Dynamic wetting and de-wetting of thin films of water under the influence of MHz surface acoustic waves

Gennady Altshuler, Ofer Manor

Published 2015-11-04Version 1

We use both theory and experiment to study the response of partially wetting films of water and surfactant solutions to a propagating MHz vibration in the solid substrate in the form of a Rayleigh surface acoustic wave (SAW). The SAW invokes a drift of mass in the liquid film that, balanced by capillary stress, may support dynamic wetting and de-wetting along the path of the SAW. The motion of the film is governed by a non-dimensional parameter, $\theta^3/{\rm We}$, where $\theta$ is the three phase contact angle and ${\rm We}\equiv \rho U^2H/\gamma$; $\rho,~\gamma$, $H$, and $U$ are the liquid density, liquid/vapour surface tension, film thickness, and the intensity of the SAW at the solid surface, respectively. We show partially wetting films of water and surfactant solutions atop a lithium niobate substrate, for which $\theta^3/{\rm We}>1$, undergo qualitatively different dynamics to fully wetting films of silicon oil, for which $\theta^3/{\rm We}\ll1$, and further explore an intermediate parameter region where $\theta^3/{\rm We}<1$.