Oscillatory yielding of a colloidal star glass

Abstract

We investigate the response of a well-characterized colloidal star glass to large-amplitude oscillatory stress and strain fields. By combining these measurements with dynamic time sweeps we demonstrate the importance of probing both strain- and stress-induced nonlinear rheology of such complex fluids in order to elucidate the yielding and fluidization behavior. We also show that, due to the strong time dependence, it is essential to perform dynamic time sweeps at different strain and stress amplitudes, which result in different departures of the glass cage from its quiescent quasiequilibrium structure. This allows for steady-state responses to be reached and for nonlinear oscillatory responses to be treated properly while also suggesting that yielding is a gradual process. Further, we use a recently published framework for analyzing nonlinear responses to large-amplitude oscillatory shear [Rogers et al., J. Rheol. 55, 435 (2011)], based on the analysis of the whole stress waveforms as a sequence of physical processes, in order to measure the points of static and dynamic yielding. By doing so, we show that the stress-amplitude dependence of the dynamic yield stress can be linked to the strain-rate-amplitude dependence via the form of the steady-state flow curve.