Acoustic Streaming Field Research Articles

An experimental acoustofluidic system for analyzing boundary-driven acoustic streaming generated by flat and curved walls

While boundary-driven acoustic streaming in fluids surrounded by flat walls has been extensively studied in the literature, theoretical studies on boundary-driven acoustic streaming generated by curved walls have recently emerged. This paper aims to present a quantitative analysis of acoustic streaming fields driven by forces induced by both flat and curved walls. A semi-circular channel made of stainless steel was designed to serve as a model channel with both flat and curved boundaries. A multi-layered glass-steel-glass device, actuated by a piezoelectric transducer, was assembled for experimental characterization of boundary-driven acoustic streaming in such scenarios. First, the various standing acoustic modes in the semi-circular channel were measured through the acoustophoretic patterning of 20 µm polystyrene particles. Next, the acoustic radiation force fields and boundary-driven acoustic streaming patterns under various resonant acoustic modes were characterized through micro-particle-image-velocimetry measurements of the motion of 20 µm and 1 µm polystyrene particles, respectively. Finally, the experimental results were explained using efficient finite element simulations of acoustofluidics and acoustophoresis in a semi-circular reduced-fluid model, with a focus on analyzing the streaming velocities driven by the flat and curved walls. Both experimental and numerical results demonstrated that the ratio of streaming velocities induced by the flat wall and the curved wall in this semi-circular channel depends on the resonant acoustic modes. This research highlights the diverse boundary-driven acoustic streaming patterns that arise in irregular channels and provides a theoretical foundation for choosing strategies for shape optimization to suppress acoustic streaming in acoustofluidic devices.

Evaluating impedance boundary conditions to model interfacial dynamics in acoustofluidics

We present a numerical study to investigate the efficacy of impedance boundary conditions in capturing the interfacial dynamics of a particle subjected to an acoustic field and study the concomitant time-averaged acoustic streaming and radiation force fields. While impedance boundary conditions have been utilized to represent fluid–solid interface in acoustofluidics, such models assume the solid material to be locally reactive to the acoustic waves. However, there is a limited understanding of when this assumption holds true, raising concerns about the suitability of impedance boundary conditions. Here, we systematically investigate the applicability of impedance boundary conditions by comparing the predictions of an impedance boundary approach against a fully coupled fluid–solid model. We contrast the oscillation profiles of the fluid–solid interface predicted by the two models. We consider different scatterer materials to identify the extent to which the differences in interfacial dynamics impact the time-averaged fields and highlight the divergence within the predictions of the two models. Our findings indicate that, although impedance boundary conditions can yield qualitatively similar results to the full model in certain situations, the predictions from the two models generally differ both qualitatively and quantitatively. These results underscore the importance of exercising caution when applying these boundary conditions to model general acoustofluidic systems.

Acoustofluidic manipulation for submicron to nanoparticles.

Particles, ranging from submicron to nanometer scale, can be broadly categorized into biological and non-biological types. Submicron-to-nanoscale bioparticles include various bacteria, viruses, liposomes, and exosomes. Non-biological particles cover various inorganic, metallic, and carbon-based particles. The effective manipulation of these submicron to nanoparticles, including their separation, sorting, enrichment, assembly, trapping, and transport, is a fundamental requirement for different applications. Acoustofluidics, owing to their distinct advantages, have emerged as a potent tool for nanoparticle manipulation over the past decade. Although recent literature reviews have encapsulated the evolution of acoustofluidic technology, there is a paucity of reports specifically addressing the acoustical manipulation of submicron to nanoparticles. This article endeavors to provide a comprehensive study of this topic, delving into the principles, apparatus, and merits of acoustofluidic manipulation of submicron to nanoparticles, and discussing the state-of-the-art developments in this technology. The discourse commences with an introduction to the fundamental theory of acoustofluidic control and the forces involved in nanoparticle manipulation. Subsequently, the working mechanism of acoustofluidic manipulation of submicron to nanoparticles is dissected into two parts, dominated by the acoustic wave field and the acoustic streaming field. A critical analysis of the advantages and limitations of different acoustofluidic platforms in nanoparticles control is presented. The article concludes with a summary of the challenges acoustofluidics face in the realm of nanoparticle manipulation and analysis, and a forecast of future development prospects.

