Mixing of a Granular Material in a Bidimensional Rotating Drum

We report experimental measurements on the mixing properties of a powder. We study a 2D model granular medium confined in a half-filled rotating drum. Using an image-processing device, we follow the trajectories of tracer particles in a monodisperse assembly of beads. Tracer particles with different size ratios exhibit a violent segregation effect: a smaller particle has a tendency to stay in the centre and a larger one will rather dwell on the edges. Furthermore, for a tracer of identical size, we evidence a specific dispersion property where the centre and the edges are competing attractors of the mixing dynamics.