(A) Sand contained equally in two compartments within a vessel is shaken vigorously. The granular gas can travel through a small hole from one side to the other, but generally the populations remain equal. At a lesser rate of shaking (B) sand on one side will begin to congregate more readily than on the other side, eventually leading to warmer sand on the one side and cooler, sand on the other side. Does the hole represent a sort of "Maxwell's Demon," which sorts hot from cold in violation of the second law of thermodynamics? Not really. Individual sand grains can absorb or dissipate heat energy, unlike the idealized gas molecules imagined by James Clerk Maxwell in his original thought experiment of 1871.
reported by: Jens Eggers in Physical Review Letters, 20 December 1999