# Inhomogeneous Fluid Motion Induced by Standing Surface Acoustic Wave (SAW): A Finite Element Study

## TL;DR

This paper explores how surface acoustic waves affect fluid movement in microchannels, revealing that they can mix fluids quickly but cannot stably relocate them.

## Contribution

The study presents the first finite element analysis of SAW-driven inhomogeneous fluid motion in microchannels.

## Key findings

- Standing SAW fields fail to achieve stable fluid relocation due to vertical pressure stratification and rapid floor-level streaming.
- SAW-induced transverse folding flows enable rapid homogenization of fluids in microchannels.
- The study highlights fundamental differences between BAW and SAW actuation in microfluidic systems.

## Abstract

Acoustofluidics has emerged as a transformative technology for contact-free manipulation of microparticles and fluids in microscale systems. Although bulk acoustic waves (BAWs) are known to displace inhomogeneous fluids through acoustic radiation force acting at fluid interfaces, the capability of surface acoustic waves (SAWs) to produce analogous relocation phenomena remains largely unexplored. This study addresses a critical gap in acoustofluidic theory by presenting the first comprehensive finite element method investigation of SAW-driven motion of inhomogeneous fluid confined within microchannels of widths equal to one full or one-half SAW wavelength. Unlike BAW-based system that generate uniform pressure fields across channel heights, SAW devices exhibit inherently nonuniform vertical pressure distributions and intense near-boundary streaming—features that fundamentally alter fluid relocation dynamics. Our simulations demonstrate that despite high-frequency operation (6.65 MHz) and strong ARF, standing SAW fields fail to achieve stable fluid relocation in both initially stable and unstable configurations due to vertical pressure stratification and rapid floor-level streaming. Nevertheless, these same characteristics generate vigorous transverse folding flows that enable exceptionally rapid homogenization, offering a distinct acoustofluidic mechanism for on-chip mixing. These findings not only elucidate fundamental physical differences between BAW and SAW actuation in multiphase microfluidic systems but also establish design principles for SAW-induced microfluidic mixers. The results provide crucial theoretical guidance for device optimization where rapid homogenization is desired over stable stratification.