Number 132 (Story #3), June 10, 1993 by Phillip F. Schewe and Ben Stein WHY DO LARGE THINGS RISE TO THE TOP? Shaking grains of different sizes in a container creates large-scale flow patterns which are responsible for separating the grains by size, new experiments have shown. An important problem in industry has been to determine why, when one shakes a pile of sand, mixed nuts, or other granular material in a container, the larger particles end up on top and the smaller ones wind up on the bottom. New experiments, performed by Sidney R. Nagel (312-702-7190) and his colleagues at the University of Chicago, uncover a previously unsuspected mechanism: the separation is caused by large-scale flow patterns, or "convection," in the grains. The researchers studied the rise of a single large glass bead through a vibrating cylinder filled with smaller beads. They found that the large bead, once it had reached the top of the pile, was unable to follow the convection cycle through a very narrow region of downward motion along the walls of the container. The researchers discovered that container boundaries play an important role in the convective process that leads to separation. (James B. Knight et al., Phys. Rev. Lett., 14 June 1993.)