This applet demonstrates the conjectured motion of particles executing successive avalanches

in a slowly rotating 2D tumbler. During each avalanche, a wedge of material falls from a

position uphill near the surface to a new position downhill near the surface. As it falls,

particles within the wedge are rearranged. Due to complicated dynamics caused by

multiple collisions between particles during the avalanche, we approximate this rearrangement

by randomizing locations of particles within the avalanching wedge.

You can change the fractional fill amount and restart the simulation. You will notice

that a core of unmixed material develops for fill levels > 50%. This occurs for the simple

reason that this core never reaches the surface, and so cannot participate in avalanches.

If you watch carefully, you will also notice that less filled containers mix more rapidly than

more filled containers. The extreme example is the 50% filled container, where mixing

occurs only diffusively and is very slow.

For details on these and related observations, see "Avalanche Mixing of Granular Solids,"

Nature 374 (1995) 39-41.