Side pressure anomalies in 2D packings of frictionless spheres

Abstract

The pressure in a dish filled with resting sand quickly saturates downward as a consequence of friction forces. This fact is often quoted to introduce granular material as a distinctive form of matter that exhibits behaviour rather different from that of ordinary liquids. One would expect that hydrostatics is fully recovered when friction is switched off. Here we demonstrate in a simple model system that even frictionless grains manifest unanticipated properties. First, wall induced layering in combination with a wedge effect leads to extremely large local pressures near the walls at the bottom of rectangular containers, and this feature does not vanish for diverging container width. Second, for monodisperse packings the average horizontal force is much weaker on the side walls than the hydrostatic value, but approaches the latter for increasing polydispersity. Third, the average sidewall force shows marked oscillations as a function of container width, which can be considered as a measure of wall-to-wall correlations.