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 Do is the Stokes-Einstein diffusion coefficient in the bulk suspension of PMMA spheres in decalin. The measured values of a are found to be in good agreement with the numerical calculation for the drag coefficient of interfacial particles. The measured values of P differ from that obtained for bulk suspensions, indicating that hydrodynamic interactions between the particles have interesting new features at the interface.