Superlattice formation in mixtures of hard-sphere colloids

We report a detailed experimental study of the superlattice structures formed in dense binary mixtures of hard-sphere colloids. The phase diagrams observed depend sensitively on the ratio alpha = R-S/R-L of the radii of the small (S) and large (L) components. Mixtures of size ratio alpha = 0.72, 0.52, 0.42, and 0.39 are studied. The structures of the colloidal phases formed were identified using a combination of light-scattering techniques and confocal fluorescent microscopy. At alpha = 0.39, ordered binary crystals are formed in suspensions containing an equal number of large and small spheres which microscopy shows have a three-dimensional structure similar to either NaCl or NiAs. At the larger size ratio, alpha = 0.52, we observe LS2 and LS13 superlattices, isostructural to the molecular compounds AlB2 and NaZn13, while at alpha = 0.72 the two components are immiscible in the solid state and no superlattice structures are found. These experimental observations are compared with the predictions of Monte Carlo simulations and cell model theories.