Void statistics and compactivity measurement in an experimental granular   pile
  We investigate experimentally the statistics of the free volume inside bidimensional granular packings, for two different kinds of grains and two levels of compaction. With the aim to gain new insight into the statistical description of granular media, we measure the free volume distribution in clusters of increasing size. Our main result is that the logarithm of the free volume distribution scales in a nonextensive way with the cluster size, while still enabling the extraction of two intensive parameters. We discuss the interpretation of these parameters, and the consequences of the nonextensivity on the possible measurements of Edwards' ''compactivity''. 
Frederic Lechenault, Olivier Dauchot and Eric Bertin