Freezing of binary mixtures of colloidal hard spheres

The freezing phase transition in a binary suspension of colloidal hard spheres of diameter ratio α=0.61 was studied by light scattering and scanning electron microscopy. The suspensions consisted of sterically stabilized poly(methyl methacrylate) spheres of diameters about 670 and 407 nm suspended in a near refractive indexed matched suspension medium composed of carbon disulphide and cis-decalin. With increasing volume fraction, binary suspensions of number fraction of larger component A xA>0.43 crystallized to give irregularly stacked close packed crystals containing almost entirely component A. As the number fraction xA decreased, the rate of crystallization decreased. Suspensions of xA≊0.28 remained amorphous and showed glassy behavior. Suspensions of xA≊0.057 showed a complex sequence of phase behavior with coexistence of crystals of component B, the ordered binary alloy phase AB13, and a binary fluid. In suspensions with xA<0.057, the only solid phase observed was irregularly stacked close packed crystals of component B. The observed phase behavior is compared with the predictions of a model for freezing of a mixture of hard spheres which are assumed to be immiscible in the solid phase.