How do grains flow out of an emptying silo? And what about sugar poured out by a pastry chef? Like liquids (2), 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.
(1) CPMOH(2) The surface of a liquid is similar to an elastic membrane under
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