Orientational Order and Kinetics in the Self-Assembly of Colloidal Ellipsoids

The axial asymmetry of oblate spheroids promotes bottom-up self-assembly into tilted structures with long-ranged orientational order. We use high-speed 3D confocal imaging to directly observe the self-assembly of a model system of monodisperse colloidal PMMA oblate spheroids with aspect ratio of 0.46, obtained by applying a uniaxial thermomechanical compression. The stable spheroids are suspended in a refractive index and density-matched solvent and a long-ranged depletion attraction is induced by the addition of polystyrene. We induce orientational aggregation in these assemblies by systematically varying the packing fraction and the depth of the attractive potential. The phase behavior of the self-assembled structures are quantified by measuring the fraction of particles in clusters with and without orientational order. In addition, we seek to deduce the mechanism of orientational aggregation by studying the cluster and orientational aggregation kinetics of these assemblies.