Physicists have identified the role of carbon-containing molecules in determining charge polarity during particle collisions in volcanic ash clouds. This discovery explains the triboelectric effect that leads to volcanic lightning. The finding comes from experiments with silicon dioxide particles.
Volcanic lightning arises when particles in ash clouds collide and exchange electric charge through the triboelectric effect, separating into positively and negatively charged groups that discharge as lightning. For decades, researchers puzzled over why identical silicon dioxide particles charge differently upon contact—one positive, one negative—breaking the expected symmetry. Candidates like humidity, surface roughness, or crystalline structure had been proposed but not confirmed. > “There are a lot of candidates,” says Galien Grosjean, now at the Autonomous University of Barcelona. “People suspect that humidity is important, or roughness, or the crystalline structure.” Grosjean, while at the Institute of Science and Technology Austria in Klosterneuburg, and colleagues tested the influence of surface contaminants. Using ultrasound to levitate silicon dioxide particles, they let each bounce once on a matching target plate and measured the charge. Cleaning samples by baking removed carbon-containing molecules, reversing charges from positive to negative. > “It might charge positive or negative. If positive, we would bake or clean it and redo the experiment – and then it would charge negative,” says Grosjean. Analysis confirmed carbon molecules as the dominant factor, overriding others. Cleaned samples regained positive charge after about a day, matching the time to reacquire airborne carbon contaminants. Daniel Lacks at Case Western Reserve University in Cleveland, Ohio, praised the work. > “People know surfaces have a lot of crap on them. But I’ve never seen that come up in triboelectric charging,” he says. Lacks noted challenges: contamination may hinder precise predictions of charging direction. The study appears in Nature (DOI: 10.1038/s41586-025-10088-w).
