Rheological chaos of frictional grains

A two-dimensional dense fluid of frictional grains is shown to exhibit time-chaotic, spatially heterogeneous flow in a range of stress values, sigma, chosen in the unstable region of s-shaped flow curves. Stress-controlled simulations reveal a phase diagram with reentrant stationary flow for small and large stress sigma. In between, no steady flow state can be reached, instead the system either jams or displays time-dependent heterogeneous strain rates (gamma) over dot(r, t). The results of simulations are in agreement with the stability analysis of a simple hydrodynamic model, coupling stress and microstructure which we tentatively associate with the frictional contact network.