The importance of travelling wave components in standing surface acoustic wave (SSAW) systems

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.

Acoustofluidic patterning in glass capillaries using travelling acoustic waves based on thin film flexible platform

Surface acoustic wave (SAW) technology has been widely used to manipulate microparticles and biological species, based on acoustic radiation force (ARF) and drag force induced by acoustic streaming, either by standing SAWs (SSAWs) or travelling SAWs (TSAWs). These acoustofluidic patterning functions can be achieved within a polymer chamber or a glass capillary with various cross-sections positioned along the wave propagating paths. In this paper, we demonstrated that microparticles can be aligned, patterned, and concentrated within both circular and rectangular glass capillaries using TSAWs based on a piezoelectric thin film acoustic wave platform. The glass capillary was placed at different angles along with the interdigital transducer directions. We systematically investigated effects of tilting angles and wave characteristics using numerical simulations in both circular and square shaped capillaries, and the patterning mechanisms were discussed and compared with those agitated under the SSAWs. We then experimentally verified the particle patterns within different glass capillaries using thin film ZnO SAW devices on aluminum (Al) sheets. Results show that the propagating SAWs can generate acoustic pressures and patterns in the fluid due to the diffractive effects, drag forces and ARF, as functions of the SAW device's resonant frequency and tilting angle. We demonstrated potential applications using this multiplexing, integrated, and flexible thin film-based platform, including patterning particles (1) inside multiple and successively positioned circular tubes; (2) inside a solidified hydrogel in the glass capillary; and (3) by wrapping a flexible ZnO/Al SAW device around the glass capillary.

The effect of microchannel height on the acoustophoretic motion of sub-micron particles

Acoustophoresis is an effective technique for particle manipulation. Acoustic radiation force scales with particle volume, enabling size separation. Yet, isolating sub-micron particles remains a challenge due to the acoustic streaming effect (ASE). While some studies confirmed the focusing ability of ASE, others reported continuous stirring effects. To investigate the parameters that influence ASE-induced particle motion in a microchannel, this study examined the effect of microchannel height and particle size. We employed standing surface acoustic wave (SSAW) to manipulate polystyrene particles suspended in the water-filled microchannel. The results show that ASE can direct particles as small as 0.31 µm in diameter to the centre of the streaming vortices, and increasing the channel height enhances the focusing effect. Smaller particles circulate in the streaming vortices continuously, with no movement towards the centres. We also discovered that when the channel height is at least 0.75 the fluid wavelength, particles transitioning from acoustic radiation-dominated to ASE-dominated share the same equilibrium position, which differs from the pressure nodes and the vortices' centres. The spatial distance between particles in different categories can lead to particle separation. Therefore, ASE is a potential alternative mechanism for sub-micron particle sorting when the channel height is accurately adjusted.

Thomson-Einstein's Tea Leaf Paradox Revisited: Aggregation in Rings

A distinct particle focusing spot occurs in the center of a rotating fluid, presenting an apparent paradox given the presence of particle inertia. It is recognized, however, that the presence of a secondary flow with a radial component drives this particle aggregation. In this study, we expand on the examination of this "Thomson-Einstein's tea leaf paradox" phenomenon, where we use a combined experimental and computational approach to investigate particle aggregation dynamics. We show that not only the rotational velocity, but also the vessel shape, have a significant influence on a particle's equilibrium position. We accordingly demonstrate the formation of a single focusing spot in a vessel center, as has been conclusively demonstrated elsewhere, but also the repeatable formation of stable ring-shaped particle arrangements.

From rectangular to diamond shape: on the three-dimensional and size-dependent transformation of patterns formed by single particles trapped in microfluidic acoustic tweezers

Generally, the pattern formed by individual particles trapped inside a microfluidic chamber by a two-dimensional standing acoustic wave field has been considered only the result of the acoustic radiation force. Previous studies showed that particles can be trapped at the local minima and maxima of the first-order pressure and velocity fields. Thus, either a rectangular or a diamond pattern can be formed solely depending on the particle size, when the acoustic field is unchanged, and the material properties of the particles and the fluid are fixed. In this paper, we report about the co-existence of different patterns with particles of the same size. The actual shape of the patterns depends mainly on the ratio between particle diameter and wavelength. In addition, particles were found to be trapped at locations that coincide with the position of antinodes, even though the particles have a positive acoustic contrast factor. These phenomena imply that the trapping of individual particles cannot be described by the acoustic radiation force solely. Hence, further research is required, taking the viscous drag force caused by the fluid flow induced by the acoustic streaming effect into account.

Manipulation of Particle/Cell Based on Compressibility in a Divergent Microchannel by Surface Acoustic Wave

The mechanical properties (compressibility or deformability) of cells are closely related to their death, migration, and differentiation. Accurate separation and manipulation of bioparticles based on these mechanical properties are still a challenging in the field of acoustofluidics. In this work, based on surface acoustic waves (SAW) and divergent microchannels, we developed a new method for separating and detecting particles or cells with different compressibility. The difference in acoustic radiation force (F_r) caused by compressibility are gradually amplified and accumulated by decreasing the flow velocity, and they are finally reflected in the particle migration distance. During the transverse migration process, the alternating dominance of the acoustic radiation force and the Stokes resistance force (F_s) drives the particles to create three typical migration patterns: intermittent migration, compound migration, and near-wall migration. In the present tilted SAW device, a 91% separation success rate of ∼10 μm polystyrene (PS) and polydimethylsiloxane (PDMS) particles can be achieved by optimizing the acoustic field input power and the fluid velocity. The application potential of the present divergent microchannel is validated by separating the myelogenous leukemia cell K562 and the natural killer cell NK92 that have similar densities and sizes (∼15 μm) but different compressibility. The results of this work are expected to provide valuable insights into the acoustofluidics separation and detection of the cells that are with different compressibility.

Three-dimensional modeling and experimentation of microfluidic devices driven by surface acoustic wave

Surface acoustic wave (SAW) technology is proving to be an effective tool for manipulating micro-nano particles. In this paper, we present a fully-coupled 3D model of standing SAW acoustofluidic devices for obtaining particle motion. The "improved limiting velocity method" (ILVM) was used to investigate the distribution of acoustic pressure and acoustic streaming in microchannel. The results show that the distribution of acoustic pressure and acoustic streaming on the piezoelectric substrate surface perpendicular to the acoustic wave propagation direction is inhomogeneous. The motion of micro-particles with diameters of 0.5-, 5-, and 10 μm is then simulated to investigate the interaction of acoustic radiation force and drag force caused by pressure and acoustic streaming. We demonstrate that micro and nanoparticles can move in three dimensions when acoustic radiation force and acoustic streaming interact. This result and method are critical for designing SAW microfluidic chips and controlling particle motion precisely.

