Shear flow of assemblies of cohesive and non-cohesive granular materials

We have studied plane shear flow of nearly homogeneous assemblies of uniformly sized, spherical particles in periodic domains. Our focus has been on the effect of interparticle attractive forces on the flow behavior manifested by dense assemblies. As a model problem, the cohesion resulting from van der Waals force acting between particles is considered. Simulations were performed for different strengths of cohesion, shear rates, particle stiffnesses, particle volume fractions and coefficients of friction. From each simulation, the average normal and shear stresses and the average coordination number have been extracted. Not surprisingly, the regimes of flow reported by Campbell [C.S. Campbell, Granular Shear Flows at the Elastic Limit, J. Fluid Mech. 465 (2002) 261-291] for the case of cohesionless particles - namely, inertial, elastic-inertial and elastic-quasistatic regimes - persist when cohesion is included. The elastic-quasistatic regime was found to correspond with the coordination number decreasing with increasing shear rate, while in the inertial regime the coordination number increased with shear rate. A striking result observed in the simulations is that the influence of cohesion on stress becomes more pronounced with decreasing particle volume fraction. Furthermore, the window in the shear rate-particle volume fraction space over which the elastic-quasistatic regime is obtained was found to expand as the strength of cohesion was increased. When the particle volume fraction is so high that even a cohesionless system would be in the elastic-quasistatic regime, the addition of cohesion had minimal effect on the stresses. At lower particle volume fractions where a cohesionless assembly would have been in the inertial regime, we present a new scaling which permits collapse of all the data at various strengths of cohesion and shear rates into a single master curve for each particle volume fraction and coefficient of friction. The regimes of flow in this master curve are discussed and the scaling for the normal stress in elastic-quasistatic flow is identified.