Flowing granular materials exhibit some peculiar features in the slow (non-inertial) flow regime that have no analogs in fluids. Two such features are independence of the stress on the shear rate, and dilatancy, or volume deformation induced by shear deformation. These features have important practical consequences: for example, rate-independence makes it is difficult to implement feedback control of the speed of an object moving through sand. However, micromechanical explanations for the features are lacking, and they have not been satisfactorily incorporated in a continuum model.

In this presentation, I shall first offer a micromechanical explanation for rate-independence of the stress. I will demonstrate using particle dynamics simulations that a granular medium undergoes shear not smoothly but via recurrent jamming-yielding dynamics, leading to large stress fluctuations. The stress is purely elastic in the jammed phases and the yielded phases occur is short bursts. I will then show that this micromechanical picture leads to a simple explanation for rate-independence of the stress and also explains the origin of dilatancy. In the second part of my talk, I will describe a continuum model for slow granular flow that has a clear physical basis, and incorporates dilatancy. I will compare the predictions of the model with experiments to demonstrate its efficacy.