We explore the crystallization of charged colloidal particles in a nonpolar solvent mixture. We simultaneously charge the particles and add counterions to the solution with aerosol-OT (AOT) reverse micelles. At low AOT concentrations, the charged particles crystallize into body-centered-cubic (bcc) or face-centered-cubic (fcc) Wigner crystals; at high AOT concentrations, the increased screening drives a thus far unobserved reentrant melting transition. We observe an unexpected scaling of the data with particle size, and account for all behavior with a model that quantitatively predicts both the reentrant melting and the data collapse. Colloidal particles can spontaneously form structures that exhibit long-range ordered states, making them a fascinating system for fundamental studies of crystal phase behavior [1][2][3][4]. The majority of studies focus on colloids which model the hard-sphere interaction, a strong repulsion that prevents particles from overlapping, whose range is restricted to contact [5]. Hard-sphere crystallization is driven through purely entropic effects, and the phase behavior is well studied [5][6][7][8][9]. Typically, the colloids used for these studies are stericallystabilized polymeric particles in nonaqueous solvents, which can match both the density ρ and refractive index n of the particles, enabling confocal microscopy to be used for these investigations. Even earlier studies focused on charged particles, where crystallization is driven by strong long-range repulsive interactions arising from Coulombic charges on the particles [1,[10][11][12][13][14][15][16][17][18][19][20][21][22]. These studies were performed on particles in aqueous solvents, which makes charge effects much easier to induce, but precludes index matching, limiting the use of optical techniques except at very low densities. Instead, x-ray scattering studies [23] showed a fascinating phase behavior of Wigner crystals, including a body-centered-cubic (bcc) phase at low concentration, and a solid-solid transition to a face-centered-cubic (fcc) phase at higher densities [15][16][17][18]24,25]. It is also possible to induce charge on particles in nonaqueous solvents through the addition of charge control agents [21,[26][27][28]. However, in this case, there is strong coupling between the charge on the particle surface and the ions in solution. Charge-induced crystallization should still be expected, although new behavior may also occur as a consequence of this coupling. Nevertheless, these systems have never been investigated experimentally, and the range of potential behavior has not yet been explored.In this Rapid Communication, we investigate the crystallization behavior of colloidal particles in nonpolar solvents. By adding aerosol-OT (AOT) reverse micelles, we both charge the particles and add ions to the solution. As AOT concentration increases from zero, the system undergoes a first transition from fluid to a charge-stabilized bcc Wigner crystal, and * Corresponding author: plu@fas.harvard.edu then a second solid-sol...