Confined Packing of Granular Rods: X-ray Tomography Experiments

We study 3D rod packings in cylindrical containers with two sizes by a computed tomography scanner. The aspect ratio of our rods is 1:8. We observe rods pack to a higher volume fraction phi in the larger container, suggesting a strong influence of the walls on the packing. In both containers, we observe that particles pack isotropically in the bulk, but pack differently near the side and bottom boundaries. Near the bottom boundary, the rods lie flat against the container boundary and thus pack with a higher phi than the bulk. In contrast, rods are loosely packed near the side boundary. For both the bottom and side boundaries, the range of influence of the boundaries is shorter than a rod length. The differences between the bottom and side boundaries show that gravity plays an important role in the packing of rods in small containers.