Shear-induced corrugation of free interfaces in concentrated suspensions

When a concentrated suspension of small inertia-less particles in a viscous fluid is sheared between two parallel-belts a free surface exists with a direction orthogonal to the plane of the simple shear flow. We observed a development of corrugation on this surface. The corrugation appears as an ensemble of disturbances driven by shear-induced diffusion and restrained by surface forces. The roughness of the surface disturbances depends on the particle size, on particle concentration and on the fluid surface tension. The corrugation has many lengthscales above and below particle size and it appears in the form of waves traveling with the local surface fluid speed. In this communication, we present a report on the phenomenon and its dependence on the physical parameters. A qualitative description is presented and the dimensionless groups governing the extent of the shear-induced corrugation are defined. We introduce the notion of a macroscopic effective surface tension, which depends on the surface particle concentration, with possible influence on the stability of the surface flow.