Dynamical impact of fractal-shaped ice grains on a cryogenic dusty plasma experiment*

Ice grains are formed and contained in a capacitively-coupled plasma with cryogenically cooled electrodes. The grains exhibit an elongated dendritic shape that grows fractally in time with a 2-D fractal dimension of about 1.5. We measure ice grain trajectories in the plasma afterglow using high-speed imaging and use laser-induced fluorescence to measure neutral temperature gradients which cause a thermophoretic force on the grains. We find that the fractal growth significantly affects the dynamics, as the scaling laws of mass, charge, and cross-section depend on the fractal dimension. These cause the thermophoretic and drag forces to dominate gravity  until the grains grow to macroscopic sizes. These insights are expected to impact models of ice dust dynamics in both astrophysical and laboratory contexts.