Collective Motion in a Dusty Plasma

Dusty plasmas containing micron-sized particles occur naturally in solar nebulae, planetary rings, and comet tails, and can be studied in controlled laboratory experiments in 2D or 3D. The organization of dust in the plasma can be amorphous or crystalline, and 2D dusty plasmas offer the opportunity to directly image the rearrangement of dust grains; these rearrangements are complex in the crystal state, and similar to the collective motion that occurs in glass forming or jamming materials. Here we take advantage of the fact that dust interactions can be quantitatively modelled using a simple Yukawa potential to simulate the dynamics of a 2D dusty plasma in amorphous and crystalline states. In the crystal, we find that dust grains diffuse through highly collective rearrangements that are similar to the string-like rearrangements occurring in glass-forming materials. By mapping the trajectory of dust grains to their corresponding energy-minimized states, we remove thermal noise which makes it possible to identify collectively rearranging dust grains without needing to employ any ad hoc criteria, eliminating the selection ambiguity that is challenging in many condensed phased materials. In doing so, we explore the interplay between structural defects and collective motion and also show that the number of defects characterizes distinct energy levels of the crystal.