Aging and flow in a complex fluid

The comprehension of the physical mechanism governing the interaction between aging dynamics and shear flow is crucial to both elucidating the nature of slow dynamics in soft materials and controlling their complex rheological behavior. Dynamic light scattering is used to probe the relaxation of the intermediate scattering function of an aging colloidal suspension and the effect of shear on the nonequilibrium structural dynamics is investigated during various protocols of applied shear. The shear flow influences significantly the aging dynamics as soon as structural relaxation enters the timescale set by the inverse shear rate. Aging is strongly reduced in this shear dominated regime, while, for a fixed system age, the average structural relaxation time scales as the inverse shear rate. The fast component in the relaxation function, usually interpreted as the single particle diffusion, shows a cross-over behavior as well, suggesting a close link between slow and fast dynamics. Shear rejuvenation of old samples is also observed leading to a faster relaxation dynamics while the subsequent aging proceeds in an apparently new regime. (c) 2006 Elsevier B.V. All rights reserved.