Surface acoustic wave resonances in the spreading of viscous fluids

Reflection and attenuation of a Rayleigh surface acoustic wave from a small stripe of viscous fluid spreading across the acoustic path have been obser ved. As the stripe spreads across the acoustic path, with an accompanying d ecrease in stripe height to conserve volume, the reflected and attenuated a coustic signals show a distinct pattern of resonances. This paper reports e xperimental results for the spreading of highly viscous poly(dimethyl)silox ane oils (100 000 cS) in the propagation path of a 169 MHz Rayleigh wave op erating in pulse mode and develops a model to explain the observed resonanc es of the transmission coefficient. Simultaneous optical observations enabl e the time evolution of the shape of the stripe to be converted into change s in geometric parameters (contact width, spherical radius, height, and con tact angle). To model the acoustic attenuation? an approach treating the fl uid as a viscoelastic fluid with a single relaxation time (Maxwell model) i s developed. This approach is able to explain the structure of the observed transmitted signal. Resonances in the transmission coefficient occur cycli cally and correspond to the stripe height matching n lambda(s)/4 where n is odd and lambda(s) is the shear wavelength in the fluid. [S0163-1829(99)087 11-1].