Adhesion of yield stress fluids

The different regimes of flow when separating two solid rough surfaces in contact via a layer of a simple yield stress fluid are identified. Generic scalings for the adhesion energy and for the geometrical characteristics of the final deposits (after separation) as a function of the initial aspect ratio of the sample are found. We show that there is a strong pinning effect which might be at the origin of an adhesion energy significantly larger (by a factor about 2) than that estimated from the lubrication theory. We also observe that the conditions of development of viscous fingering are not at all predicted by the conventional Saffman–Taylor instability theory taking into account the specific non-Newtonian character of the fluid. This again suggests that for pastes the pinning effect plays a significant stabilizing role.