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 Q1. These observations shed light on the origin of compressed exponential behaviour and help in classifying previous results in the literature on anomalous dynamics.