Nonlocal rheology of a dense granular flow in annular shear experiments

The flow of dense granular materials at low inertial numbers cannot be fully characterized by local rheological models; several nonlocal rheologies have recently been developed to address these shortcomings. To test the efficacy of these models across different particle shapes, packing fractions and shear rates, we perform experiments in a quasi-2D annular shear cell using photoelastic particles. The apparatus is designed to measure both the stress ratio μ and the inertial number through the use of a torque sensor, laser-cut leaf springs, and particle-tracking. We observe that across several different packing fractions and rotation rates, a single set of model parameters is able to capture the full range of data collected once we account for frictional drag with the bottom plate. While the model's local parameter is always approximately unity, the nonlocal parameter varies sensitively on both the particle shape and material. Our measurements confirm the prediction that there is a growing lengthscale at a finite value μ_s, associated with a frictional yield criterion. Finally, we newly identify the physical mechanism behind this transition at μ_s by observing that it corresponds to a drop in the susceptibility to force chain fluctuations.