Short-time self-diffusion of nearly hard spheres at an oil-water interface*

Optical microscopy and multi-particle tracking are used to study hydrodynamic interactions of monodisperse polymethylmethacrylate (PMMA) spheres at a decalin-water interface. The short-time self-diffusion coefficient measured at low surface coverage has the form, $D^S_S(n) = \alpha D_0 (1- \beta n)$, where $n$ is the area fraction occupied by the particles and $D_0$ is the Stokes-Einstein diffusion coefficient in the bulk suspension of PMMA spheres in decalin. The measured values of $\alpha$ are found to be in good agreement with the numerical calculation for the drag coefficient of interfacial particles. The measured values of $\beta$ differ from that obtained for bulk suspensions, indicating that hydrodynamic interactions between the particles have interesting new features at the interface.