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.