Bisymmetric coherent acoustic tweezers based on modulation of surface acoustic waves for dynamic and reconfigurable cluster manipulation of particles and cells

Acoustic tweezers based on surface acoustic waves (SAWs) have raised great interest in the fields of tissue engineering, targeted therapy, and drug delivery. Generally, the complex structure and array layout design of interdigital electrodes would restrict the applications of acoustic tweezers. Here, we present a novel approach by using bisymmetric coherent acoustic tweezers to modulate the shape of acoustic pressure fields with high flexibility and accuracy. Experimental tests were conducted to perform the precise, contactless, and biocompatible cluster manipulation of polystyrene microparticles and yeast cells. Stripe, dot, quadratic lattice, hexagonal lattice, interleaved stripe, oblique stripe, and many other complex arrays were achieved by real-time modulation of amplitudes and phase relations of coherent SAWs to demonstrate the capability of the device for the cluster manipulation of particles and cells. Furthermore, rapid switching among various arrays, shape regulation, geometric parameter modulation of array units, and directional translation of microparticles and cells were implemented. This study demonstrated a favorable technique for flexible and versatile manipulation and patterning of cells and biomolecules, and it has the advantages of high manipulation accuracy and adjustability, thus it is expected to be utilized in the fields of targeted cellular assembly, biological 3D printing, and targeted release of drugs.

Acousto-Photolithography for Programmable Shape Deformation of Composite Hydrogel Sheets

Stimuli-responsive hydrogels with programmable shapes produced by defined patterns of particles are of great interest for the fabrication of small-scale soft actuators and robots. Patterning the particles in the hydrogels during fabrication generally requires external magnetic or electric fields, thus limiting the material choice for the particles. Acoustically driven particle manipulation, however, solely depends on the acoustic impedance difference between the particles and the surrounding fluid, making it a more versatile method to spatially control particles. Here, an approach is reported by combining direct acoustic force to align photothermal particles and photolithography to spatially immobilize these alignments within a temperature-responsive poly(N-isopropylacrylamide) hydrogel to trigger shape deformation under temperature change and light exposure. The spatial distribution of particles can be tuned by the power and frequency of the acoustic waves. Specifically, changing the spacing between the particle patterns and position alters the bending curvature and direction of this composite hydrogel sheet, respectively. Moreover, the orientation (i.e., relative angle) of the particle alignments with respect to the long axis of laser-cut hydrogel strips governs the bending behaviors and the subsequent shape deformation by external stimuli. This acousto-photolithography provides a means of spatiotemporal programming of the internal heterogeneity of composite polymeric systems.

Acoustofluidic cell micro-dispenser for single cell trajectory control

The precise manipulation of individual cells is a key capability for the study of single cell physiological characteristics or responses to stimuli. Currently, only large cell populations can be transferred with certainty using expensive and laborious flow cytometry platforms. However, when approaching small populations of cells, this task becomes increasingly challenging. Here, we report an effective acoustofluidic micro-dispenser, utilising surface acoustic waves (SAWs), with the ability to trap and release cells on demand, which when combined with an external valve can guide the trajectory of individual cells. We demonstrate single cell trap and release with a single cell trapping effectiveness of 74%, enabling the capability of dispensing a highly controlled amount of cells without any harmful effects. This device has the potential to be easily integrated into a wide range of analytical platforms for applications such as single cell fluorescent imaging and single cell proteomic studies.

Residue-free acoustofluidic manipulation of microparticles via removal of microchannel anechoic corner

Surface acoustic wave (SAW)-based acoustofluidics has shown significant promise to manipulate micro/nanoscale objects for biomedical applications, e.g. cell separation, enrichment, and sorting. A majority of the acoustofluidic devices utilize microchannels with rectangular cross-section where the acoustic waves propagate in the direction perpendicular to the sample flow. A region with weak acoustic wave intensity, termed microchannel anechoic corner (MAC), is formed inside a rectangular microchannel of the acoustofluidic devices where the ultrasonic waves refract into the fluid at the Rayleigh angle with respect to the normal to the substrate. Due to the absence of a strong acoustic field within the MAC, the microparticles flowing adjacent to the microchannel wall remain unaffected by a direct SAW-induced acoustic radiation force (ARF). Moreover, an acoustic streaming flow (ASF) vortex produced within the MAC pulls the particles further into the corner and away from the direct ARF influence. Therefore, a residue of particles continues to flow past the SAWs without intended deflection, causing a decrease in microparticle manipulation efficiency. In this work, we introduce a cross-type acoustofluidic device composed of a half-circular microchannel, fabricated through a thermal reflow of a positive photoresist mold, to overcome the limitations associated with rectangular microchannels, prone to the MAC formation. We investigated the effects of different microchannel cross-sectional shapes with varying contact angles on the microparticle deflection in a continuous flow and found three distinct regimes of particle deflection. By systematically removing the MAC out of the microchannel cross-section, we achieved residue-free acoustofluidic microparticle manipulation via SAW-induced ARF inside a half-circular microchannel. The proposed method was applied to efficient fluorescent coating of the microparticles in a size-selective manner without any residue particles left undeflected in the MAC.

Three-dimensional heating and patterning dynamics of particles in microscale acoustic tweezers

Acoustic tweezers facilitate a noninvasive, contactless, and label-free method for the precise manipulation of micro objects, including biological cells. Although cells are exposed to mechanical and thermal stress, acoustic tweezers are usually considered as biocompatible. Here, we present a holistic experimental approach to reveal the correlation between acoustic fields, acoustophoretic motion and heating effects of particles induced by an acoustic tweezer setup. The system is based on surface acoustic waves and was characterized by applying laser Doppler vibrometry, astigmatism particle tracking velocimetry and luminescence lifetime imaging. In situ measurements with high spatial and temporal resolution reveal a three-dimensional particle patterning coinciding with the experimentally assisted numerical result of the acoustic radiation force distribution. In addition, a considerable and rapid heating up to 55 °C depending on specific parameters was observed. Although these temperatures may be harmful to living cells, counter-measures can be found as the time scales of patterning and heating are shown to be different.

