Put a large red sphere
on the bottom of a recipient filled with smaller blue ones, shake it
and after some the big one is on top.
The figure shows the starting of a simulation
of this "Brazil nut effect" made with
Jason Gallas , Thorsten
Pöschel and Stefan Sokolowski (J. Stat. Phys. 82, 443-450 (1996)).
next picture shows what happens after some CPU-time has been spent.
For more detail see our
paper . In two dimensions it is easier to make large scale simulations
and with Thorsten Poschel we were able to show that the large
particle is able to induce a convection roll which then drives
it upwards - some kind of bootstrap mechanism. We
published this in Europhysics Letters, Vol 29, 123-128 (1995).
With Leonardo Trujillo and
Meheboob Alam we recently developped
a kinetic gas theoretical argument for the Brazil nut effect.
It is based on the physical picture that a larger particle has a
smaller number of inelastic collisions per volume and its effective
"temperature" is therefore higher that that of the smaller one.
Buyoancy therefore will drive the larger particle to the top.
These results are presented in a
paper published in Physica A, Vol.330, p.519-542 (2003) in a
paper published in Granular Matter Vol.5, 85-89 (2003)
and in the following
paper published in Europhys. Lett. Vol.64, p.190-196 (2003).
Similar to the Brazil nut segregation is segregation flow driven
by an external fluid as can been seen from the
movie made by Fatemeh Osanloo.