Sand is strange stuff. It can flow like water, or support a house. It jams in a funnel even if the neck is wider than the grains. Now a report in the journal
'Pretend' is very much the operative word, for nature's laws are not so easily foiled. But the behaviour is striking all the same. Jens Eggers from the University of Essen in Germany, has shown that when sand is shaken in a two-compartment container with a small hole connecting the two spaces, the grains can gather preferentially in one of the compartments.
This is almost precisely the situation considered by the Scottish physicist James Clerk Maxwell in the nineteenth century. Maxwell was developing the so-called kinetic theory of gases, which explains how the random motions of molecular particles in the gas account for the relationships between its temperature, pressure and volume. Maxwell knew that the particles' motions tend to equalize the pressure of the gas everywhere. If, for instance, a gas in one compartment is separated by a partition from a vacuum in another compartment, and the two compartments are then interconnected by a hole in the partition, the gas will diffuse through the hole until its pressure is the same in both compartments.
The reverse, said Maxwell, will never happen: the equally distributed gas will never spontaneously congregate in just one compartment. The particles move at random, and so once the pressures are equal the particles are no more likely to pass in one direction than in the other. This is simply a statement of the Second Law of Thermodynamics, which says that 'entropy'-which can be crudely thought of as disorder-increases in all processes. The situation in which particles are scattered between both compartments is more disorderly than that in which particles are sequestered in just one of them.
But Maxwell postulated a way in which this law could be cheated. Suppose, he said, some tiny demon operated a shutter over the connecting hole so as to let through only particles travelling in one direction.
Physicists felt that there must be some factor that would prevent Maxwell's demon from truly instigating a violation of the Second Law. But it was not until the past few decades that they identified the flaw in the scheme: in dealing with the information needed to select the right particles, the demon itself dissipates heat and so 'gives back' the entropy. Even Maxwell's demon, it seemed, could not evade the law.
But Eggers' experiment shows sand behaving just as one would expect if a little demon was indeed letting the shaken grains through the hole in one direction only -- piling them up in a 'cold' layer on one side of the partition while grains on the other side continue to dance about like particles of a 'hot' gas.
Yet fear not for the Second Law. The crucial difference between a gas and a collection of sand grains is that the latter are big particles which heat up (and thus increase entropy) when they collide. In other words, the grains themselves do the job of upholding the law, just like the demon's energy-dissipating mental processing. This dissipation of energy in collisions is in fact responsible for the imbalance itself. Collisions in the denser 'cold' layer in one compartment are more frequent than in the less dense 'hot gas' of particles in the other compartment -- and so induce still more particles to dissipate their energy and join the dense region.