Size Segregation of Granular Mixtures in Recirculating Flows

Particle-size segregation is a common phenomenon in granular materials that has attracted increasing attention in recent years. Yet segregation in recirculating flows remains underexplored compared to segregation in gravity-driven flows. In this study, we investigated how a saturated granular mixture with a bimodal grain size distribution flowed over an inclined conveyor belt, which created recirculation in the channel. By matching the refractive indices of the fluid and the particles, we were able to visualise the internal structures of granular flow in a vertical plane lit by a laser. Applying Hough Transformation to the experimental images, we reconstructed the spatial arrangement of the particles. Our observations revealed a curved interface between large and small particles, in contrast to the typically straight interfaces in bi-dispersed granular avalanches. This curvature was attributed to the interplay between recirculation and segregation: 1) particles were recirculated, with bottom particles moving upstream and surface particles moving downstream; 2) size segregation induced particle separation, resulting in an upstream accumulation of small particles and downstream accumulation of large particles. To quantify this phenomenon, we developed a depth-averaged convective-diffusion equation, providing new insights into the coupled mechanisms of recirculation and segregation in granular materials.