Particle segregation in an annular shear cell

Particle segregation is a widespread process that affects particle composition in granular materials. Particles tend to segregate into distinct regions according to their size, density, and shape. To date, most studies investigate particles segregating in gravity direction. Less studied is the segregation whose direction deviates from gravity direction. To study this process, we conducted experiments by shearing bi-dispersed granular mixtures in an annular shear cell. Refractive-index matching (RIM) technique was applied to visualize inner particle composition. And we reconstructed three-dimensional particle velocity map from Particle Tracking Velocimetry (PTV). In experiments, radial segregation was observed apart from axial segregation. Small particles tended to accumulated radially to central regions, while large particles gathered in outer regions. We observed radial particle convection from the three-dimensional velocity map, this convection is similar to the secondary flow in the tea leaf paradox. Radial pressure gradient may be the force source for this convection, and it leads to a higher bed surface in central regions than outer regions. The segregation velocity is largest in outer regions since particles here endure a higher shear rate. Furthermore, we found that particle transport was largest when the transport contribution from small particles matched that of large particles.