Austrian scientists using powerful lasers to study the charges of individual, suspended aerosol particles like those found in clouds have made an accidental discovery, which they say may finally unravel the mystery behind what sparks potentially deadly lightning strikes.
The Institute of Science and Technology Austria (ISTA) research team behind the unexpected discovery is expanding its future research to evaluate particle charging dynamics over time, with the goal of further unraveling the unexplained mechanisms that cause lightning to form in clouds.
Aerosols are individual solid or liquid particles floating in the air, such as airborne pollen, invisible viruses, ice crystals within clouds, or the tiny salt crystals one can taste when inhaling seaside air. For their study, the ISTA team, which included former ISTA postdoc Isaac Lenton, ISTA Assistant Professor Scott Waitukaitis, and others, used model aerosols made up of tiny, transparent silica particles “to explore how these ice crystals accumulate and interact with electrical charge.”
According to PhD student Andrea Stöllner, their experiments weren’t initially intended to study aerosol particle electrification. Instead, the researcher said their setup was designed to suspend a single particle, analyze its electrical charge, and “figure out” how humidity alters its charges.
“But we never came this far,” Stöllner said.
To capture and analyze a single silica particle, the ISTA team built an experiment where several laser beams are redirected around what they described as a tabletop “obstacle course” before converging into two distinct streams. These two laser streams are funneled into a box where particles suspended in aerosols can drift past. When the two streams converge, they operate like a pair of optical tweezers capable of capturing and suspending a single particle with light.
“The first time I caught a particle, I was over the moon,” Stöllner said when describing the December 2023 event. “Scott Waitukaitis and my colleagues rushed into the lab and took a short glimpse at the captured aerosol particle.”
Although the first capture lasted only three minutes, Stöllner said the team’s improved methods can hold a particle in suspension “for weeks.”
When analyzing their trapped aerosol particle, the team found something unexpected. In addition to holding the particle in place so the team could evaluate its charge dynamics, the laser was interacting with the particle’s charge.
“We found out that the laser we are using is itself charging our aerosol particles,” Stöllner explained.
Upon further analysis, they found that the laser was charging the particle via a “two-photon” process. Specifically, the individual photons that make up the laser interact with the electrons orbiting the atoms within the particle, including “kicking out” one electron when the particle absorbs two photons simultaneously. The team said this ejection of a negatively charged electron causes the particle to gain “one elemental positive charge.”
“Step by step, it becomes increasingly positively charged,” they explained
With further experimentation, the team discovered not only that they could observe the charging caused by the lasers, but also control it.
“We can now precisely observe the evolution of one aerosol particle as it charges up from neutral to highly charged and adjust the laser power to control the rate,” Stöllner explained.
Although understanding the charge dynamics of aerosols could have several practical scientific applications, the team said one potential use is “demystifying” how clouds become electrified and “what sparks lightning.” For example, during their lab experiments, the ISTA team discovered that particles immediately begin to discharge as they are becoming charged. Because this discharge occurs in occasional, spontaneous “bursts,” the team suspects something similar may be happening in the clouds, where ice crystals and larger pellets collide and exchange electrical charge, until a massive release of energy triggers lightning.
“One theory suggests that the first little spark of a lightning bolt could be initiated at the charged ice crystals themselves,” they note. “However, the exact science behind the phenomenon of lightning formation remains a mystery.”
Although several alternate theories of lightning formation have been proposed, including linking the phenomenon to cosmic rays, Stöllner said the current scientific consensus is that the electric field in clouds is too low to cause lightning in either theory. Either way, the researcher is already preparing experiments to further investigate a potential connection to lighting, including exploring the ice crystal theory by “closely examining a particle’s charging dynamics over time.”
“Our model ice crystals are showing discharges, and maybe there’s more to that,” Stöllner said. “Imagine if they eventually create super tiny lightning sparks—that would be so cool.”
The study “Using optical tweezers to simultaneously trap, charge, and measure the charge of a microparticle in air” was published in Physical Review Letters.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.