Mixing and segregation of freely-falling granular materials through a vertical pipe

Mixing and segregation of binary mixtures of freely-falling grains are demonstrated in a cylindrical pipe with an obstacle. Previously, the same setup was used to spatially focus monodisperse granular materials. As each grain is dropped one at a time from a random position at the entrance of the cylindrical pipe, it may collide with a steady or oscillating obstacle and the wall. We performed a time-driven multi-scale simulation and use Routh’s impact model for collisions to show that the focusing of grains for monodisperse granular materials is dependent on the geometry of the pipe, the elastic properties of the grain, and the amplitude of oscillation of the obstacle. For binary granular mixtures, segregation may be achieved when the elastic properties of the grain species have a big difference. However, the efficiency of segregation is diminished when the amplitude of obstacle oscillation is increased, leading to granular mixing. Just as a high degree of segregation can be attained with a stationary obstacle, a high degree of mixing can be obtained with a moving obstacle. Graphic abstract