Size segregation of granular mixtures under recirculation

Particle-size segregation is a common phenomenon in granular materials that has attracted increasing attention in recent years. Yet segregation under recirculation remains underexplored compared to segregation in simple-sheared gravity-driven flows. In this study, we investigated the dynamics of a bi-dispersed granular mixture flowing over an inclined conveyor belt. This belt pulled particles upstream, creating a recirculating flow. We visualized the internal structure of granular flow in a vertical plane by matching the refractive indices of the fluid and particles, and then located the particles. We observed an upstream accumulation of small particles and downstream accumulation of large particles, these two regions being separated by a curved interface. We think that this separation resulted from the interplay between particle recirculation and segregation: 1) surface particles moved downstream while bottom particles moved upstream; 2) segregation led to particles separating during recirculation, with full separation achieved at the channel ends. We developed a depth-averaged advection-diffusion equation to quantify this phenomenon by treating the recirculation as convection. This study provides new insights into the coupled mechanisms of recirculation and segregation in granular materials.