system is developed by employing SAW based interdigital electrode devices. Power meters are applied to closely monitor the incident and reflected powers of the SAW device, which are associated with the separation efficiency. There exists a range of input powers to migrate the microparticles to the pressure node due to their random locations when entering the SAW field. Theoretical analysis is performed to predict a proper input power to separate mixtures of polystyrene microspheres, and the end lateral position of microspheres being acoustically separated. The separation efficiency of four sizes of microspheres, including 20 µm, 15 µm, 10 µm, and 5 µm, is calculated and compared with experimental results, which suggest the input power for separating the mixture of these microspheres. The study provides a practical guidance on operating SAW devices for bioparticle separation using the incident power as a control parameter.

Acoustic charge transport in organic semiconductor films

We demonstrate the acoustic charge transport of optically induced excitons in two organic semiconductors, P3HT and MEH-PPV, up to a distance of 3 mm. The device consists of a surface acoustic wave (SAW) resonator transmitting SAW through a polymer layer where acoustic charge transport takes place and a polymer diode at the end to collect the charges. The voltage excitation is provided using an interdigital transducer (IDT) on a piezoelectric YZ lithium niobate substrate producing Rayleigh SAW at 42 MHz. Optical illumination up to 15 mW cm−2 intensity is applied to induce excitons in the polymer layer deposited on the lithium niobate substrate. The photogenerated excitons in the polymer are ionized by SAW field resulting in free carriers that are transported to the polymer diode by the travelling SAW. A surge in photovoltaic current in the diode is observed in the presence of SAW when the carriers are optically generated away from the diode. The maximum charge capacity and transfer efficiency of the acoustic transport are calculated for various SAW power and illumination intensities. A theoretical analysis of charge carrier dynamics in the presence of a moving SAW field is also performed using a semi-classical Hamiltonian of the system.

On-chip centrifuge using spiral surface acoustic waves on a ZnO/glass substrate

On-chip centrifuges based on acoustic methods have promising applications in biomedicine, environmental protection, and food testing. Glass materials are ideal for microfluidics because of their low cost, chemical inertness, thermal stability, and excellent optical transmission. This paper demonstrates a glass-based, efficient, and controllable particle centrifuge. A circular interdigital transducer with spiral electrodes was fabricated on a composite substrate of ZnO film and quartz glass (QG), generating an omnidirectional spiral surface acoustic wave field. A vorticity field was generated in a sessile droplet, and controlled concentrations of polystyrene particles with different diameters and Hela cells in the droplet were achieved. A linear relationship was observed between the driving voltage and the field amplitude, and in turn, between the voltage and the streaming velocity. The performance of the device can be improved by tuning the driving voltage or the recipe of the solvent. The ZnO/QG-based centrifuge shows good performance in enriching micron/submicron particles and cells and is compatible with optical tools, offering good promise in enriching low abundance micro- and nanoparticles, especially in rare samples.

Acoustic Wave Sensors for Detection of Blister Chemical Warfare Agents and Their Simulants.

On-site detection and initial identification of chemical warfare agents (CWAs) remain difficult despite the many available devices designed for this type of analysis. Devices using well-established analytical techniques such as ion mobility spectrometry, gas chromatography coupled with mass spectrometry, or flame photometry, in addition to unquestionable advantages, also have some limitations (complexity, high unit cost, lack of selectivity). One of the emerging techniques of CWA detection is based on acoustic wave sensors, among which surface acoustic wave (SAW) devices and quartz crystal microbalances (QCM) are of particular importance. These devices allow for the construction of undemanding and affordable gas sensors whose selectivity, sensitivity, and other metrological parameters can be tailored by application of particular coating material. This review article presents the current state of knowledge and achievements in the field of SAW and QCM-based gas sensors used for the detection of blister agents as well as simulants of these substances. The scope of the review covers the detection of blister agents and their simulants only, as in the available literature no similar paper was found, in contrast to the detection of nerve agents. The article includes description of the principles of operation of acoustic wave sensors, a critical review of individual studies and solutions, and discusses development prospects of this analytical technique in the field of blister agent detection.

