INTERACTION OF SURFACE ACOUSTIC-WAVES WITH VISCOUS-LIQUIDS

A small amount of a viscous oil deposited on a high energy surface ado pts a spherical cap cross-section with a dynamic contact angle that ev olves 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 acous tic wave (SAW) device the rate of spreading can be monitored using cha nges in both the attenuation and the phase of the surface wave. This s ystem provides a probe of both the SAW-viscous liquid interaction and the solid-liquid interaction. In this work we report simultaneous opti cal and SAW measurements on the spreading of poly(dimethyl)siloxane oi l of viscosity 100 000 cS. Experiments using pulsed SAWs of frequency 170 MHz show an overall exponential type decrease in transmitted signa l amplitude as the oil spreads. The decreasing amplitude of the SAW is also accompanied by distinct maxima and minima indicating transmissio n resonances. A SAW signal reflected from the stripe of oil is observe d and is seen to oscillate in amplitude as the advancing front of the fluid reduces the acoustic pathlength. The optical data allows the pro file of the liquid to be constructed and confirms the simple power law behaviour of the dynamic contact angle. The geometrical information o n dynamic contact angle, contact width and height of fluid is correlat ed with the SAW signal and possible mechanisms for the transmission an d reflection resonances are discussed.