  Near-interface colloidal monolayers have often been used as model systems for
research on hydrodynamics in biophysics and microfluidic systems. Using optical
microscopy and multiparticle tracking techniques, the correlated diffusion of
particles is experimentally measured in colloidal monolayers near a water-air
interface. It is found that the characteristic lengths X1 and X2 of such a
colloidal monolayer are anisotropic in these two perpendicular directions. The
former (X1)is equal to the Saffman length of the monolayer and reflects the
continuous nature of the system in the longitudinal direction. The latter
(X2)is a function of both the Saffman length and the radius of the colloids and
reflects the discrete nature of the system in the transverse direction. This
discovery demonstrates that the hydrodynamics intrinsically follow different
rules in these two directions in this system.
