Mechanisms of two-step yielding in attractive colloidal glasses

A combination of experiments and Brownian Dynamics simulations is utilized to examine the mechanisms of yielding and flow in attractive colloidal glasses during start-up shear flow. In both experiments and simulations, the transient stress exhibits two stress peaks indicative of two-step yielding processes. The first yield depends largely on details of interparticle potential whereas the second yield is independent of the potential and takes place at strain (similar or equal to 20%), at which a purely repulsive glass yields. The stress decomposition into repulsive (hard sphere, HS) and attractive contributions reveals that there are strong contributions of both types of stresses into the first stress peak whereas the second stress peak is mainly linked with HS stresses. The transient stress during start-up shear originates from the change in the averaged pair orientation. At the first stress peak, bonded particles (causing attractive stresses) show the maximum orientation along the extension axis with colliding particles (causing HS stresses) being locally oriented along the compression axis. However, at the second stress peak, collided particles show the maximum orientation along the compression axis with particles escaping their cages along the extension axis similar to a HS glass. Analysis of particle dynamics shows that yielding takes place through a two-step shear-activated hopping process in which first shear flow takes particles out of their attractive constraints. The length scale associated to this process is at the order of attraction range (bond length). Subsequently, cage escape of particles sets the second process which leads to a complete yielding and flow. (c) 2020 The Society of Rheology.