Superlattice formation in mixtures of hard-sphere colloids

Abstract

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 ${\mathrm{LS}}_2$ and ${\mathrm{LS}}_{13}$ superlattices, isostructural to the molecular compounds ${\mathrm{AlB}}_2$ and ${\mathrm{NaZn}}_{13},$ while at $\alpha =0.72\mathrm{}$ 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.