Predicting segregation of nonspherical particles

Segregation, or demixing, of flowing mixtures of size-disperse noncohesive spherical particles is well understood. However, most particle systems in industry and geophysics involve nonspherical particles. Here, the segregation of bidisperse mixtures of millimeter-sized particles having various shapes is characterized using discrete element method simulations of gravity-driven free-surface granular heap flow. As a proxy for nonspherical particles, we study mixtures of cylindrical particles that vary widely in both their length and diameter ratios, including both disks and rods, as well as mixtures of cylindrical and spherical particles. The propensity to segregate is measured with a segregation length scale that characterizes the segregation velocity of the two species for these types of mixtures, identical to the approach for mixtures of spherical particles. Surprisingly, segregation can be predicted based solely on the volume ratio of the two species, regardless of particle shape. The segregation length scale increases linearly with the log of the volume ratio for volume ratios varying from 0.1 to 10 in the same way as it does for bidisperse mixtures of spherical particles. Thus, segregation properties based on spherical particles can be directly applied to nonspherical particles.