Nonlocal rheology of dense granular flows: the effect of particle and boundary properties

In granular rheology, one of the most promising recent advances has been the development of nonlocal rheologies, including the nonlocal rheology model proposed by Kamrin and Koval. This model extends a local Bagnold-type granular flow law to include a Laplacian term governing the diffusion of fluidity. It has been observed to successfully capture the dynamics of quasi-2D flows without the need to provide detailed particle dynamics, using a single set of experimentally-determined model parameters. For use as a modeling tool, the next step is to make predictions for particles with any particle shape/material, and to correctly model the response to various boundary conditions. We perform experiments to study particles of three different shapes and three different stiffnesses to explore their influence on the rheological parameters. We find that the nonlocal parameter varies with both particle shape and material, frictional parameter varies primarily with particle shapes, and the local parameter is approximately constant. Finally, we identify how the roughness of the boundary changes both the flow and the interparticle forces using photoelastic force measurements.