Acoustic streaming in the Cochlea resulting from periodic excitation

This paper presents a model describing acoustic streaming in the scala vestibuli and the scala tympani of the cochlea. The magnitude and spatial features of the acoustic streaming are a function of the vibration of the cochlear partition. Hence, spatial adjustment of the acoustic streaming field can be accomplished by suitable harmonic adjustment of the excitation. It is shown that the dynamic behavior of the fluid is a function of the values of two nondimensional parameters S and R. The Strouhal number S is the ratio of the typical length to the oscillatory particle displacement and the oscillatory Reynolds number R is the ratio of typical length to the oscillatory boundary layer thickness. The method of matched asymptotic expansions is used to obtain a solution that is valid in the limit as 1/S tends to zero. The solution is shown for the slow streaming R/S = O(1) and moderate streaming R/S2 = O(1) cases.

Controlled mean dynamics of red blood cells by two orthogonal ultrasonic standing waves

A computational model is developed to study the time-averaged mean dynamics of red blood cells (RBCs) driven by the time-averaged mean stress generated by two phase-shifted orthogonal ultrasonic standing waves in a viscous fluid. The cell is modelled as an ellipsoidal viscoelastic membrane enclosing the viscous fluid cytoplasm, the motion of which is described by the inclination angle of the ellipsoidal cell shape and the phase angle of the potential membrane cycle. Based on the acoustic perturbation method, the acoustic field and acoustic streaming field are solved to obtain the time-averaged mean stress, and then the temporal evolution equations of the inclination and phase angles of the cell are determined considering the torque balance and energy conservation. At a small acoustic pressure amplitude, this model reproduces the experimentally observed features of cell motion in orthogonal standing waves: the transition from steady stationary orientation to unsteady tumbling with the increase of the phase difference between the two standing waves. By turning up the acoustic pressure amplitude above a critical value, it is further predicted that the previously observed motions can be accompanied by the membrane tank-treading rotation. Observations of these motions, combined with the present computational model, can help to evaluate the mechanical properties of RBC membranes in an automated and high-throughput manner by acoustic methods.

Enhanced acoustic streaming effects via sharp-edged 3D microstructures.

Acoustofluidic micromanipulation is an important tool for biomedical research, where acoustic forces offer the ability to manipulate fluids, cells, and particles in a rapid, biocompatible, and contact-free manner. Of particular interest is the investigation of acoustically driven sharp edges, where high tip velocity magnitudes and strong acoustic potential gradients drive rapid motion. Whereas prior devices utilizing 2D sharp edges have demonstrated promise for micromanipulation activities, taking advantage of 3D structures has the potential to increase their performance and the range of manipulation activities. In this work, we investigate high-magnitude acoustic streaming fields in the vicinity of sharp-edged, sub-wavelength 3D microstructures. We numerically model and experimentally demonstrate this in fabricating parametrically configured 3D microstructures whose tip-angle and geometry influence acoustic streaming velocities and the complexity of streaming vortices, finding that the simulated and realized velocities and streaming patterns are both tunable and a function of microstructure shape. These sharp-edge interfaces hold promise for biomedical studies benefiting from precise and targeted micromanipulation.

Acoustofluidics in an equilateral triangular channel

Acoustofluidics is of great interest in many fields, such as physics, biology, chemistry, medicine, and manufacturing. Numerous research has been conducted on the basic physics of acoustofluidics in common (e.g., rectangular and circular) fluid channels. Recently, it has been reported that the resonant acoustic field in a triangular channel can be applied for rapid acoustofluidic sedimentation of microparticles. In this paper, we examine in detail the fundamentals of acoustofluidics in triangular channels and show their potential for ultrasonic particle manipulation. The variations of acoustofluidic fields, including acoustic resonances, acoustic radiation force, and outer acoustic streaming, in an equilateral triangular channel are investigated both experimentally and numerically. We develop a generalized limiting velocity solution for predicting the outer acoustic streaming fields in triangular channels of arbitrary shape. We show that it can generate both symmetric and antisymmetric acoustic resonances in triangular channels, which lead to outer acoustic streaming fields that are similar or distinct to those seen in common fluid channels. Boundary vibration-dependent asymmetric standing wave profiles can typically be excited, where spatial separation of acoustic pressure and streaming patterns is observed, which is consistent with the recent prediction of acoustofluidics in asymmetric acoustic resonances. The results demonstrate the richness of acoustic resonances and boundary-driven acoustic streaming patterns that arise in standing acoustic wave fields and open perspectives for acoustomicrofluidic applications such as heat and mass transfer enhancement and dynamic manipulation of micro- and nanoparticles.

