Convection and size segregation in a Couette flow of granular material

We have investigated the size segregation of a binary mixture of spherical glass particles in a Couette geometry, where the cylinders are made of smooth glass and the flow is generated by the shearing motion of the inner cylinder. The trajectories of 1, 2, and 3 mm glass particles, placed at the bottom of the cell, were followed as they moved through a 1 mm medium. We observe convective motion of the particles in which particles rise at the outer radius and fall at the inner radius. The horizontal and vertical speeds of 2 and 3 mm glass particles, initially at the cell bottom, as a function of the inner cylinder speed were determined and the onset of turbulent motion measured by processing images. While in all cases the larger particles rose to the top and remained on the surface as rotation continued, the particles comprising the medium appear to go through a convectivelike motion. The effect of interstitial fluid, in this case air, was studied by repeating the experiments in vacuum. We did not detect any significant changes in the results.