Rheology of soft glassy materials

We compare the linear and non-linear rheological properties of a typical soft glassy material, the colloidal glass of Laponite, with the predictions of recently developed rheological models based on glassy dynamics, in which particles are trapped in a free-energy landscape with many minima. We find qualitative agreement between theory and experiment for the linear visco-elastic properties. However, for the non-linear response, notably the viscosity under continuous shear flow, important qualitative differences are found. This shows that a developed flow affects the dynamics of relaxation of the material in a more complicated way than what is assumed in the models: not only does the flow alter the energy landscape, but it also modifies the effective temperature controlling the escape rate for a given energy barrier.