Combined size and density segregation and mixing in noncircular tumblers.

Flowing granular materials segregate due to differences in particle size (driven by percolation) and density (driven by buoyancy). For noncircular tumblers the additional interaction between the segregation mechanisms and chaotic advection complicates the physics. Experiments are performed using a bi-disperse mixture of equal volumes of different sizes of steel and glass beads in a quasi-two-dimensional square tumbler. Mixing is observed instead of segregation when the denser beads are larger than the lighter beads so that the ratio of particle sizes is greater than the ratio of particle densities. This can be expressed in terms of the particle diameters and masses as d(heavy)/d(light) >(m(heavy)/m(light))(1/4) . Segregation patterns vary from a semicircular core when the fill level is below 50% to more complicated patterns including lobes and streaks for fill levels above 50%. Temporal evolution of segregated patterns is quantified in terms of a "segregation index" (based on the area of the segregated pattern) to capture both the rate and extent of segregation at different particle properties. The circular and noncircular tumblers have no significant difference in the segregation index, even though the segregation patterns differ significantly.