Résumé:

Using particle tracking by optical microscopy, diffusion coefficient of polystyrene and silica particles were measured at the air/water interface. Diffusion is faster at interface than in the bulk water (D2D > D3D) because a significant fraction of the surface area of the particle is submitted to the friction of a low viscosity fluid. To quantitatively account for these experimental results it is crucial to precisely determine the immersion of the particle or equivalently its contact angle. To this purpose we coupled particle tracking and phase shift interferometry, so that the contact angle, the radius and diffusion coefficient are measured on the same particle. Small but significant and reproducible discrepancy between experimental results and theoretical models suggest a role of particle roughness and possible coupling with rotational diffusion.