Coarsening of granular segregation patterns in quasi-two-dimensional tumblers

A fundamental characteristic of granular flows is segregation on the basis of particle size or density. For bidisperse mixtures of particles, revolutions of the order of 10 produce a segregation pattern of several radial streaks in quasi-two-dimensional rotating tumblers with fill fractions between 50% and 70%. By extending the duration of the experiments to the order of 10 2 –10 3 tumbler revolutions, we have found the first evidence of coarsening of the radial streak pattern to as few as one streak, resulting in an unexpected wedge-shaped segregation pattern. This phenomenon occurs for a wide range of conditions including several fill fractions, particle sizes and mixtures of particles varying in both size and density in circular tumblers as well as for particles varying in size in square tumblers. Coarsening seems to be driven by transport of small (or dense) particles from streak to streak through the semicircular radial core, leading to new questions about the physics of coarsening of granular segregation patterns. Mixing two different types of grains in a revolving tumbler produces several radial streaks as the grains segregate. Unexpectedly though, after hundreds of revolutions, only one streak remains.