Hydrodynamic interactions between widely separated particles at a free surface

In this,letter, we consider a quasi-two-dimensional (Q2D) system of widely separated spheres suspended in a quiescent fluid and in contact with a planar fluid-gas interface. We construct and estimate the accuracy of two generic approximations to the translational-translational mobility, needed, e.g., in Brownian simulations. Both simplifications for Q2D systems are shown to be essentially different than their three-dimensional well-known analogs. First, we discuss the asymptotic form of the Q2D two-sphere mobility up to cubic order in the inverse inter-particle distance and compare it with the corresponding three-dimensional Rotne-Prager expression. We also explain how to avoid divergences due to lubrication forces at the contact. Next, we construct the Q2D mobility of point-particles, which-unlike in 3D-is not pairwise additive. Finally, we determine the range of validity of both long-distance approximations by computing numerically exact results.