Random Packings of Spheres and Spherocylinders Simulated by Mechanical Contraction

We introduce a simulation technique for creating dense random packings of hard particles. The technique is particularly suited to handle particles of different shapes. Dense amorphous packings of spheres have been formed, which are consistent with the existing work on random sphere packings. Packings of spherocylinders have also been simulated out to the large aspect ratio of alpha=160.0. Our method packs randomly oriented spherocylinders to densities that reproduce experimental results on anisotropic powders and colloids very well. Interestingly, the highest packing density of phi=0.70 is achieved for very short spherocylinders rather than spheres. This suggests that slightly changing the shapes of the particles forming a hard sphere glass could cause it to melt. Comparisons between the equilibrium phase diagram for hard spherocylinders and the densest possible amorphous packings have interesting implications on the crystallization of spherocylinders as a function of aspect ratio.