The mystery of how volcanic lightning occurs has been solved

Physicists have solved a long-standing mystery surrounding the process that creates volcanic lightning: When similar particles rub together, why do some become positively charged while others become negatively charged?

The exchange of electrical charge when two objects touch each other, called the triboelectric effect, is what causes hair to be drawn to a balloon after rubbing.

In a cloud of volcanic ash, swirling particles of silicon dioxide exchange electrical charge as they collide. The positively and negatively charged particles separate and lightning occurs when current flows between the two.

But physicists could not explain what breaks the symmetry between two particles of the same material and causes the charge to flow one way or the other.

“There are many candidates,” says Galien Grosjean, now at the Autonomous University of Barcelona. “People suspect that moisture is important, or roughness, or the crystalline structure.”

While working at the Institute of Science and Technology Austria in Klosterneuburg, Grosjean wondered if the answer lay in carbonaceous molecules on the surface of the particles. Such molecules are ubiquitous in nature, and materials scientists try to keep these contaminants to a minimum. But Grosjean and his colleagues kept track of what the cleaning of their samples did to the electrification.

Using ultrasound, they levitated a small particle of silicon dioxide, bounced it once on a target plate made of the same material, and then measured the charge. “It can charge positively or negatively. If it was positive, we would bake or clean it and redo the experiment – and then it would charge negatively,” says Grosjean.

Analysis of the samples showed that the removal of carbonaceous molecules was indeed the controlling factor. “We saw that this effect overcomes everything else,” says Grosjean.

Another giveaway was that a cleaned sample would be positively charged again after about a day, which is also how quickly it would get a fresh coat of carbon molecules from the air.

Daniel Lacks of Case Western Reserve University in Cleveland, Ohio, is impressed by the study. “People know that surfaces have a lot of crap on them. But I’ve never seen that come up in triboelectric charging,” he says.

The discovery could be bad news for physicists, he fears. If carbon pollution determines the charge direction, it will be very difficult to accurately calculate how particles are charged. “Prediction can only be something that will never happen,” Lacks says.

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