Study of micro-scale flow characteristics under surface acoustic waves

As an effective tool for contactless manipulation of submicrometer scale objects, the controllability of acoustic streaming velocity and flow field morphology determines the accuracy of object migration and the completeness of three dimensional (3D) imaging. This paper proposes an equivalent acoustic streaming driving force model that is applicable to both two dimensional (2D) and 3D calculations and constructs a numerical method for submicrometer microsphere migration and rotation velocity in acoustic streaming. The results show that the relationship between the peripheral vortex size Lp/wc and the relative acoustic streaming velocity vas/vf satisfies Lp/wc = 0.125vas/vf0.36 under certain geometrical conditions. Reducing the spatial confinement and increasing the inter-vortex distance will increase the energy release efficiency, reduce the pressure gradient distribution and convective dissipation rates, increase the vortex intensity and radiation range, and consequently, increase the vortex characteristic size. In complex 3D vortex flow fields, suspended objects are affected by velocity distributions and exhibit motions such as cross-flow lines and rotation. For larger vortex structure sizes, full 3D imaging is more favorable due to the increased rotation speed and period of motion along the orbit of the submicrometer microspheres. This study helps us to reveal the modulation mechanism of acoustic streaming field flow characteristics, enrich the basic theory of alternating orbital motion and forces on objects in vortex structures, and provide guidance for acoustic flow-based contactless object manipulation.

Non-contact ultrasonic manipulation for targeted micro-droplet merge by using a flat–sharp cantilever

This paper demonstrates a method for non-contact manipulation by acoustic flow to achieve selective merging of different volumes of micro-droplets. An ultrasonic cantilever with a sharp and flat tip, which worked at 84 kHz, was used to generate an acoustic streaming field in the silicone oil for non-contact manipulating individual micro-droplets. The finite element method (FEM) was used to calculate the acoustic pressure distribution around the tip of the ultrasonic cantilever and the distribution pattern of acoustic flow, respectively. To validate the calculated results, the pattern of the acoustic streaming was investigated by tracing the movement path of the polystyrene (PS) micro-particles. Subsequently, the driving voltage interval and the non-contact manipulation ability of the ultrasonic cantilever without cavitation bubbles were measured, respectively. Then, the factors affecting the manipulation ability of the ultrasonic cantilever: micro-droplet volume, vertical distance between the tip of the ultrasonic cantilever and droplets, the angle of intersection and distance between the droplet and the cantilever in the horizontal direction were experimented and discussed. Based on the above study, the automatic merging of two target droplets is achieved for a pile of micro-droplets randomly placed on the polydimethylsiloxanes (PDMS).

Controllable synthesis of silver nanoparticles using an acoustofluidic micromixer based on ultrasonic vibration

Combined liquid-phase reduction method and ultrasonic vibration mixing technology, a new method for controllable synthesis of silver nanoparticles (AgNPs) using an acoustofluidic micromixer is proposed. First, the working mechanism of the acoustofluidic micromixer and the basic principle of the liquid-phase reduction method were briefly described. Then, based on the finite element theory, the simulation software COMSOL was utilized to analyze the vibration pattern and acoustic streaming field of the device. Finally, a series of AgNPs synthesis experiments were carried out using a self-made prototype. Controllable synthesis was achieved by adjusting the control parameters of the device and changing the reagent dosage. And the synthesized AgNPs were characterized by UV spectroscopy and transmission electron microscopy (TEM). Simulation and experimental results show that rapid and homogeneous mixing reaction inside a microchannel is demonstrated via the acoustic streaming phenomenon induced by the interdigital transducers (IDT). The driving voltage, reductant, and protectant concentration have significant effects on the synthesis of AgNPs. Ultrasonic vibration combines with a proper protectant dosage can effectively inhibit particle agglomeration and ensure that the synthesized AgNPs have good homogeneity and monodispersity. When the working voltage is 30–40 V and the concentration ratio of reductant to the precursor is 2:1, the quality of the synthesized AgNPs is better. The samples are approximately spherical, without a large number of agglomerates, and high monodispersity. At this point, the average particle size is about 24.4 nm and the deviation is below 5.01 nm. The excellent and fast mixing reaction performance makes the proposed acoustofluidic micromixer a promising candidate for a wide variety of applications.

