Local Dynamics of Granular Size Segregation

We seek to quantify the local mechanisms which drive granular size segregation, using a two-dimensional system. We perform experiments using a bi-disperse mixture of disks floating on a tilted air table, agitated by bumpers at the bottom edge. A layer of large particles initially placed at the bottom of the system mixes with a layer of small particles above it, eventually resegregating to the upper surface. We record the position of each particle and measure the average segregation velocity as a function of local packing fraction $\phi$ for all particles and local concentration $c$ of small particles. The velocity of the large particles is strongly dependent on packing fraction; particles in regions of lower $\phi$ tend to move downward, while those in regions of higher $\phi$ ascend through the material. In contrast, we find that the effect of local concentration $c$ is weak.