Session D40 - Poster Session I.
POSTER session, Monday afternoon, March 12
Exhibit Hall, Washington State Convention Center
In three dimensions, granular systems settle into random structures, while in two dimensions grains can often be annealed into dense crystals. We study the shape dependence of ordering by welding ball bearings together to make clusters of spheres. We restrict our studies to two-dimensional systems by confining the shapes in a single plane. We show several measures of how well the different shapes order under a series of taps. Generally, shapes with higher rotational symmetry form more nearly perfect lattices. In most cases the nature of the disorder present is qualitatively different from that found in three dimensional random packings of spheres, with sharp boundaries dividing perfectly ordered crystallites. Only arrangements with the lowest observed densities have crystallite size comparable to the cluster size and appear random. Our data suggest a transition between random and partially crystalline arrangements at a filling factor of 84%, near the density for a similar transition found earlier for circles in two dimensions. Supported by NSF under DMR-9733898.