Active coating of a water drop by an oil film using a MHz-frequency surface acoustic wave

TL;DR

This study demonstrates the use of MHz-frequency surface acoustic waves generated by a piezoelectric actuator to actively coat water drops with a silicon oil film, highlighting potential for small-scale, low-power coating applications.

Contribution

It introduces a novel method of actively coating liquid objects using SAW-driven oil films, extending previous SAW wetting studies to dynamic coating of liquid surfaces.

Findings

SAW intensity controls oil film coverage on water drops

Oil films can traverse curved liquid surfaces

Dynamic wetting governed by acoustic, capillary, and gravitational forces

Abstract

We employ a millimeter-scale piezoelectric acoustic actuator, which generates MHz-frequency surface acoustic waves (SAWs) in a solid substrate, to actively coat a drop of water by a macroscopic film of silicon oil as a paradigm for a small scale and low power coating system. We build upon previous studies on SAW induced dynamic wetting of a solid substrate, also known as the acoustowetting phenomena, to actively drive a model low surface-energy liquid -- silicon oil -- coat a model liquid object -- a sessile drop of water. The oil film spreads along the path of the propagating SAW and comes in contact with the drop, which is placed in its path. The intensity of the SAW determines the rate and the extent to which a macroscopically thick film of oil will climb over the drop to partially or fully cover its surface. The dynamic wetting of the drop by the oil film is governed by a balance between acoustic, capillary, and gravitational contributions. Introducing a water drop as an object to be coated indicates the opportunity to coat liquid phase objects by employing SAWs and demonstrates that oil films which are actuated by SAWs may traverse curved objects and liquid surfaces.