Design, simulation, and experimental investigation on a novel multi-mode piezoelectric acoustofluidic device for ICF target manipulation

The Inertial Confinement Fusion (ICF) targets are hollow glass microspheres with strong viscosity, easy agglomeration, small diameter, and fragile structure. The size and morphology of the ICF target are crucial to the success of ICF experiment. To obtain qualified targets, the manual detection method and automatic detection systems are mainly employed. However, hard contact existed between the targets and the manipulation platform in both methods, which may cause target damage. To solve this issue, a novel multi-mode piezoelectric acoustofluidic manipulation device is proposed to achieve the non-contact manipulation of sub-millimeter size ICF targets during detection process. The proposed device mainly consists of a disk-shaped container and a four-transducer array. Standing and traveling vibration modes can be separately stimulated in the container when the four-transducer array is excited with a specific signal sequence. The modal simulation is first conducted to determine the dimensional parameters and required vibration modes. Furthermore, the acoustic streaming field simulation is used to verify the effectiveness of the modal simulation and interpret the manipulation mechanism. Then, the correctness of the simulation results is demonstrated through the experiments. In the experiments, the influences of the driving frequency, target diameter, and excitation voltage on the linear manipulation are investigated through an image recognition program, respectively. The target can be linearly manipulated, and has a maximum speed of 19.10 mm s−1 at 21.5 kHz. Furthermore, with the increase of the target diameter and excitation voltage, the speed of the target increases. Finally, the rotational manipulation of the targets are conducted, and the target can effectively rotate in the container at the driving frequency of 24.6 kHz. The proposed acoustofluidic manipulation device holds the merits of simple structure, low-frequency, multi-mode, and large particle manipulation ability, which may provide technical support for the detection and filter of ICF targets.

Outer Acoustic Streaming Flow Driven by Asymmetric Acoustic Resonances.

While boundary-driven acoustic streaming resulting from the interaction of sound, fluids and walls in symmetric acoustic resonances have been intensively studied in the literature, the acoustic streaming fields driven by asymmetric acoustic resonances remain largely unexplored. Here, we present a theoretical and numerical analysis of outer acoustic streaming flows generated over a fluid–solid interface above which a symmetric or asymmetric acoustic standing wave is established. The asymmetric standing wave is defined by a shift of acoustic pressure in its magnitude, i.e., , and the resulting outer acoustic streaming is analyzed using the limiting velocity method. We show that, in symmetric acoustic resonances (), on a slip-velocity boundary, the limiting velocities always drive fluids from the acoustic pressure node towards adjacent antinodes. In confined geometry where a slip-velocity condition is applied to two parallel walls, the characteristics of the obtained outer acoustic streaming replicates that of Rayleigh streaming. In an asymmetric standing wave where , however, it is found that the resulting limiting velocity node (i.e., the dividing point of limiting velocities) on the slip-velocity boundary locates at a different position to acoustic pressure node and, more importantly, is shown to be independent of , enabling spatial separation of acoustic radiation force and acoustic streaming flows. The results show the richness of boundary-driven acoustic streaming pattern variations that arise in standing wave fields and have potentials in many microfluidics applications such as acoustic streaming flow control and particle manipulation.

Ultrasonic trapping and collection of airborne particulate matter enabled by multiple acoustic streaming vortices

The capability of trapping and collecting airborne particulate matter is of great applications in the fields of environmental engineering, healthcare systems, energy engineering, and so forth. In this work, we show a facile strategy of trapping and collecting airborne particulate matter by a simple and compact ultrasonic device system. In this device, a radiation plate is bonded with a Langevin transducer for generating circular standing flexural waves (CSFWs) in the plate. Under the excitation of the CSFWs in the radiation plate, an acoustic field and an acoustic streaming field can be induced in the air gap formed by the radiation plate and a sampling plate. Through numerical simulations, we find that the multiple acoustic streaming vortices symmetric about the central axis in the air gap are responsible for trapping and collecting airborne particulate matter onto the sampling plate, while acoustic radiation force contributes little. Also, it is numerically found and experimentally verified that the resonant acoustic field and the accompanying acoustic streaming field can be tuned by varying the thickness of air gap. Through experimentation, we investigate and clarify the dependency of collection performance on parameters such as the air gap thickness and radius, sonication time, driving voltage, and the angle between the radiation plate and the sampling plate. Due to its contactless and mild handling attributes, our ultrasonic airborne particulate matter sampler can circumvent the clogging and secondary pollution issues and ensure device reusability and little damage to samples compared with other airborne particulate matter processing methods.

