Metastability and instability in model grain piles

Studies on the onset of layer sliding in adiabatically tilted model grain piles in 2D reveal the following physics. At tilt angles < Θaval, where Oaval is the angle at which layer sliding begins, the distribution D(s) of displacements s of the surface grains decay as s for sufficiently large s. We find that r suffers a crossover to a lower value as the surface roughness is minimized and layer sliding commences at Θαναί. The presence of restitution in granular contacts renders r = (Θaval –) and hence makes the system "fragile".