On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part I

By integrating surface acoustic waves (SAW) into microfluidic devices, microparticle systems can be fractionated precisely in flexible and easily scalable Lab-on-a-Chip platforms. The widely adopted driving mechanism behind this principle is the acoustic radiation force, which depends on the size and acoustic properties of the suspended particles. Superimposed fluid motion caused by the acoustic streaming effect can further manipulate particle trajectories and might have a negative influence on the fractionation result. A characterization of the crucial parameters that affect the pattern and scaling of the acoustically induced flow is thus essential for the design of acoustofluidic separation systems. For the first time, the fluid flow induced by pseudo-standing acoustic wave fields with a wavelength much smaller than the width of the confined microchannel is experimentally revealed in detail, using quantitative three-dimensional measurements of all three velocity components (3D3C). In Part I of this study, we focus on the fluid flow close to the center of the surface acoustic wave field, while in Part II the outer regions with strong acoustic gradients are investigated. By systematic variations of the SAW-wavelength λ_SAW and channel height H, a transition from vortex pairs extending over the entire channel width W to periodic flows resembling the pseudo-standing wave field is revealed. An adaptation of the electrical power, however, only affects the velocity scaling. Based on the experimental data, a validated numerical model was developed in which critical material parameters and boundary conditions were systematically adjusted. Considering a Navier slip length at the substrate-fluid interface, the simulations provide a strong agreement with the measured velocity data over a large frequency range and enable an energetic consideration of the first and second-order fields. Based on the results of this study, critical parameters were identified for the particle size as well as for channel height and width. Progress for the research on SAW-based separation systems is obtained not only by these findings but also by providing all experimental velocity data to allow for further developments on other sites.

On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part II

Particle separation using surface acoustic waves (SAWs) has been a focus of ongoing research for several years, leading to promising technologies based on Lab-on-a-Chip devices. In many of them, scattering effects of acoustic waves on suspended particles are utilized to manipulate their motion by means of the acoustic radiation force (F_ARF). Due to viscous damping of radiated waves within a fluid, known as the acoustic streaming effect, a superimposed fluid flow is generated, which additionally affects the trajectories of the particles by drag forces. To evaluate the influence of this acoustically induced flow on the fractionation of suspended particles, the present study gives a deep insight into the pattern and scaling of the resulting vortex structures by quantitative three-dimensional, three component (3D3C) velocity measurements. Following the analysis of translationally invariant structures at the center of a pseudo-standing surface acoustic wave (sSAW) in Part I, the focus in Part II turns to the outer regions of acoustic actuation. The impact of key parameters on the formation of the outer vortices, such as the wavelength of the SAW λ_SAW, the channel height H and electrical power P_el, is investigated with respect to the design of corresponding separation systems. As a result of large gradients in the acoustic fields, broadly extended vortices are formed, which can cause a lateral displacement of particles and are thus essential for a holistic analysis of the flow phenomena. The interaction with an externally imposed main flow reveals local recirculation regions, while the extent of the vortices is quantified based on the displacement of the main flow.

Trapping of Aqueous Droplets under Surface Acoustic Wave-Driven Streaming in Oil-Filled Microwells

Microwell arrays are ideal platforms for cell culturing, cell separation, and low-volume liquid handling. The ability to manipulate droplets in microwells could open up the opportunity for developing new biochemical assays. Here, we study the trapping of aqueous droplets in an oil-filled microwell driven by the application of nanometer amplitude vibrations called surface acoustic waves (SAW). We elucidate the dynamics of the droplet within the vortex toward the final trapping location and the physics of the trapping phenomenon using a theoretical model by considering the relevant forces. Our study revealed that the combined effect of acoustic radiation and hydrodynamic forces leads to droplet migration and trapping. We demarcate the trapping and nontrapping regimes in terms of the minimum critical input power required for the trapping of droplets of different sizes and densities. We find that the critical power varies as the square of the droplet size and is higher for a denser droplet. The effects of input power and droplet size on the trapping location and trapping time are also studied.

High-Frequency Ultrasound Boosts Bull and Human Sperm Motility

Sperm motility is a significant predictor of male fertility potential and is directly linked to fertilization success in both natural and some forms of assisted reproduction. Sperm motility can be impaired by both genetic and environmental factors, with asthenozoospermia being a common clinical presentation. Moreover, in the setting of assisted reproductive technology clinics, there is a distinct absence of effective and noninvasive technology to increase sperm motility without detriment to the sperm cells. Here, a new method is presented to boost sperm motility by increasing the intracellular rate of metabolic activity using high frequency ultrasound. An increase of 34% in curvilinear velocity (VCL), 10% in linearity, and 32% in the number of motile sperm cells is shown by rendering immotile sperm motile, after just 20 s exposure. A similar effect with an increase of 15% in VCL treating human sperm with the same setting is also identified. This cell level mechanotherapy approach causes no significant change in cell viability or DNA fragmentation index, and, as such, has the potential to be applied to encourage natural fertilization or less invasive treatment choices such as in vitro fertilization rather than intracytoplasmic injection.

Programmable motion control and trajectory manipulation of microparticles through tri-directional symmetrical acoustic tweezers

Acoustic tweezers based on travelling surface acoustic waves (TSAWs) have the potential for contactless trajectory manipulation and motion-parameter regulation of microparticles in biological and microfluidic applications. Here, we present a novel design of a tri-directional symmetrical acoustic tweezers device that enables the precise manipulation of linear, clockwise, and anticlockwise trajectories of microparticles. By switching the excitation combinations of interdigital electrodes (IDTs), various shape patterns of acoustic pressure fields can be formed to capture and steer microparticles accurately according to pre-defined trajectories. Numerical simulations and experimental tests were conducted in this study. By adjusting the input electric signals and the fluid's viscosity, the device is able to manipulate microparticles of various forms as well as brine shrimp egg cells with the accurate modulation of motion parameters. The results show that the proposed programmable design possesses low-cost, compact, non-contact, and high biocompatibility benefits, with the capacity to accurately manage microparticles in a range of motion trajectories, independent of their physical and/or chemical characteristics. Thus, our design has strong potential applications in chemical composition analysis, drug delivery, and cell assembly.

