Segregation of granular mixtures in an annular shear cell under shear, gravity, and convection

Particle-size segregation is a widespread process that affects granular materials. Under the influence of gravity and shear, particles segregate into distinct regions according to their size. To date, most experimental investigations have studied granular flows induced by gravity and shear. Less studied is the special case where the granular material is segregated under convection. We are concerned with this particular case. We conducted experiments by shearing bi-dispersed granular mixtures in an annular shear cell. Refractive-index matching (RIM) was achieved between particles and the surrounding fluid, which made it possible to visualize the granular flow when illuminated by a laser sheet. We reconstructed the particle spatial arrangement by applying the Hough Transformation to a continuous series of scans. Both axial and radial segregation was observed in experiments, i.e., small particles tended to percolate downwards and accumulated radially to the center region, while large particles were squeezed upwards and gathered in the exterior region. We found that axial segregation was related to gravity and shear, while the radial convection was related to the shear and convection. Solids volume fractions were computed as a function of time from three-dimensional scans of granular mixtures, from which segregation velocity was then derived. The experimental data provides interesting insights into segregation produced simultaneously in two directions.