VISCOELASTIC INTERFACIAL MODES IN A WETTING LAYER

Brillouin scattering from an evanescent wave enables us to study the d ynamic structure factor S (k, omega) of fluids within 1 mum of a solid wall. Two interfacial fluid modes are found in a demixed binary-fluid system where a wetting layer of the heavy phase intrudes between the lighter phase and the solid wall. Surprisingly, the propagation veloci ty of one of these modes differs significantly from that of the corres ponding bulk mode. A hydrodynamic calculation reveals that the alterat ion of the velocity cannot be explained in terms of continuity of the bulk transport properties over the interface. We propose that the shif t in the propagation velocity is due to the viscoelastic character of the fluid-fluid interface.