Disorder Foreshadows Order in Colloidal Cubes

Prior molecular simulations have revealed that dense suspensions of hard nano-cubes are remarkable in that they form a mesophase near the ordering transition which is highly unusual when compared to typical crystals, plastic crystals, or liquid crystals. Monte Carlo simulations are used to study the disorder-to-order phase transition for a bulk system of colloidal hard cubes. It is observed that the ordered phase does not form via nucleation from the disordered phase, despite the prevalence in the latter of small locally ordered domains occurring transiently and sparsely. Instead, as the isotropic phase is brought to a marginally metastable concentration, such ordered domains increase in size and number, eventually reaching a critical point where they percolate the entire system and consolidate to form the mesophase. This process, quite different from nucleation observed for other particle shapes, arises presumably from a small interfacial tension. Simulations reveal that cubes infallibly form single-grain ‘crystals’ in the mesophase. This is traced to the abundant delocalized and transient vacancies and the high particle mobility in the mesophase which lead to formation of an active ‘recrystallization zone’ next to any grain boundary.