Near-Field Capillary Interaction and Equilibrium Microstructure of Colloidal Rods: Cylinders and Ellipsoids

Owing to their anisotropic shape, rod-like particles –including ellipsoids and cylinders-adsorbed at fluid-fluid interfaces experience strong, anisotropic capillary interactions.  While far from contact, the trajectories of approach of ellipsoids and cylinders are similar, their equilibrium configurations upon assembly are not. Cylinders assemble end-to-end forming stiff linear chains. Ellipsoids assemble side-to-side to form highly flexible “worm-like” structures. We explain why small differences in shape between cylinders and ellipsoids (e.g. the presence of faceted features in the case of the cylinder, and the sharpness of its edges) can lead to dramatic differences in near-field capillary interactions, and relate these interactions to the mechanics of the particle aggregates. These results have important implications for the rheology of particle-laden monolayers.