Sand piles harden as water makes links.

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. However, rather than building bridges that hold grains together, he suspects that the water might cause increased sintering--a type of chemical bonding.

Decades ago, scientists discovered that friction between solid objects in contact increases with time, a process called aging. To test if granular materials also age, the French researchers recently poured glass beads the size of fine sand grains into a drum and let them sit before turning the drum.

As the drum begins to rotate, the initially horizontal surface of the bead pile tilts toward the vertical until the pile collapses, much like a heap of clothes tumbling in a dryer. The team conducted tests at humidities from 5 to 55 percent and rest periods of 5 seconds to 2 hours. Under such conditions, "the [aging] effect is very clear," Bocquet says.

In unpublished experiments, the researchers have returned to solids, gauging friction by the steepness at which a test block begins to slide down an incline. The rate of increase of friction with contact time at 60 percent humidity was as much as five times the rate at 10 percent, he told SCIENCE NEWS. Moreover, tests on Teflon revealed no humidity effect, consistent with water's inability to cling to it.

Teflon shows some aging, however. That's not surprising, says Bocquet. Although it may have the largest influence, he never expected the water-bridge hypothesis to be the sole explanation for the aging effect.