Colloidal hard dumbbells under gravity: structure and crystallization

We study the structure and phase behavior of hard dumbbells under gravity. The fluid shows layering near the wall, where subsequent layers of dumbbells align alternatingly parallel or perpendicular to the wall. We observe coexistence of a fluid with a plastic crystal (PC) and an aligned crystal (CP1) in a single sediment for short dumbbells. For longer dumbbells, we observe a direct fluid-CP1 coexistence, while for dumbbells of almost tangent spheres, the aperiodic crystal phase appears in between the fluid and CP1 phase. The locations of the coexistences between these phases are well described by a simple expression based on an approximation similar to the local density approximation, which has been successfully applied to hard spheres under gravity [M. Marechal and M. Dijkstra, Phys. Rev. E, 2007, 75, 061404]. The fluid-PC-CP1 coexistence could not be explained using this expression. We attribute this discrepancy to a lattice constant mismatch between the PC and CP1 phases. Finally, we show using direct simulations that the plastic crystal stacks preferably as the hexagonal close packed crystal for short dumbbells, as is the case for a bulk plastic crystal [M. Marechal and M. Dijkstra, Phys. Rev. E, 2008, 77, 061405].