Unapodization: a method to produce laterally uniform surface acoustic waves for acoustofluidics

Abstract

Surface acoustic waves (SAW) have underpinned many acoustofluidic applications. But the typical straight, full-width interdigital transducers used to generate these waves are problematic. Fresnel diffraction of the propagating SAW causes strong nonuniformity in the wave's amplitude as it propagates across the substrate, leading to flow variations, recirculation, and peculiar wetting phenomena in fluids driven by such waves. These have led to many research projects and publications, but, ultimately, uniform fluid flow from SAW-driven streaming remains elusive. By adopting a form of apodization, the shaping of the IDT, we demonstrate the ability to produce laterally uniform SAW for acoustofluidics. Unlike traditional apodization, which shapes the IDT to produce a desired electrical signal, this new approach-unapodization-instead inverts the governing equations to produce a desired spatial distribution of SAW at a defined position along the propagation direction of the SAW. Theoretical analysis and experiment demonstrate a 50% improvement in the SAW uniformity over a straight IDT for three example unapodized designs, all while maintaining the same frequency, input power, quality factor, and electromechanical coupling. The sole downside is a 50% increase in IDT area to accommodate the unapodization. A design tool is provided with tabulated unapodized IDT configurations. To illustrate the potential of unapodization in acoustofluidics, the irregular dewetting and anomalous flows of a thin fluid film induced by traditional straight IDT SAW devices is shown to become regular and uniform with the adoption of unapodized SAW devices.

A solution to the biophysical fractionation of extracellular vesicles: Acoustic Nanoscale Separation via Wave-pillar Excitation Resonance (ANSWER)

“…In this case, no significant difference is observed before and after the radio-frequency signal is applied. IDTs with a long aperture may generate nonuniform acoustic beams ( 58 , 59 ). However, the excitation resonance mechanism can ensure that the virtual wave pillars are always located in the middle of the microchannel to deflect larger particles to the channel sidewalls.…”

Acoustofluidics

“…Likewise, in many of these devices the electrodes used to generate the acoustic waves within and upon the piezoelectric substrate are typically cursory or directly derived from device designs used for telecommunications in the 1970s or even the original interdigital electrode design (White and Voltmer, 1965). However, recent work on electrode design tailored to acoustofluidics appears to offer distinct improvements in performance and capabilities, such as the remarkable multielectrode structure used to selectively trap particles described later (Gong and Baudoin, 2021), an "unapodization" approach to produce a laterally uniform acoustic wave (Zhang et al, 2021d), or even to completely remove the electrode from the substrate and using a flexible substrate simply pressed into contact with it instead (Dumčius et al, 2023).…”