Confined packing of granular rods: Bulk experiments

We conduct experiments to observe the effects surfaces have on the internal packing structure of particles. To observe this, we run an experiment using 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. The results also suggest that the surface effect on internal packing decreases with aspect ratio as well. We also conduct a second series of experiments using vibration to more closely pack the containers. This results in denser packings as expected, and also changes the influence of the boundaries.