Icy Hot Plasmas: Fluffy, Electrically Charged Ice Grains Reveal New Plasma Dynamics

When a gas is highly energized, its electrons get torn from the parent atoms, resulting in a plasma—the oft-forgotten fourth state of matter (along with solid, liquid, and gas). When we think of plasmas, we normally think of extremely hot phenomena such as the Sun, lightning, or maybe arc welding, but there are situations in which icy cold particles are associated with plasmas. Images of distant molecular clouds from the James Webb Space Telescope feature such hot–cold interactions, with frozen dust illuminated by pockets of shocked gas and newborn stars.

Now a team of Caltech researchers has managed to recreate such an icy plasma system in the lab. They created a plasma in which electrons and positively charged ions exist between ultracold electrodes within a mostly neutral gas environment, injected water vapor, and then watched as tiny ice grains spontaneously formed. They studied the behavior of the grains using a camera with a long-distance microscope lens. The team was surprised to find that extremely "fluffy" grains developed under these conditions and grew into fractal shapes—branching, irregular structures that are self-similar at various scales. And that structure leads to some unexpected physics.

The scientists describe their work in a paper in the journal Physical Review Letters. The lead author of the paper is Caltech graduate student André Nicolov (MS '22).

"It turns out that the grains' fluffiness has important consequences," says Paul Bellan, professor of applied physics at Caltech. Once such consequence is that the irregular grains, even as they grow, contain much less mass than, say, a solid spherical grain. And, indeed, when other scientists study "dusty plasma" systems they typically inject tiny solid spherical plastic grains into the plasma.

Nicolov and Bellan observed that their fluffy ice grains quickly became negatively charged because the electrons in the plasma move much faster than their positively charged ion counterparts. "They are so fluffy that their charge-to-mass ratio is very high, so the electrical forces are much more important than gravitational forces," Bellan explains. As a result, gravity—which dominates in other experiments, causing solid grains to settle to the bottom of test chambers—is no longer the primary driver of motion.

Instead, the fluffy ice grains dispersed throughout the plasma in the chamber and underwent what Nicolov describes as a "complicated motion that seems to defy gravity." The ice grains bobbed up and down, spun, and whirled in vortices throughout the plasma in ways that were difficult to predict. That remained true even of ice grains that grew to relatively large sizes, hundreds of times larger than the solid plastic spheres previously used. In fact, the researchers say, the fluffiness increases as the grains grow larger.

Nicolov specifies "the microscopic fluffy structure of the grains impacts the motion of the whole cloud of grains and the plasma." The grains are highly confined within the plasma by an inward-directed electric field, and because they are all negatively charged, they repel each other and tend to space out evenly and do not collide. Their fluffiness causes them to interact with the surrounding neutral gas like a feather in the wind.

Bellan says this behavior might help explain how similarly charged fluffy grains interact in astrophysical environments, such as the rings of Saturn and molecular clouds. He adds that because the grains have large surface areas and high charge-to-mass ratios, they may act as intermediaries capable of transferring momentum from electric fields to the neutral gas around them. "You could make a wind where the electric field pushes the dust grains, which then push the neutral gas," he says. The tiny fluffy grains, therefore, might even be responsible for gas and dust streaming across the galaxy.

The findings might also be useful in semiconductor manufacturing, where dust spontaneously formed inside industrial plasmas can deposit on tiny features of the electronic chips being fabricated and so render the chips useless. Understanding the fractal growth and motion of grains within plasma systems could improve strategies for controlling or removing them. "If you want to control the grains, you have to take into account this fractal nature," Nicolov says.

Along with Bellan and Nicolov, former Caltech postdoctoral scholar Seth Pree is also an author of the paper, "Dynamics of Fractal Ice Grains in Cryogenic Plasmas." The work was supported by the National Science Foundation (NSF) and the NSF/Department of Energy Partnership in Plasma Science and Engineering.