Shear banding phenomena in ultrasoft colloidal glasses

We explore the nonlinear rheological response of a soft gel formed by a crowded colloidal star polymer and focus on the occurrence of a stress plateau, marked hysteresis, and yield stress in its flow curve. With the aid of nuclear magnetic resonance velocimetry we find evidence for fluctuations in the flow behavior across the gap of the concentric cylindrical Couette device, in association with a degree of apparent slip at the inner wall. The time scale of the these fluctuations appears rapid (with respect to the measurement time per shear rate in the flow curve), on the order of tens to hundreds of milliseconds, with the speed of fluctuations associated with the flow history of the sample. Our velocity profile analysis suggests a qualitative model in which intermittent changes due to jamming/unjamming transitions occur, analogous to cage dynamics in colloidal glasses. (C) 2004 The Society of Rheology.