The Influence of the Wall on Confined Random Packing of Rods*

We experimentally study the random packing of rods into small containers, and in particular, examine the influence of the container surface on the packing. Our experiments use cylindrical containers of different radii, and rods of aspect ratio 8. As previously seen, rods packed into smaller cylindrical containers yielded lower volume fractions than in larger containers. In this current work, we coat the inner vertical surface of the containers with sandpaper to change the enhance friction between the wall and the rods. We find a linear relationship between the volume fraction f and 1/R (using the radius R of the container). The intercept gives the infinite-radius container extrapolated volume fraction, and the slope quantifies how strongly the walls affect the packing. As might be expected, rougher sandpaper results in a stronger influence from the walls. Surprisingly, sandpaper also influences the infinite-radius extrapolation volume fraction.