Finite Packing of Shape-Anisotropic Colloidal Particles

Small aggregates (or clusters) of silica doublet or dumbbell particles were produced inside oil-in-water or water-in-oil emulsions. First, forced aggregation of silica microsphere suspension was induced under a high concentration of ammonia, followed by the addition of tetraethylorthosilicate for bonding and fixation of the particle aggregates. Then, density gradient centrifugation was applied for the fractionation of silica doublets or dumbbell particles. The resultant shape-anisotropic particles, which were re-dispersed in oil phase such as hexane or toluene after surface modification with octadecyltrmethoxysilane, were emulsified and small clusters of silica doublets or dumbbells were obtained by evaporation-driven self-assembly. Most configurations of the doublet clusters are similar to minimal second-moment clusters from singlet particles. However, for a few particular cases of n = 6, 8, and 12, we observed some isomeric structures. In addition, the pear-shaped microparticles as asymmetric dimers were also assembled by self-organization of the shape-anisotropic particles inside aerosol droplets by electrospray-assisted atomization, and the resultant configurations were quite similar to the clusters of simple microspheres.