Spatial distribution functions of random packed granular spheres obtained by direct particle imaging

We measure the two-point density correlations and Voronoi cell distributions of cyclically sheared granular spheres obtained with a fluorescence technique and compare them with random packing of frictionless spheres. We find that the radial distribution function g(r) is captured by the Percus-Yevick equation for initial volume fraction ϕ=0.59. However, small but systematic deviations are observed because of the splitting of the second peak as ϕ is increased toward random close packing. The distribution of the Voronoi free volumes deviates from postulated Γ distributions, and the orientational order metric Q6 shows local order but no long range order. Overall, these measures show significant similarity of random packing of granular and frictionless spheres, but some systematic differences as well.