Ageing, shear rejuvenation and avalanches in soft glassy materials

We study the nonlinear rheological behaviour and the microscopic particle dynamics for a colloidal glass. The measurements allow us to relate the microscopic diffusion dynamics to the macroscopic viscosity of the system. We demonstrate that the competition between the spontaneous restructuration (ageing) and the destruction of the internal structure by the shear ('shear rejuvenation') leads to a bifurcation in rheological behaviour. For a stress smaller than a (time-dependent) critical value, the viscosity increases in time and the material eventually stops flowing. For slightly larger stresses the viscosity decreases continuously with time and the flow accelerates. Thus the viscosity jumps discontinuously to infinity at the critical stress.