Controlling rheology via boundary conditions in dense granular flows

Boundary shape, particularly roughness, strongly controls the amount of
wall slip in dense granular flows. We aim to quantify and understand which
aspects of a dense granular flow are controlled by the boundary conditions
and to incorporate these observations into a cooperative nonlocal model
characterizing slow granular flows. To examine the influence of boundary
properties, we perform experiments on a quasi-2D annular shear cell with a
rotating inner wall and a fixed outer wall; the latter is selected among 6
walls with various roughnesses, local concavity, and compliance. Measuring
flow field and stress field throughout the material in experimental
studies of granular materials is an ongoing challenge due to the complex
nature of these materials. Here, we use innovative techniques to quantify
stress field and flow field when different boundaries were used. We find
that we can successfully capture the full flow profile using a single set
of empirically determined model parameters, with only the wall slip
velocity set by direct observation. Through the use of photoelastic
particles, we observe how the internal stresses fluctuate more for rougher
boundaries, corresponding to a lower wall slip, and connect this
observation to the propagation of nonlocal effects originating from the
wall. Our measurements indicate a universal relationship between
dimensionless fluidity and velocity.