Free volume distributions inside a bidimensional granular medium

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We investigate experimentally within a two-dimensional cell the distribution of the free volume associated either to single grains or to clusters of grains. Our main result is that the logarithm of the probability to find a free volume per grain vₙᶠ in a cluster of N grains scales as Nᵃlpha g(vₙᶠ), with alpha = 0.75. We interpret this non extensive scaling factor Nᵃlpha as an evidence for the onset of long range correlations between the free volumes of individual grains. We also discuss the possible relation between g(vₙᶠ) and Edwards entropy.