Suspending droplets beyond the Rayleigh limit: The interplay of acoustic and gravity forces

TL;DR

This study explores how standing acoustic waves can suspend larger droplets beyond the Rayleigh limit, revealing size-dependent effects and proposing a new acoustic-based droplet sorting method.

Contribution

It demonstrates the size-dependent suspension behavior of droplets beyond the Rayleigh limit and introduces a novel acoustic power-based sorting technique.

Findings

Suspension depends on droplet size and acoustic power.

Critical acoustic power increases with droplet size.

Proposed a new droplet sorting method based on acoustic power.

Abstract

In this work, we experimentally investigate the suspension behavior of droplets subjected to standing acoustic waves. We focus on the droplet sizes beyond the Rayleigh limit, i.e., when the droplet size is comparable to the wavelength of the acoustic wave. We show that an acoustic field can disrupt the uniform motion of aqueous droplets in oil and cause them to either suspend or settle, depending on the interplay between acoustic and gravity forces. Remarkably, in contrast to droplets within the Rayleigh limit, the critical acoustic power or minimum pressure amplitude required to suspend droplets beyond the Rayleigh limit is dependent on droplet size. As the droplet size increases, the critical acosutic power increases significantly. Building upon this understanding, a novel sorting method is proposed based on critical acoustic power.