Interaction of surface acoustic waves with viscous liquids

A small amount of a viscous oil deposited on a high energy surface adopts a spherical cap cross-section with a dynamic contact angle that evolves following simple power laws of t(-3/10) for a droplet and t(-2/7) for a stripe. If the surface being wet forms part of a surface acoustic wave (SAW) device the rate of spreading can be monitored using changes in both the attenuation and the phase of the surface wave. This system provides a probe of both the SAW-viscous liquid interaction and the solid-liquid interaction. In this work we report simultaneous optical and SAW measurements on the spreading of poly(dimethyl)siloxane oil of viscosity 100 000 cS. Experiments using pulsed SAWs of frequency 170 MHz show an overall exponential type decrease in transmitted signal amplitude as the oil spreads. The decreasing amplitude of the SAW is also accompanied by distinct maxima and minima indicating transmission resonances. A SAW signal reflected from the stripe of oil is observed and is seen to oscillate in amplitude as the advancing front of the fluid reduces the acoustic pathlength. The optical data allows the profile of the liquid to be constructed and confirms the simple power law behaviour of the dynamic contact angle. The geometrical information on dynamic contact angle, contact width and height of fluid is correlated with the SAW signal and possible mechanisms for the transmission and reflection resonances are discussed.