Hydrodynamic coupling in diffusion of quasi–one-dimensional Brownian particles

We report a study of the diffusion of weakly interacting colloidal spheres in a one-dimensional groove using digital video microscopy. At a time much shorter than the direct interaction time, τ I , the spheres diffuse according to Fick's law. The packing fraction dependence of the short-time diffusion coefficient is much stronger than in unbound bulk fluids, indicative of an enhanced hydrodynamic coupling between particles in a groove. At a later time (but still shorter than τ I ) a subdiffusion (⟨Δ x 2 (τ)⟩ ∝ τ γ , where γ < 1) evolves, suggesting that the hydrodynamic coupling induces the subdiffusion. Correlated motion of spheres is identified both in the trajectories of the spheres and in the magnitude of the velocity pair-correlation.