How do grains of similar size behave as they flow out of a hole?
A very thin rectangular “silo” is created out of plates of clear glass separated by a small space. All sides except the top are closed. There is a small hole in the bottom. The silo is filled partway with tiny glass beads. See figures 1 and 2. The process of the beads emptying through the hole, driven by the force of gravity, is captured on video. At different times in the emptying process, it is observed that some of the beads are moving and others are not. Also, the surface shape of the bead pile changes. Initially, a gentle depression is formed, but the beads on the surface don t slide. Over time, however, the surface gradually forms a deeper and deeper “v” shape. When the angle of the sides of the “v” reaches a certain threshold, called the angle of repose, the beads on the surface start to slide toward the point of the “v”. From then on, the overall height of the surface continues to drop, but its shape stays the same. Digital imaging also allows physicists to monitor the flow pat
