Anisotropic scaling lengths of colloidal monolayers near a water–air interface

Near-interface colloidal monolayers are often used as model systems for research on hydrodynamics in biophysics systems and in the chemical industry. Using microrheological methods, the correlated diffusion of particles is experimentally measured in colloidal monolayers near a water–air interface. The results show that the scaling lengths of such colloidal monolayers are anisotropic in two orthogonal directions within the monolayer, which are parallel and perpendicular to the line connecting the centres of a particle pair. The former is the Saffman length of the monolayer, while the latter is a function of both the Saffman length and the radius of the colloids. The size of the colloids is involved in but not , which reflects the discrete nature of the monolayer in the transverse direction and the continuous nature of the monolayer in the longitudinal direction. From the scaling lengths, the viscosities of the colloidal monolayers are obtained, which agree with those obtained from the single-particle diffusion coefficients. The influence of the boundary condition imposed by the nearby interface on the hydrodynamic interactions is in a power-law behaviour of the distance z.