Rheology of frictional grains

Abstract

This thesis deals with the description of flow and arrest of granular matter. Granular matter can undergo a rigidity transition — called jamming — that is mainly controlled by the applied stresses and the packing fraction of the grains that constitute the medium. In addition to stress and packing fraction, interparticle friction greatly affects the rheology of granular matter. Using numerical simulations and analytical modeling, we show how novel behavior in dense flow and jamming regimes arises in the presence of friction. In particular, frictionless jamming is continuous with a critical point at zero stress. In contrast, frictional jamming is shown to exhibit a discontinuous phase transition with a critical point at finite stress. The fact that the critical point resides at finite stress gives rise to remarkable flow behavior, called reentrant flow. Explicitly, there is an interval of packing fractions above the critical packing fraction in which large or low stress leads to flow but intermediate stress jams the medium. The behavior close to jamming depends substantially on the system size, i.e., there is a critical system size above which unsteady flow emerges. Unsteady flow is rationalized by large-scale structures in the stress fields. Both, the large-scale structures and the accompanied unsteady flow, are novel phenomena regarding the flow of dry granular matter and can be attributed to interparticle friction.