Confined Packing of Granular Rods*

We conduct a series of experiments and simulations to observe the effects surfaces have on the packing structure of randomly packed rods. Our experiments use cylindrical containers of different diameters, and rods of aspect ratios ranging from 4 to 32. We find that the rods packed into smaller cylindrical containers yielded lower volume fractions than in larger containers. Our results are extrapolated to an infinite container size, and the subsequent volume fraction decreases with increasing aspect ratios, in agreement with previous simulations. We also do x-ray tomography experiments and study simulated rod packings, which reveal boundary layers where the packing differs from the bulk. At the bottom, rods form layers, although their average volume fraction is similar to that of the bulk. The sides and top layers are both packed with lower volume fractions than the bulk. In particular, the top boundary layer has the strongest perturbation to the overall volume fraction.