Granular materials give off a zap just before slipping

By Rachel Ehrenberg

Web edition: June 11, 2012
Print edition: July 14, 2012; Vol.182 #1 (p. 13)

Ordinary baking flour isn’t the most electrifying substance, but spilling a box of the stuff yields a jolt of voltage that has scientists excited about their prospects for sensing catastrophic events like earthquakes and industrial accidents.

Scientists have known for years that materials including rock, crystals and adhesives like ordinary office tape can produce an electrical signal as they fracture or crack under a load. It’s also known that before a granular material can flow, the space it takes up has to enlarge — think of a traffic jam in which another lane opens up and cars begin to move again. The voltage measured in the flour may be a signal of this ‘dilation,’ which indicates flow is about to happen.

“We’ve known about dilation and that there’s an electrical signal when things fail, but nobody has put these two together before,” says chemical engineer Joseph McCarthy of the University of Pittsburgh, who wasn’t involved in the work. “This is a really, really interesting observation.”

Researchers at Rutgers University in Piscataway, N.J., took a cylindrical tumbler — the sort used to thoroughly mix powders — and filled it first with a blend of ground-up acetaminophen and cellulose, and then unbleached white flour. After running a static eliminator over the whole set-up to clear out any static electricity, the researchers fixed a voltage probe to different locations on the tumbler and slowly spun it. Seconds before the powder dumped from one side of the tumbler to another — essentially right before the avalanche — the researchers detected an impressive zap of electricity. They report the findings online June 11 in the Proceedings of the National Academy of Sciences.

“Our expectation was that we weren’t going to see anything, but we found hundreds of volts from just tumbling flour,” says Troy Shinbrot, a granular materials specialist at Rutgers University in Piscataway, N.J. “It seemed very strange, like something was wrong with the flour.”

Puzzled, he made the setup even simpler: The team built an acrylic box, filled it with flour and slowly tipped it. Again, there was a detectable voltage spike in 15 of the 18 trials. This spike occurred less than one second before the flour began to slip, the team reports.

While the spike in voltage was often very close in time to start of the grain cascade, the researchers think the voltage is released from the crack that precedes the event. Such a signal might be harnessed for predicting the impending failure of granular materials, McCarthy says, perhaps in huge industry vats, silos or even earthquakes.