Granular flow: Friction and the dilatancy transition

Abstract

Molecular-dynamics simulations of noncohesive granular assemblies under shear are described. At low shear rate \ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{}, 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 \ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{} where 6--12 layers of grains flow over a compact, static assembly. A phase boundary, characterized by a discontinuity in \ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{} 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 \ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{}.