Acoustically manipulating internal structure of disk-in-sphere endoskeletal droplets

Manipulation of micro/nano particles has been well studied and demonstrated by optical, electromagnetic, and acoustic approaches, or their combinations. Manipulation of internal structure of droplet/particle is rarely explored and remains challenging due to its complicated nature. Here we demonstrated the manipulation of internal structure of disk-in-sphere endoskeletal droplets using acoustic wave. We developed a model to investigate the physical mechanisms behind this interesting phenomenon. Theoretical analysis of the acoustic interactions indicated that these assembly dynamics arise from a balance of the primary and secondary radiation forces. Additionally, the disk orientation was found to change with acoustic driving frequency, which allowed on-demand, reversible adjustment of the disk orientations with respect to the substrate. This dynamic behavior leads to unique reversible arrangements of the endoskeletal droplets and their internal architecture, which may provide an avenue for directed assembly of novel hierarchical colloidal architectures and intracellular organelles or intra-organoid structures.

Endoskeletal droplets are a class of complex colloids containing a solid internal phase cast within a liquid emulsion droplet. Here, authors show acoustic manipulation of solid disks inside liquid droplets whose orientation can be externally controlled with the frequency.

Influences of microparticle radius and microchannel height on SSAW-based acoustophoretic aggregation

The use of acoustic waves for microfluidic aggregation has become widespread in chemistry, biology and medicine. Although numerous experimental and analytical studies have been undertaken to study the acoustophoretic aggregation mechanisms, few studies have been conducted to optimise the device design. This paper presents a numerical investigation of the acoustophoresis of microparticles suspended in compressible liquid. The wall of the rectangular microchannel is made of Polydimethylsiloxane (PDMS), and Standing Surface Acoustic Waves (SSAW) are introduced into the channel from the bottom wall. First, the relative amplitude of the acoustic radiation force and the viscous drag force is evaluated for particles of different radii ranging from 0.1μm to 15μm. Only when the particle size is larger than a critical value can the particles accumulate at acoustic pressure nodes (PNs). The efficiency of the particle accumulation depends on the microchannel height, so an extensive parametric study is then undertaken to identify the optimum microchannel height. The optimum height, when normalised by the acoustic wavelength, is found to be between 0.57 and 0.82. These findings provide insights into the design of acoustophoretic devices.

Periodic Rayleigh streaming vortices and Eckart flow arising from traveling-wave-based diffractive acoustic fields

Recent studies have demonstrated that periodic time-averaged acoustic fields can be produced from traveling surface acoustic waves (SAWs) in microfluidic devices. This is caused by diffractive effects arising from a spatially limited transducer. This permits the generation of acoustic patterns evocative of those produced from standing waves, but instead with the application of a traveling wave. While acoustic pressure fields in such systems have been investigated, acoustic streaming from diffractive fields has not. In this work we examine this phenomenon and demonstrate the appearance of geometry-dependent acoustic vortices, and demonstrate that periodic, identically rotating Rayleigh streaming vortices result from the imposition of a traveling SAW. This is also characterized by a channel-spanning flow that bridges between adjacent vortices along the channel top and bottom. We find that the channel dimensions determine the types of streaming that develops; while Eckart streaming has been previously presumed to be a distinguishing feature of traveling-wave actuation, we show that Rayleigh streaming vortices also results. This has implications for microfluidic actuation, where traveling acoustic waves have applications in microscale mixing, separation, and patterning.

Valley Vortex Assisted and Topological Protected Microparticles Manipulation with Complicated 2D Patterns in a Star-like Sonic Crystal

Arranging microparticles into desired patterns, especially in a complicated pattern with a reliable and tunable manner, is challenging but highly desirable in the fields such as biomedicine and tissue engineering. To overcome these limitations, here, by using the concept of topology in acoustics, the valley vortex is utilized to manipulate particles on a large scale with complicated 2D patterns in the star-like sonic crystals at different frequencies. A topologically protected edge state is obtained at the interface of the crystals with different valley Hall phases, which shows the ability of reliable microparticles control along the sharp corner and the capability of robust particles cluster aggregation in a defective system. The results may provide intriguing resources for future microfluidic systems in a complicated and brittle environment.

Quantitative assessment of parallel acoustofluidic device

The advantage of ultrasonic fields in harmless and label-free applications intrigued researchers to develop this technology. The capability of acoustofluidic technology for medical applications has not been thoroughly analyzed and visualized. Toward efficient design, in this research, flowing fluid in a microchannel excited by acoustic waves is fully investigated. To study the behavior of acoustic streaming, the main interfering parameters such as inlet velocity, working frequency, displacement amplitude, fluid buffer material, and hybrid effect in a rectangular water-filled microchannel actuated by standing surface acoustic waves are studied. Governing equations for acoustic field and laminar flow are derived employing perturbation theory. For each set of equations, appropriate boundary conditions are applied. Results demonstrate a parallel device is capable of increasing the inlet flow for rapid operations. Frequency increment raises the acoustic streaming velocity magnitude. Displacement amplitude amplification increases the acoustic streaming velocity and helps the streaming flow dominate over the incoming flow. The qualitative analysis of the hybrid effect shows using hard walls can significantly increase the streaming power without depleting excessive energy. A combination of several effective parameters provides an energy-efficient and fully controllable device for biomedical applications such as fluid mixing and cell lysis.

Optimal design of a driver of interdigital transducers used to generate standing surface acoustic waves for cell sorting

A compact driver based on current feedback amplifiers is designed to drive interdigital transducers (IDTs) that generate standing surface acoustic waves for cell sorting. Compared with commercial RF amplifiers, this driver can be used to drive a wider range of loads without impedance matching. Furthermore, the driver works in a switch mode triggered by target cells, which significantly reduces power consumption in the system. A Butterworth-Van Dyke equivalent circuit was fabricated to study the electrical characteristics of the IDTs, and the driver was designed and optimized by circuit simulations. A cell sorter was constructed and tested experimentally to demonstrate that the driver meets sorting requirements. The driver allows the cell sorter to extract rare cells while otherwise consuming low power.

