What if sand flowed like water? Researchers at Centre de Physique Moléculaire Optique et Hertzienne (CNRS/ Université Bordeaux have just demonstrated that even without an attractive force between grains in flowing sand, they have a cohesion similar to that of liquids.
How do grains flow out of an emptying silo? Or And what about sugar poured out by a pastry chef? Like liquids, grains can flow, but there is no attraction between the grains to ensure 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 surfaces. This “skin” on flowing grain flows is its surface tension.
The surface of a liquid is similar to an elastic membrane under tension, which causes, for example, the pressure on the interior of soap bubbles. This “surface tension” is due to cohesion forces between molecules in the liquid.
By measuring wave propagation speed, the researchers have shown that this cohesion effect is a result of reduces 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 pressure is reduced within the flow, which bringing pulls straying grains back towards the mass. These results are goingshould improve our understanding of the details of what happens on a very small scale in grain flows – these materials may bewhich are common, but they are not yet well understood.
- Amarouchene et al. Capillarylike Fluctuations at the Interface of Falling Granular Jets. Physical Review Letters, 2008; 100 (21): 218001 DOI: 10.1103/PhysRevLett.100.218001
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