Synchrotron X-ray phase-contrast imaging of ultrasonic drop atomization

TL;DR

This study uses synchrotron X-ray phase-contrast imaging to analyze ultrasonic drop atomization, revealing how fluid properties, structure interactions, and cavitation influence droplet size and ejection dynamics.

Contribution

It provides a detailed understanding of the physical mechanisms governing ultrasonic atomization, including the roles of fluid-structure interaction and cavitation, with experimental validation.

Findings

Droplet sizes can be predicted by linear Navier-Stokes equations.

Fluid-structure interaction influences ejection onset.

Cavitation accelerates ejection but reduces size control.

Abstract

Ultrasonic atomization is employed to generate size-controllable droplets for a variety of applications. Here, we minimize the number of parameters dictating the process by studying the atomization of a single drop pending from an ultrasonic horn. Spatiotemporally resolved X-ray phase-contrast imaging measurements show that the number-median sizes of the ejected droplets can be predicted by the linear Navier-Stokes equations, signifying that the size distribution is controlled by the fluid properties and the driving frequency. Experiments with larger pendant water drops indicate that the fluid-structure interaction plays a pivotal role in determining the ejection onset of the pendant drop. The atomization of viscoelastic drops is dictated by extended ligament formation, entrainment of air, and ejection of drop-encapsulated bubbles. Existing scaling laws are used to explain the required higher input amplitudes for the complete atomization of viscoelastic drops as compared to inviscid drops. Finally, we elucidate the differences between capillary wave-based and cavitation-based atomization and show that inducing cavitation and strong bubble oscillations quickens the onset of daughter drop ejection but impedes their size control.