This paper describes the phase separation of millimetre-scale spheres based on electrostatic charge. Initially, polymeric (Teflon, T;6, N) and metallic (gold-coated 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.This communication describes the separation of a mixture of ~3mm spheres 6, N; Teflon, T; and gold-coated Nylon, Au N )-on agitation in two-dimensions-into an ionic crystalline, two-dimensional (2D) phase having the composition T -N + , and a disordered phase comprising Au N spheres. 1 Teflon and Nylon spheres, when agitated together, charged oppositely by contact electrification. 2,3 These polymeric spheres aggregated and separated from the gold-coated spheres, which were, on average, electrically neutral (Fig. 1). The objectives of this research were: (i) to test our prediction that mixtures of oppositely charged objects would crystallize and separate from non-charged objects when agitated, and (ii) to explore the effects of experimental parameters (e.g., amplitude and frequency of agitation, and density of spheres) on the separation.Contact electrification is the process by which electrostatic charge transfers between objects when their surfaces contact and separate. 2,3 When charged, one object has a positive charge and the other a negative charge. From our work with ionic electrets-materials that have a covalently bound ion and a mobile counterion-we have shown that the transfer of ions can lead to contact electrification. 3,4 We have demonstrated that contact electrification can direct the self-assembly of micron-sized ionic electrets into ordered three-dimensional microstructures, 4 and millimetre-sized spheres into ordered 2D rings and lattices. 5,6 † This paper is part of a Soft Matter theme issue on Self-Assembly. Guest editor: Bartosz Grzybowski. ‡ Electronic supplementary information (ESI) available: Experimental procedures; derivation of eqn (1), the limits of separation (Φ), and the scaled separation (Φ*); Fig. S1 This work, which describes the separation of ordered 2D Coulombic crystals of charged, millimetre-sized T and N spheres from less-charged Au N spheres, is important for at least three reasons: (i) the system of charged and uncharged spheres provides a simple, physical model for nucleation of crystals, and thus for phase separation by formation of a cr...