Brittle yielding in supercooled liquids below the critical temperature of mode coupling theory.

Abstract

Molecular dynamics computer simulations of a polydisperse soft-sphere model under shear are presented. The starting point for these simulations are deeply supercooled samples far below the critical temperature, Tc, of mode coupling theory. These samples are fully equilibrated with the aid of the swap Monte Carlo technique. For states below Tc, we identify a lifetime τlt that measures the time scale on which the system can be considered as an amorphous solid. The temperature dependence of τlt can be well described by an Arrhenius law. The existence of transient amorphous solid states below Tc is associated with the possibility of brittle yielding, as manifested by a sharp stress drop in the stress-strain relation and shear banding. We show that brittle yielding requires, on the one hand, low shear rates and, on the other hand, the time scale corresponding to the inverse shear rate has to be smaller or of the order of τlt. Both conditions can only be met for a large lifetime τlt, i.e., for states far below Tc.