Self-ordering to imposed ordering of dust—A continuous spatial phase transition experiment in MDPX

Previous experiments conducted in the Magnetized Dusty Plasma eXperiment revealed an intriguing phenomenon first referred to as imposed ordering. This occurs when micrometer-sized dust particles become aligned with the geometry of a conducting mesh placed above the dust (at a distance much larger than the plasma Debye length or the ion–neutral or electron–neutral mean free paths) in the presence of a strong magnetic field perpendicular to the mesh. In this work, results of a transition experiment are presented wherein starting from a classical two-dimensional Coulomb crystal with hexagonal symmetry in an unmagnetized plasma (B=0 T), dust transitions to a state in which it flows along the geometry of a conducting mesh placed above it, mapping out the fourfold symmetry of the boundary condition. It is hypothesized that beyond a certain magnetization, elongated electric potential structures emanating from the mesh grow strong enough to drive the dust motion to reflect the mesh morphology, gradually transitioning from a sixfold self-ordering to fourfold imposed ordering. The various dust phases are quantified, and a critical value of magnetic field is identified in the transition experiment, indicating the onset of imposed ordering over the whole dust cloud.