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 boundaries. Near the bottom boundary, the rods lie flat against the container boundary but nearly maintain a same volume fraction as bulk area. In contrast, rods are much loosely packed near the side boundary and top boundary. For all three boundary layers, the layer thickness is shorter than a rod length. The differences between the bottom and top boundaries show that gravity plays an important role in the packing of rods.