Sure, wizards can suspend objects in midair with wingardium leviosa. But researchers in Dr. Margaret Tolbert’s lab don’t need spells; they have science.
Nestled in the corner of the Cooperative Institute for Research in Environmental Sciences (CIRES), graduate students study atmospheric chemistry by levitating small specks. The Tolbert Research Group studies clouds and atmospheric pollution one droplet at a time by holding them in midair with lasers.
Using “optical trapping”—commonly called optical tweezers—to study micro-scale objects is an invaluable tool that is gaining momentum in several fields, from biophysics to geoscience. Researchers use this technology by putting a small particle between two laser beams, like a pair of chopsticks.
“Optical tweezers” can trap a small particle in place. (Image: Thor Labs)
Light is made up of photons that all carry a small amount of energy and momentum. This means if someone is shining a laser beam at you, they are also pushing on you with a very small force. Photons that are all the same wavelength in a laser are called coherent, meaning the crests and troughs of light waves are aligned perfectly, which enables them to exert force together. Coherent beams of light can be precisely controlled and generate forces large enough to move tiny objects like nanoparticles and biological cells.
The Tolbert group’s optical tweezer system has two lasers, one on top of the other. One laser applies an upward force against gravity, and the other applies a force down to keep the particle from escaping. The forces generated are reaction forces based on the change in momentum that results from changing the direction of the laser beam. The light starts by traveling straight line as it approaches a tiny particle. After passing through it, the light has acquired some momentum to the left or right by refracting, but the total momentum in the system must remain the same so the particle is also pushed to the left or right. When there are two lasers pointed at each other, the forces to the left and right cancel each other out, leading to a force that pushes and pulls the particle into the focus of the beam.
The Tolbert group are experts in precise control of lasers. “We look at aerosols, which are little droplets of water, or liquid, or salt particles,” explains graduate student Shuichi Ushijima. The phase state of different chemicals in our air—in other words, whether they are liquid or solid—have a huge effect on our climate. Ushijima’s research focuses on inorganic materials that are soluble in water.
In the context of Earth’s atmosphere, Ushijima studies salts that become dissolved in the humidity in the air. Whether these salts are solid or liquid depends on the relative humidity of the environment, but Tolbert’s research group investigates how pollutants that hog atmospheric space influence the phase state of salts. The phases of aerosols have drastic effects on the climate, such as the formation of clouds.
Ushijima uses these light tweezers to hold one salty water droplet at a time by itself in a humidity-controlled environment. He then slams a speck of dust into the droplet to see if the salt becomes a solid, which is called crystallization. Observing how pollution changes the crystals of salt in clouds is the gateway to understanding the exponentially worsening conditions in our atmosphere.
The Tolbert lab uses this equipment to levitate aerosols.
“There’s another method used to look at single levitated particles, using something called electrodynamic balance,” said Ushijima, “but to do that you need to supply a lot of charge, and use huge magnetic plates. All we need is a laser.” T
his creative levitation method opens the door to studying small or large salt particles at a variety of pressures and widely varying relative humidity, all at relevant timescales to the interaction of atmospheric particles with air pollution. As an added bonus, Ushijima does all of his phase-state analysis by observing how reflective the particles are with a camera, producing unique images and videos of phase changes. This special package of experimentation and analysis has led to four major publications in the last three years for the Tolbert group.
This tool’s popularity is snowballing. This years’ Nobel Prize in Physics was awarded to three exceptional scientists, one of which is Arthur Ashkin for the development of optical tweezers. This technology illustrates the precise control that lasers bring to science. Ashkin invented the tool itself back in the 1970s at Bell Labs, but it’s the application of the tools to study complex systems makes these particular lasers revolutionary. The types of tools available to suspend objects without touching them is limited, making the Tolbert group’s investment in optical tweezers a natural yet savvy choice.
So where are they headed next? “We want to figure out if liquid water is stable on Mars,” Ushijima said, armed with a shiny new graduate fellowship from NASA. Satellite evidence points to liquid water on the Red Planet, but the temperature and pressure forces water to be either gas or ice. However, the high prevalence of salt in the soil may provide an explanation for Mars’ relationship with water.
Ushijima and the Tolbert research group at CU Boulder are looking to build the story of water on Mars, one drop at a time.
By Kaitlin McCreery
Science Buffs 