Stability of monomer-dimer piles

We present an experimental and theoretical study of piles consisting of monodisperse spherical grains mixed with a weight fraction nu(d) of dimer grains made by the rigid bonding of two such spherical grains. The maximum static angle of stability tantheta(c) of the pile increases from 0.45 to 1.1 and the grain packing fraction Phi decreases from 0.58 to 0.52 as nu(d) is increased from 0 to 1. The stability of these piles appears to be controlled by the grains sitting on the surface, which roll out of their local "traps" as the tilt angle is increased. We attribute the increase in tan theta(c)(nu(d)) to the enhanced stability of dimers on the surface, such that at higher tilt angles, there are sufficiently many stable surface traps available to accommodate the reduced density of monomers on the surface. A full characterization of the grain-scale roughness of the surface is required to quantitatively account for the changes in theta(c) with nu(d).