Acoustic-Induced Atomization of a Sessile Droplet on Hydrophobic, Hydrophilic, and Hybrid Surfaces via Fully Compressible VOF Simulation

Abstract

This study numerically investigates the non-contact atomization of a sessile droplet subjected to acoustic excitation from the gaseous phase. A fully compressible two-phase geometric Volume of Fluid (VOF) solver was developed within the OpenFOAM (Open-source Field Operation and Manipulation) framework to resolve the coupled dynamics of sound-wave propagation, interface deformation, and droplet breakup. Acoustic radiation from the gas side induces surface instabilities on the droplet, leading to the ejection of smaller secondary droplets. The effects of substrate wettability—hydrophilic, hydrophobic, and hybrid (hydrophilic–hydrophobic)—and excitation frequency (200, 300, and 400 kHz (kilohertz)) were systematically examined. Results show that increasing the excitation frequency shortens surface wavelength, accelerates the onset of atomization, and decreases the atomization threshold time by approximately 26% on hybrid and 20% on hydrophilic surfaces, while remaining nearly unchanged on the hydrophobic surface. Higher excitation frequencies also produce finer atomized droplets: increasing the frequency from 200 to 400 kHz reduces the mean droplet size by about 14% on the hybrid surface, 11% on the hydrophobic surface, and 32% on the hydrophilic surface. Overall, the hybrid surface led to the production of finer droplets compared to the other two.