Professor Philippe Coussot, Université Paris-Est

A wide range of materials (emulsions, gels, foams, suspensions, etc) are yield stress fluids, deforming only in a finite way below some critical stress, but flowing indefinitely, like liquids, beyond this stress. Here we focus on the mechanical behavior of these materials in the solid regime and on the characteristics of the solid-liquid transition.

We first show that for a set of materials (emulsions, gels, colloids) whose behavior is governed by the jamming of the suspended elements and their interactions, the solid state of the material is associated with a persistent (basic) elastic network of constant elastic modulus up to yielding, while progressively more additional elastoplastic elements are involved. The main features of these elastoplastic elements, i.e. increase of both the plastic and the elastic deformation components with the square of the shear stress, reveal the fundamental characteristics of a simple generic model, i.e. independent of material structure, describing the different components of the mechanical behavior of such systems in their solid regime. The solid-liquid transition can then be associated with the breakage (collapse) of the basic elastic network occurring for a (relatively large) critical deformation. The existence of a single model for various soft-jammed structures suggests a generalization to standard solids.

Another type of yield stress fluids behaves in a completely different way. Thanks to the existence of a continuous (percolating) network of weak interparticle bonds in a liquid, wax suspensions, which may be considered as model materials of waxy crude oils, behave as “soft breakable (brittle) solids”. Under the action of either a large stress over a short time or oscillating low stress (fatigue test), the initially solid network of these materials is broken and dispersed in the liquid, which makes them turn abruptly (collapse) and irreversibly to a low viscous fluid. In that case, this collapse, which occurs for a very low deformation, is more dramatic as the concentration increases. This suggests that such materials might be used as model systems to simulate and explain natural catastrophic events such as landslides and avalanches.