The Dynamics of Fractal Ice Grains in Cryogenic Plasma

Ice grains formed within the Ice Dusty Plasma Experiment at Caltech have unique fractal morphologies. This complicates dynamical behavior, as their collisional cross-sections, masses, and electric charges differ from those of spherical grains. By tracking grain trajectories in the plasma afterglow, we measure grain mass and cross-section and find that they follow fractal scaling laws. This alters the fundamental force balance in a plasma such that gravitational settling is reduced and drag forces are enhanced. These effects become increasingly prominent with grain growth and drive dynamics unattainable by compact grains. Consequently, fractal grains couple more strongly to both surrounding gas and ion flows, mediating momentum transfer between the ions and the gas. Our findings quantify the impact of fractal morphology on grain behavior in plasma and reveal implications ranging from nanoparticle synthesis to astrophysical dust transport.