In 1939, the Japanese researcher Yositisi Oyama, showed that a rotating drum half-filled with beads of different sizes causes the beads to demix forming into various patterns of segregation. This is a potentially interesting way to separate such mixtures. This and other work kickstarted an entirely new field focused on the strange behaviour of granular fluids.

Now, Frank Rietz and Ralf Stannarius at the Department of Nonlinear Phenomena at the University of Magdeburg in Germany have found a curious corollary to this work.

Instead of a rotating drum, they confine their mixture of small and large beads to a flat box which they then set rotating at slow speed so inertial effects are minimised. And instead of half filling the box, they almost totally fill it with beads.

You might imagine that the beads would jam, preventing any separation but what actually happens is quite extraordinary. Above some filling threshold, the bead separation flow begins to show a rich pattern of convection.

All this is beautifully filmed and explained in a fluid dynamics video which is well worth looking at (if only to see how scientific publication is changing.)

Rietz and Stannarius say they have been unable to explain these patterns using the known mechanisms of granular convection. So they’re left with a puzzle: how do these patterns emerge?

The answer may be of more than passing interest to earth scientists since similar convection patterns emerge on large scale in Earth’s atmosphere. Whether there is any link, however, has yet to be established.

An interesting puzzle for fluid dynamicists with a few hours to spare.

Ref: arxiv.org/abs/0910.4897: Convection Rolls in a Rotating Box Filled with Beads