Using surface acoustic waves to drive thin film flow over an obstacle

TL;DR

This study demonstrates how surface acoustic waves can effectively drive thin oil films over obstacles, revealing complex interactions between ultrasonic forces, capillarity, and gravity, with a validated simplified theoretical model.

Contribution

The paper introduces a new experimental paradigm for ultrasonic-driven coating and a simplified 2D model that captures key physics of oil film dynamics over obstacles.

Findings

SAWs propel oil films to climb obstacles in experiments.

Theoretical simulations qualitatively agree with experimental observations.

The model incorporates obstacle geometry into thin-film evolution equations.

Abstract

We study a new paradigm for ultrasonic driven object coating by using a model system where MHz-level surface acoustic waves (SAWs) drive the spreading of a silicone oil film atop topographical obstacles. We use experiments to show that nanometer-amplitude SAWs, propagating in the substrate of a piezoelectric actuator, propel macroscopic oil films to climb and traverse solid obstacles placed on the actuator. The oil dynamics reveal rich coupling between ultrasonic forcing, capillarity, and gravity; the balance of which determines coating success. We formulate a simplified two-dimensional theoretical model that incorporates obstacle geometry directly in the oil thin-film evolution equation, introducing a new representation of acoustic streaming in the presence of substrate height variations. Despite the simplifications inherent in the modeling, simulations show qualitative agreement with the experiments, providing evidence that the model captures the key physics.