Magazine article Science News

Sand Piles Harden as Water Makes Links

Magazine article Science News

Sand Piles Harden as Water Makes Links

Article excerpt

Granular materials such as sands and powders behave in mysterious ways, for instance acting partly like solids and partly like liquids. They also play important roles in industries ranging from agriculture to drug manufacturing (SN: 9/20/97, p. 186).

A new study by French researchers finds that piles of undisturbed granular materials stick together more tightly as time passes. They also offer a new explanation for the phenomenon in these materials: Water from humid air slowly condenses into films bridging the spaces between grains. Surface tension then binds the grains together, increasing friction in the pile. The French team's experimental results match predictions based on this model.

"The more you wait, the more you create liquid bridges, and the more you have an adhesion force," says Lyderic Bocquet of the Ecole Normale Superieure in Lyon. He and his colleagues there and at the Universite Claude Bernard-Lyon I in Villeurbanne describe their findings in the Dec. 24/31, 1998 NATURE.

"I think it's neat. It's an interesting result," says Peter E. Schiffer of the University of Notre Dame in Indiana. The French data complement his own lab's finding that a few drops of moisture, in this case oil, will dramatically stiffen a liter of granular materials.

"If it's really what's happening, it's quite fascinating," says Jacob N. Israelachvili at the University of California, Santa Barbara. Water-bridge formation could be important in many areas, such as chemical and electric behavior of thin films, he adds. …

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