Hydrodynamic Interactions May Enhance the Self-Diffusion of Colloidal Particles

We report experimental evidence for an enhancement, due to the hydrodynamic interactions (HI), of the self-diffusion function $D_s\left(t\right)$ of colloidal particles at intermediate and long times. Monolayers of paramagnetic polystyrene spheres confined to an air/water interface are studied using digital videomicroscopy. The interparticle potential tuned by an external magnetic field is accurately calibrated by comparing measured radial distribution functions with computer simulation results. This allows one to separate the effects of HI on $D_s\left(t\right)$ from those of the direct interactions.