Trapping particles to explain lightning

Using lasers as tweezers to understand cloud electrification might sound like science fiction but at the Institute of Science and Technology Austria (ISTA) it is a reality. By trapping and charging micron-sized particles with lasers, researchers can now observe their charging and discharging dynamics over time. This method, published in Physical Review Letters, could provide key insights into what sparks lightning.

Aerosols are liquid or solid particles that float in the air. They are all around us. Some are large and visible, such as pollen in spring, while others, such as viruses that spread during flu season, cannot be detected by the naked eye. Some we can even taste, like the airborne salt crystals we breathe in at the seaside. 

PhD student Andrea Stöllner, part of the Waitukaitis and Muller groups at the Institute of Science and Technology Austria (ISTA), focuses on ice crystals within clouds. The Austrian scientist uses model aerosols—tiny, transparent silica particles—to explore how these ice crystals accumulate and interact with electrical charge.

Stöllner, alongside former ISTA postdoc Isaac Lenton, ISTA Assistant Professor Scott Waitukaitis and others, has developed a way to catch, hold, and electrically charge a single silica particle using two laser beams. This approach holds potential for application in different areas, including demystifying how clouds become electrified and what sparks lightning.

Laser tweezers lock aerosol particle in place

Andrea Stöllner stands in front of a large desk covered with shiny metal gadgets. Green laser beams cut across the space, bouncing around through a series of small mirrors. A squishing sound comes from the table, like air escaping from a tire. “It’s an anti-vibration table,” Stöllner explains, noting its crucial role in absorbing any vibrations from the room and nearby equipment—essential for precision work with lasers.

The beams zigzag around a type of obstacle course, eventually converging into two streams that funnel into a container. Here, the two beams meet and create a ‘trap,’ where tiny objects are held steadily by light alone, acting as “optical tweezers.” Inside this magical box, particles drift past these tweezers. Suddenly, boom! A green flash appears, signaling success: A perfectly round, vibrant green glowing aerosol particle has been caught and is being held tightly by the tweezers.

“The first time I caught a particle, I was over the moon,” Stöllner says as she recalls her Eureka moment two years ago, just before Christmas. “Scott Waitukaitis and my colleagues rushed into the lab and took a short glimpse at the captured aerosol particle. It lasted exactly three minutes, then the particle was gone. Now we can hold it in that position for weeks.”

It took Stöllner almost four years to get the experiment to the point where it could provide reliable data, starting with a previous version of the setup developed by her former ISTA colleague Lenton. “Originally, our setup was built to just hold a single particle, analyze its charge, and figure out how humidity changes its charges,” explains Stöllner. “But we never came this far. We found out that the laser we are using is itself charging our aerosol particles.”

Kicking out electrons

The scientist and her colleagues discovered that lasers charge the particle through a “two-photon process.”

Typically, aerosol particles are close to neutrally charged, with electrons (negatively charged entities) swirling around in every atom of the particle. The laser beams consist of photons (particles of light traveling at the speed of light), and when two of these photons are absorbed simultaneously, they can ‘kick out’ one electron from the particle. In this way, the particle gains one elemental positive charge. Step by step, it becomes increasingly positively charged.

For Stöllner, uncovering this mechanism is an exciting discovery that she can leverage in her research. “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.”

This observation also reveals that, as the particle becomes positively charged, it begins to discharge, meaning that it occasionally releases charge in spontaneous bursts.

Way above our heads, something similar might also be happening in clouds. 

Lifting the lid on lightning?

Thunderstorm clouds contain ice crystals and larger ice pellets. When these collide, they exchange electric charges. Eventually, the cloud becomes so charged that lightning forms. One theory suggests that the first little spark of a lightning bolt could be initiated at the charged ice crystals themselves. However, the exact science behind the phenomenon of lightning formation remains a mystery. Alternative theories meanwhile suggest cosmic rays initiate the process as the charged particles they create accelerate from pre-existing electric fields. According to Stöllner, however, the current understanding in the scientific community is that – in either case – the electric field in clouds seems too low to cause lightning.

“Our new setup allows us to explore the ice crystal theory by closely examining a particle’s charging dynamics over time,” Stöllner explains. While ice crystals in clouds are much larger than the model ones, the ISTA scientists are now aiming to decode these microscale interactions to better understand the big picture. “Our model ice crystals are showing discharges and maybe there’s more to that. Imagine if they eventually create super tiny lightning sparks—that would be so cool,” Stöllner says with a smile. 

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