Nonequilibrium Structure for Shocks in a 2D Dusty Plasma

A single horizontal layer of negatively charged microspheres was levitated in a glow-discharge plasma, to make a 2D dusty plasma. Under steady conditions, the microspheres self-organized into a crystal, which had a hexagonal lattice, composed of equilateral triangles. This equilibrium condition was then disturbed by causing a compressional shock wave to propagate through it. The shock was generated by a moving exciter, which was a wire that had a negative bias, and moved at a supersonic speed while repelling nearby microspheres. Using a high-speed video camera viewing from above, we recorded images of the layer of microspheres, and we analyzed these images to obtain the x-y coordinates of microspheres. We then characterized the microstructure using Voronoi diagrams and polygon analysis. We found that as the hexagonal lattice deformed, under the considerable compression of the shock, the microspheres were no longer arranged in a lattice with hexagonal structure, but instead tended toward a trapezoidal lattice, with a nearly square symmetry. This nearly square lattice was observed within the shock layer, and immediately behind it. We note that a square lattice is never observed under steady conditions because it is unstable.