Frictional indeterminancy of forces in hard-disk packings

We study the statical indeterminacy of contact forces in 2D random frictional packings of perfectly rigid disks. Based on contact dynamics simulations we perform a random walk in the force space in order to explore the equilibrium force-states for a fixed packing structure. Our measurement is in agreement with the isostaticity of frictionless hard particles, in that case forces are fully determined. For non-zero friction coefficient the problem gets undetermined, the possible force fluctuations are growing with increasing friction up to a maximum at friction coefficient around 0.1. Further increase of friction reduces the force fluctuations on the average.