Kinematics - Wide shear zones in granular bulk flow

Granular matter does not flow homogeneously like a fluid when submitted to external stress, but usually forms rigid regions that are separated by narrow shear bands where the material yields and flows1,2,3,4,5,6,7,8,9,10,11,12,13 (examples include geological faults9,10,11, avalanches12 and silo discharges2,13). Shear bands are narrow (five to ten grains in diameter1,2,3,4,5,6,7,8,9,10,11,12,13) and dependent on the particle shape5, and often localize near a boundary4,5,6,7,8,12,13; they hinder mixing and make grain flows difficult to predict or describe1,2,3. Here we show that the shear zones created in the bulk of the material are wider than those near the walls, and that their bulk velocity profiles lie on a universal curve. This finding challenges the accepted picture of shear banding in granular media.