Shear Banding of Soft Glassy Materials in Large Amplitude Oscillatory Shear.

We study shear banding in soft glassy materials subject to a large amplitude oscillatory shear flow (LAOS). By numerical simulations of the widely used soft glassy rheology model, supplemented by more general physical arguments, we demonstrate strong banding over an extensive range of amplitudes and frequencies of the imposed shear rate γ[over ˙](t)=γ[over ˙]_{0}cos(ωt), even in materials that do not permit banding as their steady state response to a steadily imposed shear flow γ[over ˙]=γ[over ˙]_{0}=const. Highly counterintuitively, banding persists in LAOS even in the limit of zero frequency ω→0, where one might a priori have expected a homogeneous flow response in a material that does not display banding under conditions of steadily imposed shear. We explain this finding in terms of an alternating competition within each cycle between glassy aging and flow rejuvenation. Our predictions have far-reaching implications for the flow behavior of aging yield stress fluids, suggesting a generic expectation of shear banding in flows of even arbitrarily slow time variation.