  The size-segregation of granular materials, a process colloquially known as
the Brazil Nut Effect, has generally been thought to proceed faster the greater
the size difference of the particles. We experimentally investigate sheared
bidisperse granular materials as a function of the size ratio of the two
species, and find that the mixing rate at low confining pressure behaves as
expected from percolation-based arguments. However, we also observe an
anomalous effect for the re-segregation rates, wherein particles of both
dissimilar and similar sizes segregate more slowly than intermediate particle
size ratios. Combined with the fact that increasing the confining pressure
significantly suppresses both mixing and segregation rates of particles of
dissimilar size, we propose that the anomalous behavior may be attributed to a
species-dependent distribution of forces within the system.
