Viscoelastic interfacial modes in a wetting layer

Brillouin scattering from an evanescent wave enables us to study the dynamic structure factor S(k,{ω}) of fluids within 1 {μ}m 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 velocity of one of these modes differs significantly from that of the corresponding bulk mode. A hydrodynamic calculation reveals that the alteration of the velocity cannot be explained in terms of continuity of the bulk transport properties over the interface. We propose that the shift in the propagation velocity is due to the viscoelastic character of the fluid-fluid interface.