The mechanics of yield stress fluids: similarities, specificities and open questions

A wide range of materials encountered in our everyday life, such as clay suspensions, foams, concentrated emulsions, cement pastes, paints, glues, purees, creams, can flow like simple liquids under certain conditions and behave like solids under other conditions. This is the specificity of yield stress fluids which makes them so useful in various applications. In their “liquid regime” these materials exhibit typical flow properties of simple fluids such as a transition to turbulence, the roll wave instability, the hydraulic jump, etc. The specific properties occur when the solid regime is involved, either in a part of the material or as a whole. In that case one may for example observe plug flow, flow stoppage over steep slopes, no sedimentation of dense particles, cylindrical drips, Saffman-Taylor instability at vanishing velocity, etc. In addition yield stress fluids are often thixotropic, i.e. their viscosity may vary in time. The physical origin of this phenomenon and the mechanical model appropriate for describing it remain the most challenging aspects of these fluids.