Complex oscillatory yielding in model hard sphere glasses

The yielding behaviour of hard sphere glasses under large amplitude oscillatory shear has been studied by experimental rheology and Brownian Dynamics simulations. Here we focus on varying the frequency of oscillation probing the interplay between Brownian motion and shear-induced diffusion. Stress, structure and dynamics are followed by rheology and BD simulations Two frequency regimes are revealed: At low frequencies Brownian motion is dominant, assisting particles to escape their cage during a single step yielding process, identified by a peak of G'' at around the G'=G'' crossover. At high frequencies shear induced particle collisions causes cage breaking with the maximum energy dissipation marked by the G'' peak taking place beyond the G'=G'' crossover. Intermediate frequencies present a complex yielding behaviour influenced by both mechanisms that leads to a double peak in G'' that has not been reported before in HS glasses. The nonlinear response is quantified by the higher harmonics present in the stress signal. While at low frequencies the strength of higher harmonics reaches a constant value at high strains, in the intermediate and high frequency regime a non-monotonic behaviour is detected with characteristic large amplitude strains exhibiting apparent harmonic response.