(99c) Rheology of Homogeneous and Inhomogeneous Granular Materials

This study is from an ongoing investigation of the effect of different particle properties like friction, softness and cohesion on the compaction of sheared soft granular materials. We compare dry non-cohesive, dry cohesive and wet moderately-to-strongly cohesive frictionless to frictional soft granular materials and study the local volume fraction at steady state. We compare the results from two different geometries: (i) an inhomogeneous three dimensional, unconfined, slowly sheared split-bottom ring shear cell, where materials while sheared are subject to compression under the weight of the material above, and (ii) homogeneous stress controlled simple shear in a 3D cuboidal box with periodic boundaries. In addition, we have also considered two cohesive contact models: (i) a linear reversible cohesive model for dry particles resembling van der Waals forces, and (ii) a non-linear irreversible cohesive model for wet particles resembles liquid bridges.

The local volume fractions at steady state all agree between the two geometries for frictional particles as well as for the different particle properties. The inter-particle cohesion has an interesting impact on the rheology and compaction of sheared soft granular materials. Cohesion causes additional stresses due to attractive forces between particles, causing an increase in volume fraction. This effect is not visible in a system of infinitely stiff particles but is significant for soft particles. Acting oppositely, we observe a general decrease in volume fraction due to increased cohesion for frictional particles, which we attribute to the role of contact friction enhanced by cohesion, which is causing dilation. These two consequences of inter-particle cohesion are further highlighted in a particle friction – bond number phase diagram for different softness, in which one can predict sample dilation/compaction.