Shear banding transition in granular materials under uniform and boundary shear

We experimentally study shear band formation in a 2D granular material subjected to quasistatic shear using Couette apparatus with a base consisting of 21 rotating concentric rings. The rings can independently be controlled to interpolate between uniform shear and boundary shear in a layer of photo-elastic disks resting on them. Previous uniform shear experiments using this setup [PhysRevLett.123.158001] showed that a steady localized shear band forms at sufficiently large strains for packing fractions φ >0.78. In the present work, we study how this transition is affected by the specific choice of the relative velocities of the rings. For φ<0.78, the flow roughly follows the basal velocity profile. For φ>0.78, we find that the steady flow profile is insensitive to the basal velocity profile, though the times needed to reach the steady flow regime are different. For φ~0.78, a localized shear band forms and dissolves irregularly in association with the formation and yielding of a strong force network.