Confined Packing of Granular Rods: Bulk Experiments*

We conduct experiments to observe the effects surfaces have on the internal packing structure of randomly packed rods. We 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. The results are extrapolated to an infinite container size, leading to a volume fraction which decreases for increasing aspect ratio, in agreement with previous simulations. The results also suggest that the surface effect on internal packing decreases with aspect ratio as well.  By comparing containers with different heights and diameters, our results suggest that the influence of the top and bottom boundaries is less significant than the influence of the side boundaries.  The implication is that gravity plays a role in these experimental packings.