Effect of particle shape on the density and microstructure of random packings.

We study the random packing of non-spherical particles by computer simulation to investigate the effect of particle shape and aspect ratio on packing density and microstructure. Packings of cut spheres (a spherical segment which is symmetric about the centre of the sphere) are simulated to assess the influence of a planar face on packing properties. It turns out that cut spheres, in common with spherocylinders and spheroids, pack more efficiently as the particle's aspect ratio is perturbed slightly from unity (the aspect ratio of a sphere) to reach a maximum density at an aspect ratio of approximately 1.25. Upon increasing the aspect ratio further the cut spheres pack less efficiently, until approximately an aspect ratio of 2, where the particles are found to form a columnar phase. The amount of ordering is sensitive to simulation parameters and for very thin disks the formation of long columns becomes frustrated, resulting in a nematic phase, in marked contrast to the behavior of long thin rods which always randomly pack into entangled isotropic networks. With respect to coordination numbers it appears that cut spheres always pack with significantly fewer contacts than required for isostatic packing.