Methodologies, technologies, and strategies for acoustic streaming-based acoustofluidics

Abstract

Acoustofluidics offers contact-free manipulation of particles and fluids, enabling their uses in various life sciences, such as for biological and medical applications. Recently, there have been extensive studies on acoustic streaming-based acoustofluidics, which are formed inside a liquid agitated by leaky surface acoustic waves (SAWs) through applying radio frequency signals to interdigital transducers (IDTs) on a piezoelectric substrate. This paper aims to describe acoustic streaming-based acoustofluidics and provide readers with an unbiased perspective to determine which IDT structural designs and techniques are most suitable for their research. This review, first, qualitatively and quantitatively introduces underlying physics of acoustic streaming. Then, it comprehensively discusses the fundamental designs of IDT technology for generating various types of acoustic streaming phenomena. Acoustic streaming-related methodologies and the corresponding biomedical applications are highlighted and discussed, according to either standing surface acoustic waves or traveling surface acoustic waves generated, and also sessile droplets or continuous fluids used. Traveling SAW-based acoustofluidics generate various physical phenomena including mixing, concentration, rotation, pumping, jetting, nebulization/atomization, and droplet generation, as well as mixing and concentration of liquid in a channel/chamber. Standing SAWs induce streaming for digital and continuous acoustofluidics, which can be used for mixing, sorting, and trapping in a channel/chamber. Key challenges, future developments, and directions for acoustic streaming-based acoustofluidics are finally discussed.

Acoustic micro‐beam vortex generator for flow actuation inside droplets

“…Then, the surface acoustic waves travel in the substrate until they make contact with the fluid in the droplet or inside the micro‐channel. The acoustic streaming‐induced drag forces on the particles are controlled by the design of the IDT, the relative position of the IDT with respect to the droplet, or by altering the boundary conditions on the substrate, for example, with reflectors or shielding structures 7 . Alternative concepts for controlling the three‐dimensional flow structures inside the droplet have been proposed.…”

2D acoustofluidic distributions in micro-chambers modulated by Sierpiński-type structural plates

“…For the time being, almost all ultrasonic micro/nano manipulation methods require the utilization of travelling/standing acoustic waves [31], focused Gaussian beams [32], and special sound beams like vortex-or Bessel-beams [33], which are commonly generated by Langevin-type energy converters [34], commercialized piezoelectric buzzers [35], or customized interdigital transducers (IDTs) [36]. Moreover, combining assorted ultrasonic waves with miniaturized lab-on-a-chip devices and integrated micro-electro-mechanical systems (MEMS) has been becoming a trend from sophisticated manufacturing to biochemical engineering [37,38].…”

Rapid acoustofluidic mixing by ultrasonic surface acoustic wave-induced acoustic streaming flow

“…Recently, acousto-microfluidic mixers have been proposed to address the limitations of previous micromixers [21] , [22] . The acoustic approaches utilized acoustic streaming flow (ASF) for mixing, which derives from the attenuation of acoustic waves leading to momentum flux within the fluid [23] . The acoustofluidic mixing techniques have been widely developed being incorporated not only with microchannel but also with sessile droplets [24] , [25] or various geometrical channels [26] , [27] for the enhancement of mixing effect and their practical applications.…”

“…The boundary streaming within the viscous boundary layer induces counter-rotating flow in the outer boundary region, consecutively resulting in periodic patterns of micro-vortices within the fluid [28] . These micro-vortices of acoustic streaming require high power consumption as the loss of acoustic energy is relatively greater near the viscous boundary, and the vortex regime is small, typically less than the acoustic wavelength, which could be unfavorable for fluid mixing [23] , [29] . On the other hand, Eckart streaming is a vortex-like flow propelled by the acoustic jet, resulting from the propagation of the acoustic wave and its acoustic pressure gradient in the fluid [30] .…”