Fluctuations in the flow of disordered solids and complex fluids: the ups and downs of simple models
Alexandre Nicolas (LPTMS)
Disordered solids, such as foams, concentrated emulsions and metallic glasses, are examples of complex systems made of complex entities. However, when describing or modelling their macroscopic response, most of the complexity of the entities under study can often be left aside. This applies in particular to the fluctuations in their flow. In this talk, I will present some successes and caveats associated with simple descriptions of these fluctuations.
A very broad class of materials, disordered solids span all damping regimes, from overdamped to underdamped. This fact calls for a clarification of the effect of inertia on their flow. The first part of my talk will be dedicated to showing that, macroscopically, the effect of inertia is identical to that of simple (but shear-rate-dependent) thermal fluctuatiions.
But this does not imply that any fluctuations can be modelled as thermal fluctuations. Accordingly, I will then insist on a caveat associated with these simple models, by questioning the popular description of the mechanical noise (i.e., the stress fluctuations induced local microruptures in the deformed material) as an effective activation temperature.
Finally, if time allows, I will very briefly touch upon fluctuations in a very different type of complex systems, namely, crowds of pedestrians, and the statistical fluctuations in evacuation times.