Dynamic measurement of the acoustic streaming time constant utilizing an optical tweezer.

The combination of a bulk acoustic wave device and an optical trap allows for studying the buildup time of the respective acoustic forces. In particular, we are interested in the time it takes to build up the acoustic radiation force and acoustic streaming. For that, we measure the trajectory of a spherical particle in an acoustic field over time. The shape of the trajectory is determined by the acoustic radiation force and by acoustic streaming, both acting on different time scales. For that, we utilize the high temporal resolution (Δt=0.8μs) of an optical trapping setup. With our experimental parameters the acoustic radiation force on the particle and the acoustic streaming field theoretically have characteristic buildup times of 1.4μs and 1.44ms, respectively. By choosing a resonance mode and a measurement position where the acoustic radiation force and acoustic streaming induced viscous drag force act in orthogonal directions, we can measure the evolution of these effects separately. Our results show that the particle is accelerated nearly instantaneously by the acoustic radiation force to a constant velocity, whereas the acceleration phase to a constant velocity by the acoustic streaming field takes significantly longer. We find that the acceleration to a constant velocity induced by streaming takes in average about 17 500 excitation periods (≈4.4ms) longer to develop than the one induced by the acoustic radiation force. This duration is about four times larger than the so-called momentum diffusion time which is used to estimate the streaming buildup. In addition, this rather large difference in time can explain why a pulsed acoustic excitation can indeed prevent acoustic streaming as it has been shown in some previous experiments.

Three dimensional acoustic tweezers with vortex streaming

Acoustic tweezers use ultrasound for contact-free manipulation of particles from millimeter to sub-micrometer scale. Particle trapping is usually associated with either radiation forces or acoustic streaming fields. Acoustic tweezers based on single-beam focused acoustic vortices have attracted considerable attention due to their selective trapping capability, but have proven difficult to use for three-dimensional (3D) trapping without a complex transducer array and significant constraints on the trapped particle properties. Here we demonstrate a 3D acoustic tweezer in fluids that uses a single transducer and combines the radiation force for trapping in two dimensions with the streaming force to provide levitation in the third dimension. The idea is demonstrated in both simulation and experiments operating at 500 kHz, and the achieved levitation force reaches three orders of magnitude larger than for previous 3D trapping. This hybrid acoustic tweezer that integrates acoustic streaming adds an additional twist to the approach and expands the range of particles that can be manipulated.

Principle analysis for the micromanipulation probe-type ultrasonic nanomotor

The micromanipulation probe-type (MMP-type) ultrasonic nanomotor (UNM) has proven to be a type of robust and versatile tool for controllable rotary driving, dynamic trapping and high-precision orientation of a single one-dimensional (1D) nano object. The manipulation functions of the MMP-type UNM are implemented in the probe-liquid-substrate (PLS) system, in which a MMP is inserted into a liquid film of nano suspension on a stationary substrate. When the MMP elliptically vibrates parallel to the substrate surface, the acoustic streaming can be engendered to rotate a single silver nanowire (AgNW) at the liquid-substrate interface. Although the experimental results have demonstrated the effectiveness and high performance of the developed UNM, there have been no simulation results and quantitative analysis method to show the details of acoustic streaming field and to perform principle analysis for the MMP-type UNM, which has hindered a deeper understanding of the underlying physical mechanisms as well as optimization for the MMP-type UNM. In this work, to deeply analyze the principle for our developed UNM, we carry out numerical investigations on the thermo-viscous acoustic field and acoustic streaming field in the PLS system of the MMP-type UNM based on the finite element method (FEM). The simulation result shows that the elliptical vibration of the MMP parallel to the substrate surface can induce the reversed acoustic streaming vortex (compared to the MMP’s vibration trajectory direction) at the liquid-substrate interface, which enables controllable rotary driving of a single AgNW. With the simulation results of acoustic streaming fields, we theoretically predict the driving angular velocities of the AgNW at the liquid-substrate interface, and verify that the quantitative simulation results agree well with the reported experimental results. Moreover, the effects of device’s parameters and working conditions such as the distance between the MMP’s tip and substrate surface, the MMP’s length and radius and the liquid film’s thickness on the acoustic streaming field and the angular velocity of the AgNW are analyzed and clarified through both simulation results and experimental verifications, and the effect of the MMP’s material is also predicted via simulations.