Mechanisms and Applications of Neuromodulation Using Surface Acoustic Waves-A Mini-Review

The study of neurons is fundamental for basic neuroscience research and treatment of neurological disorders. In recent years ultrasound has been increasingly recognized as a viable method to stimulate neurons. However, traditional ultrasound transducers are limited in the scope of their application by self-heating effects, limited frequency range and cavitation effects during neuromodulation. In contrast, surface acoustic wave (SAW) devices, which are producing wavemodes with increasing application in biomedical devices, generate less self-heating, are smaller and create less cavitation. SAW devices thus have the potential to address some of the drawbacks of traditional ultrasound transducers and could be implemented as miniaturized wearable or implantable devices. In this mini review, we discuss the potential mechanisms of SAW-based neuromodulation, including mechanical displacement, electromagnetic fields, thermal effects, and acoustic streaming. We also review the application of SAW actuation for neuronal stimulation, including growth and neuromodulation. Finally, we propose future directions for SAW-based neuromodulation.

High DNA integrity sperm selection using surface acoustic waves

Male infertility is a global reproductive issue, several clinical approaches have been developed to tackle it, but their effectiveness is limited by the labour-intensive and time-consuming sperm selection procedures used. Here, we present an automated, acoustic based continuous-flow method capable of selecting high quality sperm with considerably improved motility and DNA integrity compared to the initial raw bull semen. The acoustic field translates larger sperm and guides highly motile sperm across the channel width. The result is the selection of sperm with over 50% and 60% improvement in vitality and progressive motility and more than 38% improvement in DNA integrity, respectively, while providing a clinically relevant volume and selected sperm number for the performance of in vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI) by selecting over 60 000 sperm in under an hour.

Versatile biomimetic array assembly by phase modulation of coherent acoustic waves

A high-throughput cell-assembly method, with the advantages of adjustability, ease of operation, and good precision, is remarkable for artificial tissue engineering. Here, we present a scientific solution by introducing high rotational symmetrical coherent acoustic waves, in order to enable the shape and arrangement of the acoustic potential wells to be flexibly modulated, and therefore to assemble on a large area diverse biomimetic arrays on a microfluidic platform. Ring arrays, honeycomb, and many other biomimetic arrays are achieved by real-time modulation of the wave vectors and phase relation of acoustic beams from six directions. In the experiments, human umbilical vein endothelial cells (HUVECs), arranged in ring structures, tend to connect with the adjacent cells and reach confluency, thus directing the in vitro two-dimensional vascular network formation. Higher rotational symmetry of the six coherent acoustic waves provides much more flexibility and diversity for acoustic cell assembly. With the advantages of efficiency, diversity and adjustability, this acoustic chip is expected to fulfill many applications, such as in biochemistry, bioprinting and tissue engineering related research.

Exosome trapping and enrichment using a sound wave activated nano-sieve (SWANS)

Exosomes, a form of extracellular vesicle, are an important precursor in regenerative medicine. Microfluidic methods exist to capture these sub-micrometer sized objects from small quantities of sample, ideal for multiple diagnostic applications. To address the challenge of extraction from large volumes, we use the visual access offered by microfluidic techniques to probe the physical mechanisms behind a method which is compatible with future upscaling. The sound wave actuated nano-sieve uses resonant modes in a packed bed of microparticles to exert trapping forces on nanoparticles. Here, we examine the role of the microparticle size, demonstrating better performance from 15 μm particles than 7 μm particles. When applied to biological samples, we demonstrate for the first time that a packed bed of these larger particles is capable of capturing exosomes and liposomes, the captured particles being on average 20 to 40 times smaller than the pores within the trapped bed.

Diffraction-based acoustic manipulation in microchannels enables continuous particle and bacteria focusing

Acoustic fields have shown wide utility for micromanipulation, though their implementation in microfluidic devices often requires accurate alignment or highly precise channel dimensions, including in typical standing surface acoustic wave (SSAW) devices and resonant channels. In this work we investigate an approach that permits continuous microscale focusing based on diffractive acoustics, a phenomenon where a time-averaged spatially varying acoustic pressure landscape is produced by bounding a surface acoustic wave (SAW) transducer with a microchannel. By virtue of diffractive effects, this acoustic field is formed with the application of only a single travelling wave. As the field is dictated by the interplay between a propagating substrate-bound wave and a channel geometry, the pressure distribution will be identical for a given channel orientation regardless of its translation on a SAW substrate, and where small variations in channel size have no substantive effect on the pressure field magnitude or overall particle migration. Moreover, in the case of a channel with dimensions on the order of the diffractive fringe pattern spacing, the number of focusing positions will be identical for all channel orientations, with acoustic radiation forces pushing suspended particles to the channel edges. We explore this highly robust particle manipulation technique, determining two distinct sets of streaming and acoustic radiation dominant concentration positions, and show the continuous focusing of polystyrene 1 μm and 0.5 μm diameter particles and fluorescently labeled E. coli bacteria cells at flow rates exceeding those of previous microfluidic implementations for micron and submicron sized particles.

Development of a Piezoelectric Actuator for Separation and Purification of Biological Microparticles

The technique of a larger volume of microparticle continuous separation in the acoustic field is proposed in this paper. This technique has got considerable potential with regard to the development of technologies for the portable, low-cost and non-biodegradable procedures of energy-efficient separation/purification of microparticles in biological suspension. Using a disk-shaped piezo transducer-bimorph (DSPTB) mounted on the bottom of a plastic conical fluid container (CFC), the acoustic waves (AWs) in the fluid and the eigenmodes of the CFC were excited. The AWs, induced by piezo transducer in the CFC, pushed the suspension with the microparticles upwards, which accumulated in the nodal zones of the resonating CFC walls by purifying the suspension that accumulates in the anti-nodal zones. The outlets distributed in the nodal and anti-nodal zones of the conical fluid container walls resonating on the fourth eigenmode allowed the collection of enriched or purified from microparticles suspension.

A deep learning approach for designed diffraction-based acoustic patterning in microchannels

Acoustic waves can be used to accurately position cells and particles and are appropriate for this activity owing to their biocompatibility and ability to generate microscale force gradients. Such fields, however, typically take the form of only periodic one or two-dimensional grids, limiting the scope of patterning activities that can be performed. Recent work has demonstrated that the interaction between microfluidic channel walls and travelling surface acoustic waves can generate spatially variable acoustic fields, opening the possibility that the channel geometry can be used to control the pressure field that develops. In this work we utilize this approach to create novel acoustic fields. Designing the channel that results in a desired acoustic field, however, is a non-trivial task. To rapidly generate designed acoustic fields from microchannel elements we utilize a deep learning approach based on a deep neural network (DNN) that is trained on images of pre-solved acoustic fields. We use then this trained DNN to create novel microchannel architectures for designed microparticle patterning.