Imaging of Love Waves and Their Interaction with Magnetic Domain Walls in Magnetoelectric Magnetic Field Sensors

AbstractThe complex behavior of horizontally polarized surface shear waves in magnetoelectric surface acoustic wave based magnetic field sensor devices is revealed by time‐resolved magnetooptical microscopy with picosecond temporal and submicron spatial resolution. The imaging of the propagating waves in the magnetoelectric composites is realized through the functional soft‐magnetic layer by coupled magnetoelastic interactions. Partial surface wave reflections, wave front dephasing, and secondary wave generation occur, which originate from structures and magnetic domain walls. Closure domain structures bend and reflect the magnetic surface waves. Strain stimulated magnetic domain walls display dynamic periodic expansions, which propagate along the domain walls and change the magnetomechanical response also in the surrounding regions. The revealed spatial and temporally varying nondeterministic response restricts the noise performance of the surface acoustic wave based magnetic field sensors and thus confines the sensor's limit of detection. Magnetic time‐resolved optical imaging is shown to be a powerful method for the operando characterization of magnetoelectric devices and in‐plane displacement surface acoustic wave fields that are not accessible by other methods.

A Physics-Informed Neural Network for Quantifying the Microstructural Properties of Polycrystalline Nickel Using Ultrasound Data: A promising approach for solving inverse problems

We employ physics-informed neural networks (PINNs) to quantify the microstructure of a polycrystalline Nickel by computing the spatial variation of compliance coefficients (compressibility, stiffness and rigidity) of the material. The PINN is supervised with realistic ultrasonic surface acoustic wavefield data acquired at an ultrasonic frequency of 5 MHz for the polycrystalline material. The ultrasonic wavefield data is represented as a deformation on the top surface of the material with the deformation measured using the method of laser vibrometry. The ultrasonic data is further complemented with wavefield data generated using a finite element based solver. The neural network is physically-informed by the in-plane and out-of-plane elastic wave equations and its convergence is accelerated using adaptive activation functions. The overarching goal of this work is to infer the spatial variation of compliance coefficients of materials using PINNs, which for ultrasound involves the spatially varying speed of the elastic waves. More broadly, the resulting PINN based surrogate model shows a promising approach for solving ill-posed inverse problems, often encountered in the non-destructive evaluation of materials.

Fluorescence Detection of miRNA-21 Using Au/Pt Bimetallic Tubular Micromotors Driven by Chemical and Surface Acoustic Wave Forces.

In this study, surface acoustic wave (SAW) systems are described for the removal of molecules that are unbound to micromotors, thereby lowering the detection limit of the cancer-related biomarker miRNA-21. For this purpose, in the first step, mass production of the Au/Pt bimetallic tubular micromotor was performed with a simple membrane template electrodeposition. The motions of catalytic Au/Pt micromotors in peroxide fuel media were analyzed under the SAW field effect. The changes in the micromotor speed were investigated depending on the type and concentration of surfactants in the presence and absence of SAW streaming. Our detection strategy was based on immobilization of probe dye-labeled single-stranded probe DNA (6-carboxyfluorescein dye-labeled-single-stranded DNA) to Au/Pt micromotors that recognize target miRNA-21. Before/after hybridization of miRNA-21 (for both w/o SAW and SAW streaming conditions), the changes in the speed of micromotors and their fluorescence intensities were studied. The response of fluorescence intensities was observed to be linearly varied with the increase of the miRNA-21 concentration from 0.5 to 5 nM under both w/o SAW and with SAW. The resulting fluorescence sensor showed a limit of detection of 0.19 nM, more than 2 folds lower compared to w/o SAW conditions. Thus, the sensor and behaviors of Au/Pt tubular micromotors were improved by acoustic removal systems.