Numerical Simulation of Boundary-Driven Acoustic Streaming in Microfluidic Channels with Circular Cross-Sections.

While acoustic streaming patterns in microfluidic channels with rectangular cross-sections have been widely shown in the literature, boundary-driven streaming fields in non-rectangular channels have not been well studied. In this paper, a two-dimensional numerical model was developed to simulate the boundary-driven streaming fields on cross-sections of cylindrical fluid channels. Firstly, the linear acoustic pressure fields at the resonant frequencies were solved from the Helmholtz equation. Subsequently, the outer boundary-driven streaming fields in the bulk of fluid were modelled while using Nyborg’s limiting velocity method, of which the limiting velocity equations were extended to be applicable for cylindrical surfaces in this work. In particular, acoustic streaming fields in the primary (1, 0) mode were presented. The results are expected to be valuable to the study of basic physical aspects of microparticle acoustophoresis in microfluidic channels with circular cross-sections and the design of acoustofluidic devices for micromanipulation.

Diversity of 2D Acoustofluidic Fields in an Ultrasonic Cavity Generated by Multiple Vibration Sources.

Two-dimensional acoustofluidic fields in an ultrasonic chamber actuated by segmented ring-shaped vibration sources with different excitation phases are simulated by COMSOL Multiphysics. Diverse acoustic streaming patterns, including aggregation and rotational modes, can be feasibly generated by the excitation of several sessile ultrasonic sources which only vibrate along radial direction. Numerical simulation of particle trajectory driven by acoustic radiation force and streaming-induced drag force also demonstrates that micro-scale particles suspended in the acoustofluidic chamber can be trapped in the velocity potential well of fluid flow or can rotate around the cavity center with the circumferential acoustic streaming field. Preliminary investigation of simple Russian doll- or Matryoshka-type configurations (double-layer vibration sources) provide a novel method of multifarious structure design in future researches on the combination of phononic crystals and acoustic streaming fields. The implementation of multiple segmented ring-shaped vibration sources offers flexibility for the control of acoustic streaming fields in microfluidic devices for various applications. We believe that this kind of acoustofluidic design is expected to be a promising tool for the investigation of rapid microfluidic mixing on a chip and contactless rotational manipulation of biosamples, such as cells or nematodes.

Acoustic Streaming and Temperature Field in the Cavity with Isothermal and Adiabatic Boundary Conditions at the Ends

A cylindrical cavity filled with air is considered. The cavity carries out the vibration movement along its axis. Adiabatic boundary conditions are specified on the lateral surface of the cavity. The boundary conditions at the ends of the cavity are considered in two types—adiabatic and isothermal. The features of acoustic streaming and temperature field are described when the cavity radius increases and the boundary conditions at the cavity ends are changed from adiabatic to isothermal.

Standard and inverse transducer-plane streaming patterns in resonant acoustofluidic devices: Experiments and simulations

• A new transducer-plane streaming pattern is observed and characterised. • A mathematical model is developed for the modelling of transducer-plane streaming patterns. • Modelled standard and inverse transducer-plane streaming patterns are consistent with those observed in experiments. • Physics of standard and inverse transducer-plane streaming patterns have been analysed. • Mathematical model has been verified to show physics of acoustic streaming fields in other acoustofluidic systems. In this paper, we report a new transducer-plane streaming pattern, which we call here inverse four-quadrant transducer-plane streaming , in an ultrasound-excited thin-layer glass capillary device, illustrate the importance of the travelling wave component and emphasize its phase difference with the standing wave component of three-dimensional acoustic fields excited in thin-layer resonant acoustofluidic particle manipulation devices on the formation of transducer-plane streaming patterns through numerical simulations. A mathematical model was created for solving the three-dimensional acoustic streaming fields from limiting velocities derived from predefined acoustic fields, capturing both the standing wave and travelling wave components and their phase relations. This model was then successfully applied to interpret the unusually reversed in-plane streaming patterns seen in a silicon-based fluid channel. It was found that, by tuning the phase difference between the standing and travelling wave components, φ , the transducer-plane streaming vortices could be shifted along the fluid channel and reversed streaming patterns can be excited when φ differ by π, which could provide insights for the control of transducer-plane streaming in thin-layer acoustofluidic devices and may have promising potential for acoustofluidic applications such as nanoscale particle manipulation.