Structural characterization of two-dimensional granular systems during the compaction

We examine numerically the density relaxation of frictional hard disks in two dimensions (2D), subjected to vertical shaking. Dynamical recompression of the packing under the action of gravity is based on an efficient event-driven molecular-dynamics algorithm. To quantify the changes in the internal structure of packing during the compaction, we use the Voronoï tessellation and a certain shape factor which is a clear indicator of the presence of different domains in the packing. It is found that the narrowing of the probability distribution of the shape factor during the compaction is in accordance with the fact that the packings of monodisperse hard disks spontaneously assemble into regions of local crystalline order. An interpretation of the memory effects observed for a sudden perturbation of the tapping intensity is provided by the analysis the accompanying transformations of disk packings at a “microscopic” level. In addition, we investigate the distributions of the shape factor in a 2D granular system of metallic disks experimentally, and compare them with the simulation results.