Transducer-Plane Streaming Patterns in Thin-Layer Acoustofluidic Devices

Abstract

While classical Rayleigh streaming, whose circulations are perpendicular to the transducer radiating surfaces, is wellknown, transducer-plane streaming patterns, in which vortices circulate parallel to the surface driving the streaming, have been less widely discussed. Previously, a four-quadrant transducer-plane streaming pattern has been seen experimentally and subsequently investigated through numerical modelling. In this paper, we show that by considering higher order threedimensional cavity modes of rectangular channels in thin-layered acoustofluidic manipulation devices, a wider family of transducer-plane streaming patterns are found. As an example, we present a transducer-plane streaming pattern, which consists of eight streaming vortices with each occupying one octant of the plane parallel to the transducer radiating surfaces, which we call here eight-octant transducer-plane streaming. An idealised modal model is also presented to highlight and explore the conditions required to produce rotational patterns. It is found that both standing and travelling wave components are typically necessary for the formation of transducer-plane streaming patterns. In addition, other streaming patterns related to acoustic vortices and systems in which travelling waves dominate are explored with implications for potential applications. DOI:I. INTRODUCTION Acoustic streaming is steady fluid motion driven by the absorption of acoustic energy due to the interaction of acoustic waves with the fluid medium or its solid boundaries. Understanding the driving mechanisms of acoustic streaming patterns within acoustofluidic devices is important in order to precisely control it for the enhancement or suppression of acoustic streaming for applications such as particle/cell manipulation [1-8], heat transfer enhancement [9-12], noncontact surface cleaning [13][14][15][16][17], microfluidic mixing [18][19][20][21][22][23][24][25][26][27], and transport enhancement [28][29][30][31][32][33][34][35].In most bulk micro-acoustofluidic particle and cell manipulation systems of interest, the acoustic streaming fields are dominated by boundary-driven streaming [36], which is associated with acoustic dissipation in the viscous boundary layer [37]. Theoretical work on boundary-driven streaming was initiated by Rayleigh [38], and developed by a series of modifications for particular cases [39][40][41][42][43][44], which have paved the fundamental understanding of acoustic streaming flows.While Rayleigh streaming patterns (which have streaming vortices with components perpendicular to the driving boundaries) have been extensively studied [45][46][47][48], we have recently explored the mechanisms behind fourquadrant transducer-plane streaming [49] which generates streaming vortices in planes parallel to the driving boundary, and modal Rayleigh-like streaming [50] in which vortices have a roll size greater than the quarter wavelength of the main acoustic resonance and are driven by limiting velocities (the value of the streaming velocity just outside...

Unveiling of the mechanisms of acoustic streaming induced by sharp edges

“…Therefore, while typical cost-effective transducers and associated amplifiers are generally in a range of a few kHz to a few tens of kHz, they should in principle fail to generate AS in microchannels, as the acoustic field would then be homogeneous in space. Although a few studies could circumvent this limitation by tuning the excitation of immersed bubbles [27], by using micropillars [28] or flexural waves on a flexible wall [13], by prescribing a wavy channel geometry [29][30][31], or by tuning streaming modes within the transducer plane [32], the majority of them were carried out under ideal geometries such as infinite or semi-infinite domains. Still, remaining issues concern the influence of geometry, for instance the presence of obstacles or nonstraight profiles like constrictions, or in situations of confinement when δ can be comparable to one of the channel dimensions [25].…”

Ultrasonic Particle Manipulation in Glass Capillaries: A Concise Review

“…Generally speaking, the Reynolds stress method is more accurate as it takes into account the thin boundary layer and solves acoustic streaming from its genesis, the Reynolds stress force, i.e., the left-hand-side of Equation (9), while the limiting velocity method is more computationally efficient and is suitable for three-dimensional (3D) simulations. In the past decade, with the assistance of the limiting velocity method, a number of acoustic streaming patterns, including the classical Rayleigh-type streaming [ 28 ] and new (i.e., those that cannot be explained by classical Rayleigh streaming theory [ 29 ]) streaming such as Modal Rayleigh-like streaming [ 30 ] and transducer-plane streaming (e.g., four-quadrant [ 31 , 32 , 33 ] and eight-octant [ 34 ] patterns, which are usually seen in planar resonant devices [ 35 ]) in glass capillaries, have been modeled and elucidated through 3D simulations.…”

On‐Chip Generation of Vortical Flows for Microfluidic Centrifugation

“…The undulating boundary due to the bulk substrate vibration gives rise to a number of streaming phenomena across different length scales as a consequence of the divergence in the momentum flux. Within the viscous boundary layer of Equation , boundary layer or Rayleigh streaming arises due to velocity continuity at the solid–liquid interface; naturally, the boundary conditions as well as the confinement geometry has a large influence on the nature and intensity of the streaming vortices ( Figure ) . If the characteristic system dimension of the boundaries confining the liquid are sufficient to support the leakage of sound waves in the fluid (i.e., ), wherein λ f is the sound wavelength in the fluid at the frequency associated with that excited in the substrate, viscous dissipation and hence the attenuation of the sound wave in the bulk liquid drives a longer range flow known as Eckart streaming over length scales comparable to the attenuation length in which c represents the speed of sound in the fluid.…”