Massively Multiplexed Submicron Particle Patterning in Acoustically Driven Oscillating Nanocavities

Nanoacoustic fields are a promising method for particle actuation at the nanoscale, though THz frequencies are typically required to create nanoscale wavelengths. In this work, the generation of robust nanoscale force gradients is demonstrated using MHz driving frequencies via acoustic-structure interactions. A structured elastic layer at the interface between a microfluidic channel and a traveling surface acoustic wave (SAW) device results in submicron acoustic traps, each of which can trap individual submicron particles. The acoustically driven deformation of nanocavities gives rise to time-averaged acoustic fields which direct suspended particles toward, and trap them within, the nanocavities. The use of SAWs permits massively multiplexed particle manipulation with deterministic patterning at the single-particle level. In this work, 300 nm diameter particles are acoustically trapped in 500 nm diameter cavities using traveling SAWs with wavelengths in the range of 20-80 µm with one particle per cavity. On-demand generation of nanoscale acoustic force gradients has wide applications in nanoparticle manipulation, including bioparticle enrichment and enhanced catalytic reactions for industrial applications.

On-demand sample injection: combining acoustic actuation with a tear-drop shaped nozzle to generate droplets with precise spatial and temporal control

An on-demand droplet injection method for controlled delivery of nanolitre-volume liquid samples to scientific instruments for subsequent analysis is presented. We employ pulsed focussed surface acoustic waves (SAW) to eject droplets from an enclosed microfluidic channel into an open environment. The 3D position of individual droplets and their time of arrival can be precisely controlled to within 61 μs in a 500 μm square target region 40 μm wide. The continuous ejection rate of 16 000 droplets per second can be tuned to produce pulsed trains of droplets from 0 up to 357 Hz. The main benefit of this technique is its ease of integration with complex microfluidic processing steps, such as droplet merging, sorting, and encapsulation, prior to sample delivery. With its ability to precisely deliver a small quantity of fluid to a pre-defined location this technology is applicable in X-ray based molecular studies, including the rapidly expanding field of X-ray free electron lasers. Fabrication procedures for this device, the underlying forcing mechanism, the role of nozzle design, and demonstration of the performance in both continuous and on-demand modes are reported.

Cell Adhesion, Morphology, and Metabolism Variation via Acoustic Exposure within Microfluidic Cell Handling Systems

Acoustic fields are capable of manipulating biological samples contained within the enclosed and highly controlled environment of a microfluidic chip in a versatile manner. The use of acoustic streaming to alter fluid flows and radiation forces to control cell locations has important clinical and life science applications. While there have been significant advances in the fundamental implementation of these acoustic mechanisms, there is a considerable lack of understanding of the associated biological effects on cells. Typically a single, simple viability assay is used to demonstrate a high proportion of living cells. However, the findings of this study demonstrate that acoustic exposure can inhibit cell attachment, decrease cell spreading, and most intriguingly increase cellular metabolic activity, all without any impact upon viability rates. This has important implications by showing that mortality studies alone are inadequate for the assessment of biocompatibility, but further demonstrates that physical manipulation of cells can also be used to influence their biological activity.

Acoustic fields and microfluidic patterning around embedded micro-structures subject to surface acoustic waves

Recent research has shown that interactions between acoustic waves and microfluidic channels can generate microscale interference patterns with the application of a traveling surface acoustic wave (SAW), effectively creating standing wave patterns with a traveling wave. Forces arising from this interference can be utilized for precise manipulation of micron-sized particles and biological cells. The patterns that have been produced with this method, however, have been limited to straight lines and grids from flat channel walls, and where the spacing resulting from this interference has not previously been comprehensively explored. In this work we examine the interaction between both straight and curved channel interfaces with a SAW to derive geometrically deduced analytical models. These models predict the acoustic force-field periodicity near a channel interface as a function of its orientation to an underlying SAW, and are validated with experimental and simulation results. Notably, the spacing is larger for flat walls than for curved ones and is dependent on the ratio of sound speeds in the substrate and fluid. Generating these force-field gradients with only travelling waves has wide applications in acoustofluidic systems, where channel interfaces can potentially support a range of patterning, concentration, focusing and separation activities by creating locally defined acoustic forces.

Acoustomicrofluidic separation of tardigrades from raw cultures for sample preparation

Tardigrades are microscopic animals widely known for their ability to survive in extreme conditions. They are the focus of current research in the fields of taxonomy, biogeography, genomics, proteomics, development, space biology, evolution and ecology. Tardigrades, such as Hypsibius exemplaris, are being advocated as a next-generation model organism for genomic and developmental studies. The raw culture of H. exemplaris usually contains tardigrades themselves, their eggs, faeces and algal food. Experimentation with tardigrades often requires the demanding and laborious separation of tardigrades from raw samples to prepare pure and contamination-free tardigrade samples. In this paper, we propose a two-step acoustomicrofluidic separation method to isolate tardigrades from raw samples. In the first step, a passive microfluidic filter composed of an array of traps is used to remove large algal clusters in the raw sample. In the second step, a surface acoustic wave-based active microfluidic separation device is used to deflect tardigrades continuously from their original streamlines inside the microchannel and thus isolate them selectively from algae and eggs. The experimental results demonstrated the efficient separation of tardigrades, with a recovery rate of 96% and an impurity of 4% algae on average in a continuous, contactless, automated, rapid and biocompatible manner.

Standing Surface Acoustic Wave-Assisted Fabrication of Region-Selective Microstructures via User-Defined Waveguides

Polymer-based substrate with region-selective microstructures are crucial for many biomedical applications. Here, we explored a novel method based on standing surface acoustic waves (SSAWs) for the fabrication of localized polymer-based microstructures via a predefined waveguide. When the SSAWs are excited, the generated acoustic pressure field can be controlled in a predetermined region of the fluid surface through controlling the size and shape of the waveguide geometry. On the basis of the capillary wave motion, the generated acoustic pressure field can excite microwavy patterned structures on the surface. Then with use of ultraviolet (UV) solidification, the polymer-based substrates with region-selective patterned microstructures can be successfully fabricated. Both finite element modeling and experimental studies demonstrated that the polymer substrate with different region-selective microstructures can be achieved by selecting the pairs of interdigital transducers (IDTs) and shapes of the predefined waveguides. The results showed that the proposed method is effective for fabricating polymer-based substrate with region-selective microstructures and may have potential in cell-laden chips for tissue engineering, cell-cell interactions, and other biomedical applications.

