Segregation of Free-Sifting Fine Particles

Flowing dense mixtures of bidisperse particles tend to segregate by size, and the large to small particle diameter ratio, R, determines whether the segregation is shear-induced (small R) or free-sifting (large R). Free-sifting segregation remains largely unexplored and exhibits qualitatively different segregation dynamics from shear-driven segregation, where segregation occurs only when large and small particles flow together. To consider free-sifting, we use discrete element method simulations of gravity-driven motion of fine particles through static random packings of large particles for 4 ≤ R ≤ 7.5. For R > 6.464 (the geometric trapping threshold), fine particles flow freely through the voids between large particles, and the percolation velocity is constant and decreases only slightly with decreasing R. For R decreasing from 6.464 to 4, fine particles flow a decreasing distance before being trapped in the bed of static large particles, but up to the point where they are trapped their velocity is nearly as large as that for R > 6.464. Preliminary results for dense flows of both particles with R > 4 indicate that the fine particles freely percolate between the flowing large particles leading to the fine particles flowing independently from the large particles.