Physicists have cracked a long-standing puzzle regarding the process behind volcanic lightning: how identical particles, upon friction, end up with opposite electrical charges. The findings of this new study have been published in the journal Nature.
The transfer of electrical charge when two objects come into contact, known as the triboelectric effect, is what makes hair cling to a balloon after rubbing.
Within a volcanic ash cloud, swirling silicon dioxide particles exchange electrical charges upon collision. Once positively and negatively charged particles segregate, lightning occurs as a current flows between them.
However, physicists were unable to explain what breaks the symmetry between two particles consisting of the same material and dictates the direction in which the charge flows.
“There are numerous potential candidates,” states Ghaylen Grosjean from the Autonomous University of Barcelona. “People have suggested that humidity, surface roughness, or crystal structure plays a role.”
Working at the Austrian Institute of Science and Technology in Klosterneuburg, Grosjean wondered if the answer lay in the carbon-containing molecules present on the particles’ surfaces. Such molecules are ubiquitous in nature, and materials scientists typically strive to minimize these contaminants. Yet, Grosjean and his colleagues tracked how cleaning the samples affected their electrification.
Using ultrasound, they levitated a minute silicon dioxide particle, allowed it to bounce once off a target made of the identical material, and subsequently measured its charge. “It could become positive or negative. If it was positive, we would heat or clean it and repeat the experiment—and then it would charge negatively,” Grosjean explains.
Analysis of the samples revealed that the removal of carbon-containing molecules was, in fact, the decisive factor. “We observed that this effect overshadowed everything else,” Grosjean remarks.
Further evidence was provided by the fact that a cleaned sample resumed a positive charge after about 24 hours, aligning with the rate at which a new layer of carbon-containing molecules formed from the air.
Daniel Lacks of Case Western Reserve University in Cleveland, Ohio, finds this research compelling. “People are aware that surfaces accumulate a lot of debris. But I’ve never seen it manifest so clearly in triboelectric charging,” he notes.
He expresses concern that this discovery might be unwelcome news for physicists. If carbon contamination governs the direction of charging, precisely calculating how particles charge up will become extremely difficult. “Prediction might simply never materialize,” Lacks concludes.
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