Free volume distributions and compactivity measurement in a bidimensional granular packing

We investigate experimentally the distribution of the free volume inside a bidimensional granular packing associated either with single grains or with clusters of grains. This is done for two different types of grains and two levels of compaction. The logarithm of the free volume distribution scales in a non-extensive way with the cluster size for cluster sizes up to a few hundred grains. Having factorized this size dependence, we characterize the distributions by two intensive parameters. We discuss the interpretation of these parameters and their possible relation to Edwards' compactivity.