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.
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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.
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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.
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.
