GRANULAR FLOW - FRICTION AND THE DILATANCY TRANSITION

Molecular-dynamics simulations of noncohesive granular assemblies under shear are described. At low shear rate-gamma, the assembly is unstable to uniform motion and exhibits stick-slip dynamics involving periodic dilatancy transitions and gravitational compactification. Steady-state motion occurs at larger gamma where 6-12 layers of grains flow over a compact, static assembly. A phase boundary, characterized by a discontinuity in gamma and an enhanced normal stress, separates the static and flowing regions. In the steady-state regime, dilatancy leads to a shear stress that is independent of gamma.