Universal Wide Shear Zones in Granular Bulk Flow

Summary Granular matter exhibits an intricate mix of solid and liquid-like phenomena, some familiar, others remarkable, but almost always poorly understood [1–5]. In particular, when submitted to external stress, granular matter does not flow homogeneously like an ordinary fluid would. Instead, it forms rigid regions separated by narrow shear bands where the material yields and flows [1–7]. This shear localization may also be relevant for dense colloids, emulsions and foams [8–10], but for granular media it is ubiquitous- think of geological faults [11,12], avalanches [13,14] and silo discharges [2,15–17]. Empirically, shear bands are observed to be narrow, particle-shape dependent and often localize near a wall [1,2,6,7,11–21]. Here in contrast to this behaviour, we present experiments in which much wider and universal shear zones can be created in the bulk of the material. These shear zones exhibit Gaussian strain rate profiles, with position and width tunable by the experimental geometry and particle properties.