The packing and aggregation of colloidal particles is important for a wide variety of applications, including biological assays , sensors, paints, ceramics, and photonic crystals. [1±4] Over the years, different methods have been developed for controlling the structure and aggregation of large numbers of colloi-dal particles, thereby enabling the fabrication of coatings, artificial opals, and complex ceramic bodies. By contrast, relatively few methods exist for controlling the aggregation and structure of small numbers of colloidal particles. Motivated by recent interest in colloidal self-assembly for optical applications , a few groups have started developing schemes for making aggregates consisting of a small number of monodisperse colloidal microspheres. [5±7] These small colloidal aggregates, which include dimers, tetrahedra, and more complex polyhe-dra, possess lower symmetry than the spheres from which they are made and offer the possibility of forming more complex colloidal phases and structures than can be realized using simple spheres, just as molecules form more complex phases and structures than do atoms. It has been suggested, for example, that tetrahedral colloidal clusters might be useful in developing schemes for assembling colloidal crystals in the diamond structure. [5,6] Colloidal crystals with the diamond structure are predicted to exhibit a full photonic bandgap with many desirable properties. [8] We recently demonstrated a process that is capable of making a large number of identical clusters, approximately 10 8 ±10 10 in the original experiments, [6] where the number of spheres (n) in each cluster can be varied between approximately 2 and 15. The process is based on emulsifying a suspension of lightly crosslinked polystyrene microspheres in toluene with an aqueous surfactant solution. This yields a toluene-in-water emulsion with the polystyrene microspheres bound by surface tension to the droplet interfaces. When the toluene is removed by evaporation, the particles form stable clusters of colloidal particles suspended in water. The particles within clusters are strongly bound together by the van der Waals' force, while cluster±cluster aggregation is prevented by the surface charge the clusters acquire when the sulfate groups covalently bonded to the particle surfaces dissociate in water. Clusters of different aggregation number are readily fractio-nated using density gradient centrifugation to produce mono-disperse suspensions of clusters. The shapes of the different aggregates for 2 £ n £ 11 correspond to compact packings that minimize the second moment of the mass distribution, defined as M P n i1
r i À r cm 2 (1) …