Physicists have finally unraveled a long-standing enigma concerning the process that generates lightning within volcanic eruptions. The question has been precisely why, when particles of similar composition interact, some acquire a positive electrical charge while others gain a negative one.
This phenomenon is rooted in the triboelectric effect, which describes the transfer of electric charge when two objects come into contact. It’s the same principle that causes hair to cling to a balloon after they’ve been rubbed together.
Within a volcanic ash cloud, particles composed of silicon dioxide are in constant motion. As these particles collide, they exchange electrical charges. The subsequent separation of these positively and negatively charged entities is what ultimately leads to lightning when an electric current flows between them.
However, scientists struggled to explain the underlying mechanism that breaks the symmetry between identical particles, dictating the direction of charge flow. Previous hypotheses suggested various contributing factors.
“There are numerous potential explanations,” stated Galien Grosjean, currently affiliated with the Autonomous University of Barcelona. “Researchers have suspected that humidity, surface roughness, or the crystalline structure of the particles play significant roles.”
While working at the Institute of Science and Technology Austria, Grosjean explored a different avenue, postulating that carbon-containing molecules present on the particle surfaces might hold the key. These molecules are prevalent in natural environments and are often considered contaminants by materials scientists, who typically aim to minimize their presence.
Grosjean and his colleagues observed the impact of cleaning their samples on the resulting electrification. Using an ultrasound device, they suspended a small silicon dioxide particle. This particle was then allowed to make a single impact with a target plate made of the same material, after which its electrical charge was measured.
“The particle could charge either positively or negatively,” Grosjean explained. “If it charged positively, we would then bake or clean the sample and repeat the experiment. Following this, it would predictably charge negatively.”
Subsequent analysis of the treated samples revealed that the elimination of carbon-containing molecules was indeed the decisive factor. “We observed that this effect superseded all other influences,” Grosjean noted.
Further evidence emerged from the observation that a cleaned sample would regain a positive charge after approximately a day. This timeframe coincided with how quickly the sample assimilated a fresh layer of carbon molecules from the surrounding air.
The findings of this study have garnered appreciation from experts. Daniel Lacks at Case Western Reserve University described the research as impressive. “It’s widely understood that surfaces carry a significant amount of extraneous material. However, I had not previously encountered this factor being identified in triboelectric charging,” he commented.
Lacks expressed concern that this discovery might present challenges for physicists. If the direction of electrical charging is dictated by carbon contamination, then precisely predicting how particles become charged could become exceptionally difficult. “Accurate prediction may simply be an unattainable goal,” Lacks concluded.