FREEZING OF BINARY-MIXTURES OF HARD-SPHERE COLLOIDS

Binary mixtures of suspended colloidal spheres of size ratio alpha = 0.58, whose interaction is close to that of hard spheres, undergo freezing transitions into ordered superlattice structures. At different number ratios of the two species, both the AB2 (atomic analogue AlB2) and the AB13 (atomic analogue NaZn13) structures are found. The structures were identified by both ''powder light crystallography'' and electron microscopy of dried samples. The phase diagram contains three eutectics. The freezing transitions in this conceptually simple binary mixture are driven entropically: put simply, at a given concentration and number ratio, the system adopts the ''most efficient'' packing of the spheres.