A NEUTRON-SCATTERING STUDY OF THE STRUCTURE OF A BIMODAL COLLOIDAL CRYSTAL

We have studied the freezing of a binary mixture of colloidal poly(methyl methacrylate) spheres of size ratio 0.31 and composition AB4 (here A refers to the larger spheres). When suspended in a suitable liquid these particles interact via a steeply repulsive (approximately hard sphere) potential. The structure of the colloidal crystals formed in this binary system has been established from a combination of small-angle neutron and light scattering measurements. We find that there is an almost complete size separation on freezing. The crystalline phase contains almost exclusively large spheres while the smaller spheres are excluded from the crystal into a coexisting binary fluid. This observation is in agreement with recent density functional calculations for the freezing of hard sphere mixtures.