Anisotropy of Brownian motion caused only by hydrodynamic interaction with a wall

The diffusivity of spherical colloidal particles close to a planar hard wall is studied by dynamic light scattering with evanescent illumination. A novel setup allows us to independently vary the scattering vector components parallel Q(parallel to) and normal Q(perpendicular to) to the wall. An expression for the initial decay rate Gamma of the time autocorrelation functions is derived as a function of both Q(parallel to) and Q(perpendicular to), as well as the penetration depth of the evanescent wave, where hydrodynamic interactions of particles with the wall are included. This makes it possible to study the viscous wall drag effect quantitatively for particles as small as 85 nm in radius.