Tangential forces between nontouching colloidal particles

By measuring the orientation of colloidal doublets suspended in water under the opposing alignment forces of gravity and electrophoresis, we observed tangential forces between nontouching particles. We found a new phenomenon of time-dependent switching between “slipping” (no tangential force) and “sticking” (tangential forces producing rigid-body behavior of the doublet) for heterodoublets of silica/polystyrene. Tangential forces between particle surfaces across a fluid gap and their transient nature are important to the dynamics of colloidal suspensions and the evolution of floc morphology.