Aging and effective delays in colloidal glasses under shear

In this paper we analyze the changes in the microscopic dynamics of a colloidal glass submitted to an oscillatory shear. We use Multispeckle diffusing Wave Spectroscopy to monitor the transient dynamical regimes following a shear application. We show that the system displays a spontaneous aging that is amplified by a low amplitude oscillation (overaging) but stopped by a high amplitude one (rejuvenation). Intermediate amplitudes drive the system into a dynamical state that cannot be reached through spontaneous evolution. We demonstrate that theses systems present a weak long term memory and that the all observed phenomenology can be explained by a broadening of the relaxation time distribution under oscillatory strain. We discussed the similarities of our results with the behavior of spin glasses upon temperature shifts. We eventually propose a scenario based on the existence of harder and softer regions in the material.