We study static packings of frictionless and frictional spheres in three dimensions, obtained via molecular dynamics simulations, in which we vary particle hardness, friction coefficient, and coefficient of restitution. Although frictionless packings of hard spheres are always isostatic (with six contacts) regardless of construction history and restitution coefficient, frictional packings achieve a multitude of hyperstatic packings that depend on system parameters and construction history. Instead of immediately dropping to four, the coordination number reduces smoothly from z=6 as the friction coefficient u between two particles is increased.
Silbert, Leo, Ertas, Deniz, Grest, Gary S., Halsey, Thomas C. and Levine, Dov. "Geometry of Frictionless and Frictional Sphere Packings." (Feb 2002).