Directional dependence of nonlinear piezoelectric surface acoustic waves in lithium niobate

A model for the propagation of finite-amplitude surface acoustic waves (SAWs) in a piezoelectric material [Hamilton et al., J. Acoust. Soc. Am. 100, 2567(A) (1996)] is used to investigate the dependence of SAW nonlinearity on crystal cut and propagation direction in lithium niobate (LiNbO3). The model is based on Hamiltonian formalism, in which the boundary conditions at the free surface need only be evaluated at linear order, thus simplifying computation of the nonlinear problem. In contrast with the scalar coefficient of nonlinearity that appears in evolution equations for bulk planewaves, nonlinearity of SAWs is described by a matrix for harmonic interaction due to the variation of depth dependence of the SAW field with frequency. Components of the nonlinearity matrix are generally complex-valued as a result of material anisotropy and piezoelectricity. Variation with propagation direction of the (1,1) component of the nonlinearity matrix, which governs second-harmonic generation, is presented for LiNbO3 crystals that are X-cut, Y-cut, and Z-cut. Relative contribution of each nonlinear mechanism (nonlinear elasticity, nonlinear piezoelectricity, electrostriction, and nonlinear dielectricity) to the nonlinearity matrix is examined. Distortion of an initially sinusoidal waveform during propagation is demonstrated by integrating coupled spectral evolution equations for the harmonic amplitudes.

Measurement of the acoustic streaming pattern in a standing surface acoustic wave field

The application of standing surface acoustic waves (sSAW) has enabled the development of many flexible and easily scalable concepts for the fractionation of particle solutions in the field of microfluidic lab-ona-chip devices. In this context, the acoustic radiation force (ARF) is often employed for the targeted manipulation of particle trajectories, whereas acoustically induced flows complicate efficient fractionation in many systems [Sehgal and Kirby (2017)]. Therefore, a characterization of the superimposed fluid motion is essential for the design of such devices. The present work focuses on a structural analysis of the acousticallyexcited flow, both in the center and in the outer regions of the standing wave field. For this, experimental flow measurements were conducted using astigmatism particle tracking velocimetry (APTV) [Cierpka et al. (2010)]. Through multiple approaches, we address the specific challenges for reliable velocity measurements in sSAW due to limited optical access, the influence of the ARF on particle motion, and regions of particle depletion caused by multiple pressure nodes along the channel width and height. Variations in frequency, channel geometry, and electrical power allow for conclusions to be drawn on the formation of a complex, three-dimensional vortex structure at the beginning and end of the sSAW.

Piezoelectric Modulation of Excitonic Properties in Monolayer WSe2 under Strong Dielectric Screening.

We investigate the interaction of excitons in monolayer WSe2 with the piezoelectric field of surface acoustic wave (SAW) at room temperature using photoluminescence (PL) spectroscopy and report a large in-plane exciton polarizability of 8.43 ± 0.18 × 10-6 Dm/V. Such large polarizability arises due to the strong dielectric screening from the piezoelectric substrate. In addition, we show that the exciton-piezoelectric field interaction and population distribution between neutral excitons and trions can be optically manipulated by controlling the field screening using photogenerated free carriers. Finally, we model the broadening of the exciton PL line width and report that the interaction is dominated by type-II band edge modulation, because of the in-plane electric field in the system. The results help understand the interaction of excitons in monolayer transition-metal dichalcogenides that will aid in controlled manipulation of excitonic properties for applications in sensing, detection, and on-chip communication.

