How do grains flow out of an emptying silo? And what about sugar poured out by a pastry chef?

Researchers at Centre de Physique Moléculaire Optique et Hertzienne (CPMOH) of CNRS/ Université Bordeaux 1 have just demonstrated that even without an attractive force between grains in flowing sand, they have a cohesion similar to that of liquids. These results were published in Physical Review Letters.

The surface of a liquid is similar to an elastic membrane under tension, which causes things like the pressure on the interior of soap bubbles. This “surface tension” is due to cohesion forces between molecules in the liquid.

Like liquids, grains can flow, but there is no attraction between the grains to trigger cohesion. However, by studying the waves that form and propagate on the surface of flowing sand, the physicists have observed telltale signs of cohesion. Like the very small ripples that form on the surface of water, these waves point to the existence of a “taut elastic skin” on the surface of volumes of grain. This “skin” on flowing grain is its surface tension.

By measuring wave propagation speed, the researchers have shown that this cohesion effect is a result of a decrease in air pressure between flowing grains.

Therefore, when a mass of grains flows, there is a depressed area at the middle of the flow, which pulls straying grains back towards the mass. These results should improve our understanding of the details of what happens in grain flows –materials which are common, but not yet well understood.

Article: Y. Amarouchene, J-F Boudet and H. Kellay, “Capillarylike Fluctuations at the Interface of Falling Granular Jets” Phys. Rev published online May 27, 2008, and in print May 30.