Phase separation of 2D meso-scale Coulombic crystals from meso-scale polarizable "solvent"

This paper describes the phase separation of millimetre-scale spheres based on electrostatic charge. Initially, polymeric (Teflon, T; Nylon-6,6, N) and metallic (gold-coated Nylon-6,6, Au-N) spheres are uniformly mixed in a two-dimensional (2D) monolayer on a gold-coated plate. Oscillating the plate vertically caused the spheres to charge by contact electrification (tribocharging). Positively charged N and negatively charged T spheres attracted each other more strongly than they attracted the capacitively charged, Au-N spheres. The T and N spheres formed 2D Coulombic crystals, and these crystals separated from the Au-N spheres. The extent and rate of separation increased with increasing amplitude of agitation during tribocharging, and with decreasing density of spheres on the surface. At high surface density, the T and N spheres did not separate from the Au-N spheres. This system models the 2D nucleation of an ionic crystal from a polarizable liquid.