Influence of shape on ordering of granular systems in two dimensions - art. no. 061303

We investigate ordering properties of two-dimensional granular materials us ing several shapes created by welding ball bearings together. Ordered domai ns form much more easily in two than in three dimensions, even when configu rations lack long-range order. The onset of ordered domains occurs near a p acking density of 0.8, a phenomenon observed previously for disks. One of o ur shapes. the trapezoid, has packings that remain disordered and near the transition density even after annealing by shaking. Although random packing s are unstable for disks and many other shapes in two dimensions, trapezoid packings provide an approach to studying two-dimensional randomness. We al so find that the rotational symmetry of a shape is an excellent predictor o f how easily it orders, and a potential guide to identifying two-dimensiona l shapes that remain random after annealing.