Angle of repose and angle of marginal stability: molecular dynamics of granular particles

The authors present an implementation of realistic static friction in molecular dynamics (MD) simulations of granular particles. In their model, to break contact between two particles, one has to apply a finite amount of force, determined by the Coulomb criterion. Using a two-dimensional model, they show that piles generated by avalanches have a finite angle of repose theta R (finite slopes). Furthermore, these piles are stable under tilting by an angle smaller than a non-zero tilting angle theta T, showing that theta R is different from the angle of marginal stability theta MS, which is the maximum angle of stable piles. These measured angles are compared to a theoretical approximation. They also measure theta MS by continuously adding particles on the top of a stable pile.