Numerical Determination of the Secondary Acoustic Radiation Force on a Small Sphere in a Plane Standing Wave Field

Two numerical methods based on the Finite Element Method are presented for calculating the secondary acoustic radiation force between interacting spherical particles. The first model only considers the acoustic waves scattering off a single particle, while the second model includes re-scattering effects between the two interacting spheres. The 2D axisymmetric simplified model combines the Gor’kov potential approach with acoustic simulations to find the interacting forces between two small compressible spheres in an inviscid fluid. The second model is based on 3D simulations of the acoustic field and uses the tensor integral method for direct calculation of the force. The results obtained by both models are compared with analytical equations, showing good agreement between them. The 2D and 3D models take, respectively, seconds and tens of seconds to achieve a convergence error of less than 1%. In comparison with previous models, the numerical methods presented herein can be easily implemented in commercial Finite Element software packages, where surface integrals are available, making it a suitable tool for investigating interparticle forces in acoustic manipulation devices.

Tailoring surface acoustic wave atomisation for cryo-electron microscopy sample preparation

Surface acoustic wave (SAW) atomisation has been widely explored for use in pharmacological delivery, hence performance is characterised predominately in terms of droplet size and maximum delivery of fluid, to ensure sufficient dosage is delivered to the right location. For the application of cryo electron microscopy grid preparation, however, what is required is the transfer of very little fluid onto the grid in a well-defined manner. To meet this requirement, the analysis of SAW atomisation needs to focus on very different characteristics. Specifically, we examine the aerosol jet geometry, in terms of width, cone angle, and elevation angle, and its stability at low power, and hence low flow rates. The variables used are the width and the location of the channel delivering the fluid to the site of atomization. From the experiments, it is observed that we can reach a flowrate as low as 0.55 μl s-1 with reasonable aerosol jet stability, a jet width of 0.5 mm wide and an elevation angle variation as low as 2°.

On-chip cell mechanophenotyping using phase modulated surface acoustic wave

A surface acoustic wave (SAW) microfluidic chip was designed to measure the compressibility of cells and to differentiate cell mechanophenotypes. Polystyrene microbeads and poly(methylmethacrylate) (PMMA) microbeads were first tested in order to calibrate and validate the acoustic field. We observed the prefocused microbeads being pushed into the new pressure node upon phase shift. The captured trajectory matched well with the equation describing acoustic radiation force. The compressibility of polystyrene microbeads and that of PMMA microbeads was calculated, respectively, by fitting the trajectory from the experiment and that simulated by the equation across a range of compressibility values. Following, A549 human alveolar basal epithelial cells (A549 cells), human airway smooth muscle (HASM) cells, and MCF-7 breast cancer cells were tested using the same procedure. The compressibility of each cell from the three cell types was measured also by fitting trajectories between the experiment and that from the equation; the size was measured by image analysis. A549 cells were more compressible than HASM and MCF-7 cells; HASM cells could be further distinguished from MCF-7 cells by cell size. In addition, MCF-7 cells were treated by colchicine and 2-methoxyestradiol to disrupt the cell microtubules and were found to be more compressible. Computer simulation was also carried out to investigate the effect of cell compressibility and cell size due to acoustic radiation force to examine the sensitivity of the measurement. The SAW microfluidic method is capable of differentiating cell types or cells under different conditions based on the cell compressibility and the cell size.

Versatile platform for performing protocols on a chip utilizing surface acoustic wave (SAW) driven mixing

We present and demonstrate a dextrous microfluidic device which features a reaction chamber with volume flexibility. This feature is critical for developing protocols directly on chip when the exact reaction is not yet defined, enabling bio/chemical reactions on chip to be performed without volumetric restrictions. This is achieved by the integration of single layer valves (for reagent dispensing) and surface acoustic wave excitation (for rapid reagent mixing). We show that a single layer valve can control the delivery of fluid into, an initially air-filled, mixing chamber. This chamber arrangement offers flexibility in the relative volume of reagents used, and so offers the capability to not only conduct, but also develop protocols on a chip. To enable this potential, we have integrated a SAW based mixer into the system, and characterised its mixing time based on frequency and power of excitation. Numerical simulations on the streaming pattern inside the chamber were conducted to probe the underlying physics of the experimental system. To demonstrate the on-chip protocol capability, the system was utilised to perform protein crystallization. Furthermore, the effect of rapid mixing, results in a significant increase in crystal size uniformity.

Acoustophoretic Control of Microparticle Transport Using Dual-Wavelength Surface Acoustic Wave Devices

We present a numerical and experimental study of acoustophoretic manipulation in a microfluidic channel using dual-wavelength standing surface acoustic waves (SSAWs) to transport microparticles into different outlets. The SSAW fields were excited by interdigital transducers (IDTs) composed of two different pitches connected in parallel and series on a lithium niobate substrate such that it yielded spatially superimposed and separated dual-wavelength SSAWs, respectively. SSAWs of a singltablee target wavelength can be efficiently excited by giving an RF voltage of frequency determined by the ratio of the velocity of the SAW to the target IDT pitch (i.e., f = c_SAW/p). However, the two-pitch IDTs with similar pitches excite, less efficiently, non-target SSAWs with the wavelength associated with the non-target pitch in addition to target SSAWs by giving the target single-frequency RF voltage. As a result, dual-wavelength SSAWs can be formed. Simulated results revealed variations of acoustic pressure fields induced by the dual-wavelength SSAWs and corresponding influences on the particle motion. The acoustic radiation force in the acoustic pressure field was calculated to pinpoint zero-force positions and simulate particle motion trajectories. Then, dual-wavelength SSAW acoustofluidic devices were fabricated in accordance with the simulation results to experimentally demonstrate switching of SSAW fields as a means of transporting particles. The effects of non-target SSAWs on pre-actuating particles were predicted and observed. The study provides the design considerations needed for the fabrication of acoustofluidic devices with IDT-excited multi-wavelength SSAWs for acoustophoresis of microparticles.

