Dichotomic aging behaviour in a colloidal glass

An unexpected dichotomic long time aging behaviour is observed in a glassy colloidal clay suspension investigated by X-ray photon correlation spectroscopy and dynamic light scattering. In the long time aging regime the intensity autocorrelations are non-exponential, following the Kohlrausch–Williams–Watts functional form with an exponent bQ. We show that for spontaneously aged samples a stretched behaviour (bQ < 1) is always found. Surprisingly a compressed exponent (bQ > 1) appears only when the system is rejuvenated by application of a shear field. In both cases the relaxation times scale as Q 1. These observations shed light on the origin of compressed exponential behaviour and help in classifying previous results in the literature on anomalous dynamics.