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