The size dependant behaviour of particles driven by a travelling surface acoustic wave (TSAW)

The use of travelling surface acoustic waves (TSAW) in a microfluidic system provides a powerful tool for the manipulation of particles and cells. In a TSAW driven system, acoustophoretic effects can cause suspended micro-objects to display three distinct responses: (1) swirling, driven by acoustic streaming forces, (2) migration, driven by acoustic radiation forces and (3) patterning in a spatially periodic manner, resulting from diffraction effects. Whilst the first two phenomena have been widely discussed in the literature, the periodic patterning induced by TSAW has only recently been reported and is yet to be fully elucidated. In particular, more in-depth understanding of the size-dependant nature of this effect and the factors involved are required. Herein, we present an experimental and numerical study of the transition in acoustophoretic behaviour of particles influenced by relative dominance of these three mechanisms and characterise it based on particle diameter, channel height, frequency and intensity of the TSAW driven microfluidic system. This study will enable better understanding of the performance of TSAW sorters and allow the development of TSAW systems for particle collection and patterning.

Microparticle Manipulation by Standing Surface Acoustic Waves with Dual-frequency Excitations

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip (LOC) system. The simultaneous excitation of the fundamental frequency and its third harmonic, which is termed as dual-frequency excitation, to a pair of interdigital transducers (IDTs) could generate a new type of standing acoustic waves in a microfluidic channel. Varying the power and the phase in the dual-frequency excitation signals results in a reconfigurable field of the acoustic radiation force applied to the microparticles across the microchannel (e.g., the number and location of the pressure nodes and the microparticle concentrations at the corresponding pressure nodes). This article demonstrates that the motion time of the microparticle to only one pressure node can be reduced ~2-fold at the power ratio of the fundamental frequency greater than ~90%. In contrast, there are three pressure nodes in the microchannel if less than this threshold. Furthermore, adjusting the initial phase between the fundamental frequency and the third harmonic results in different motion rates of the three SSAW pressure nodes, as well as in the percentage of microparticles at each pressure node in the microchannel. There is a good agreement between the experimental observation and the numerical predictions. This novel excitation method can easily and non-invasively integrate into the LOC system, with a wide tenability and only a few changes to the experimental set-up.

Surface acoustic wave diffraction driven mechanisms in microfluidic systems

Acoustic forces arising from high-frequency surface acoustic waves (SAW) underpin an exciting range of promising techniques for non-contact manipulation of fluid and objects at micron scale. Despite increasing significance of SAW-driven technologies in microfluidics, the understanding of a broad range of phenomena occurring within an individual SAW system is limited. Acoustic effects including streaming and radiation force fields are often assumed to result from wave propagation in a simple planar fashion. The propagation patterns of a single SAW emanating from a finite-width source, however, cause a far richer range of physical effects. In this work, we seek a better understanding of the various effects arising from the incidence of a finite-width SAW beam propagating into a quiescent fluid. Through numerical and experimental verification, we present five distinct mechanisms within an individual system. These cause fluid swirling in two orthogonal planes, and particle trapping in two directions, as well as migration of particles in the direction of wave propagation. For a range of IDT aperture and channel dimensions, the relative importance of these mechanisms is evaluated.

Delivery of femtolitre droplets using surface acoustic wave based atomisation for cryo-EM grid preparation

Cryo-Electron Microscopy (cryo-EM) has become an invaluable tool for structural biology. Over the past decade, the advent of direct electron detectors and automated data acquisition has established cryo-EM as a central method in structural biology. However, challenges remain in the reliable and efficient preparation of samples in a manner which is compatible with high time resolution. The delivery of sample onto the grid is recognized as a critical step in the workflow as it is a source of variability and loss of material due to the blotting which is usually required. Here, we present a method for sample delivery and plunge freezing based on the use of Surface Acoustic Waves to deploy 6-8 µm droplets to the EM grid. This method minimises the sample dead volume and ensures vitrification within 52.6 ms from the moment the sample leaves the microfluidics chip. We demonstrate a working protocol to minimize the atomised volume and apply it to plunge freeze three different samples and provide proof that no damage occurs due to the interaction between the sample and the acoustic waves.

A rapid and meshless analytical model of acoustofluidic pressure fields for waveguide design

Acoustofluidics has a strong pedigree in microscale manipulation, with particle and cell separation and patterning arising from acoustic pressure gradients. Acoustic waveguides are a promising candidate for localizing force fields in microfluidic devices, for which computational modelling is an important design tool. Meshed finite element analysis is a popular approach for this, yet its computation time increases rapidly when complex geometries are used, limiting its usefulness. Here, we present an analytical model of the acoustic pressure field in a microchannel arising from a surface acoustic wave (SAW) boundary condition that computes in milliseconds and provide the simulation code in the supplementary material. Unlike finite element analysis, the computation time of our model is independent of microchannel or waveguide shape, making it ideal for designing and optimising microscale waveguide structures. We provide experimental validation of our model with cases including near-field acoustic patterning of microparticles from a travelling SAW and two-dimensional patterning from a standing SAW and explore the design of waveguides for localised particle or cell capture.

Acoustofluidic particle dynamics: Beyond the Rayleigh limit

In this work a numerical model to calculate the trajectories of multiple acoustically and hydrodynamically interacting spherical particles is presented. The acoustic forces are calculated by solving the fully coupled three-dimensional scattering problem using finite element software. The method is not restricted to single re-scattering events, mono- and dipole radiation, and long wavelengths with respect to the particle diameter, thus expanding current models. High frequency surface acoustic waves have been used in the one cell per well technology to focus individual cells in a two-dimensional wave-field. Sometimes the cells started forming clumps and it was not possible to focus on individual cells. Due to a lack of existing theory, this could not be fully investigated. Here, the authors use the full dynamic simulations to identify limiting factors of the one-cell-per-well technology. At first, the authors demonstrate good agreement of the numerical model with analytical results in the Rayleigh limiting case. A frequency dependent stability exchange between the pressure and velocity was then demonstrated. The numerical formulation presented in this work is relatively general and can be used for a multitude of different high frequency applications. It is a powerful tool in the analysis of microscale acoustofluidic devices and processes.