Disks vs. Spheres: Contrasting Properties of Random Packings

Collections of random packings of rigid disks and spheres have been generated by computer using a previously described concurrent algorithm. Particles begin as infinitesimal moving points, grow in size at a uniform rate, undergo energynonconserving collisions, and eventually jam up. Periodic boundary conditions apply, and various numbers of particles have been considered (N~<2000 for disks, N~< 8000 for spheres). The irregular disk packings thus formed are clearly polycrystalline with mean grain size dependent upon particle growth rate. By contrast, the sphere packings show a homogeneously amorphous texture substantially devoid of crystalline grains. This distinction strongly influences the respective results for packing pair correlation functions and for the distributions of particles by contact number. Rapidly grown disk packings display occasional vacancies within the crystalline grains; no comparable voids of such distinctive size have been found in the random sphere packings. "Rattler" particles free to move locally but imprisoned by jammed neighbors occur in both the disk and sphere packings.