Rheology of cohesive granular flows
Nicolas Berger, Emilien Azéma, Jean-Francois Douce and Farhang Radjai
The shear strength of dense granular flows is generally described by an effective friction coefficient, defined as the ratio of shear and normal stresses, as a function of the inertial number. However, this ratio depends on the normal stress when the particles interact via both friction and adhesion forces, and in this sense it does not properly represent a Coulomb-like friction. For the same reason, it is not a unique function of the inertial number. We use contact dynamics simulations to isolate the cohesive strength from the purely frictional strength in dense inertial flows for a broad range of shear rates and adhesion forces between particles. We find that the cohesive strength is a decreasing function of the inertial number. More generally, we show that a single dimensionless parameter, combining the inertial number and adhesion, controls not only the cohesive strength but also the normalized solid fraction and granular texture described in terms of the contact network connectivity and anisotropy.
Sheared granular packings in the steady state for (top) I=0.23, eta=0 and (bottom) I=0.03, eta=15. Compressive forces are in red, tensile forces are in green and line thickness is proportional to the normal force.