In situ surface acoustic wave field probing in microfluidic structures using optical transmission interferometry

The generation of mechanical driving forces in fluids at the microscale can be efficiently realized using acoustic actuators. For this purpose, bulk or surface acoustic waves (SAWs) are typically excited by an electroacoustic transducer, and the acoustic energy is subsequently coupled to the fluid. The resultant acoustic pressure field in the fluid allows for precise manipulation of immersed objects and also for the agitation of the fluid itself. In general, the fluidic actuation capability is mainly determined by the mechanical displacement amplitude at the interface between the fluid and the acoustically active surface. In the case of SAW-based actuators, the fluid most often is directly attached to the substrate surface along which the surface waves propagate. Hence, the lateral distribution of surface displacement amplitude, i.e., the surface acoustic wave field, at the fluid–substrate interface is of particular interest in order to achieve full control of the fluidic actuation. Here, we present a reliable experimental method for the in situ determination of the SAW field on fluid loaded substrate surfaces based on laser interferometry. The optical accessibility of the fluid–substrate interface is realized via transmission through the anisotropic, piezoelectric substrate material requiring only an additional calibration procedure in order to compensate the parasitic influence of effects based on different indices of refraction as well as on complex acousto-optic effects. Finally, the proposed method is demonstrated to yield reliable results of displacement amplitude on the fluid loaded surface and thus, to provide a valuable insight into acoustofluidic coupling that was not available so far.

Theory of acoustophoresis in counterpropagating surface acoustic wave fields for particle separation.

Acousotophoretic particle separations in counterpropagating surface acoustic wave (SAW) fields, e.g., standing SAWs (SSAWs), phase modulated SSAWs, tilted angle SSAWs, and partial standing SAWs, have proven successful. But there still lacks analytical tools for predicting the particle trajectory and optimizing the device designs. Here, we study the acoustophoresis of spherical Rayleigh particles in counterpropagating SAW fields and find that particle motions can be characterized into two distinct modes, the drift mode and the locked mode. Through theoretical studies, we provide analytical expressions of particle trajectories in different fields and different moving patterns. Based on these, we obtain theory-based protocols for designing such SAW acoustofluidic particle separation chips, which are demonstrated through finite-element simulations. The results here provide theoretical guidelines for designing high throughput and high efficiency particle separation devices.

Multiple outcome particle manipulation using cascaded surface acoustic waves (CSAW)

On-chip fluorescent activates cell sorting (FACS) requires the optical identification of a cell’s type followed by the selective displacement of that particle from the incoming streamline, the number of displacement choices available dictate the number of different sort outcomes which are obtained. Surface acoustic waves (SAW) offer an effective way of generating a sound field which is capable of exerting forces on suspended particles and cells. The SAW couples into the fluid contained within a microfluidic channel. A single SAW source can cause a travelling wave which pushes particles in the direction of propagation, whilst a pair of sources can give rise to a standing wave which moves particles to pressure nodes. By pulsing the SAW field on, both approaches have been used to displace selected particles, either by pushing particles to the channel wall furthest along the propagation direction or moving them to the nearest pressure node. However, this work shows that multiple outcomes can be obtained by successive source pairs, each spatially offset from each other, the result is a cascaded manipulation yielding, in this demonstration, four possible outcomes.

Unraveling the Autonomous Motion of Polymer‐Based Catalytic Micromotors Under Chemical−Acoustic Hybrid Power

Artificial nano‐ and microswimmers are promising as versatile nanorobots for applications in biomedicine, environmental chemistry, and materials science. Herein, a hybrid micromotor containing a conjugated polymer (poly(3,4‐ethylenedioxythiophene) (PEDOT), and a catalytic structure composed of platinum (Pt) synthesized using a template‐supported electrochemical deposition process is reported. The movement of this PEDOT/Pt micromotor is characterized under chemical power generated by hydrogen peroxide catalysis, and acoustic power generated by surface acoustic waves (SAWs). The acoustic radiation force acting between the bubbles, the secondary Bjerknes force, is shown to increase the micromotor speed. The movement of the micromotor is precisely controllable using the acoustic field, providing excellent response time and reproducibility over a wide dynamic range. A theoretical model is developed to understand and predict the micromotor propulsion under the hybrid chemical and acoustic power. Predicted micromotor speeds are in excellent agreement with experiment as a function of peroxide fuel concentration, SAW field strength, and SAW frequency. The model allows for design of micromotor geometries and acoustic field strengths to achieve desired speed with excellent on/off control.