Anomalous hydrodynamic interaction in a quasi-two-dimensional suspension

We have studied the correlated Brownian motion of micron-sized particles suspended in water and confined between two plates. The hydrodynamic interaction between the particles exhibits three anomalies. (i) The transverse coupling is negative, i.e., particles exert "anti-drag" on one another when moving perpendicular to their connecting line. (ii) The interaction decays with inter-particle distance r as 1/r^2, faster than in unconfined suspensions but slower than near a single wall. (iii) At large distances the pair interaction is independent of concentration within the experimental accuracy. The confined suspension thus provides an unusual example of long-range, yet essentially pairwise correlations even at high concentration. These effects are shown to arise from the two-dimensional dipolar form of the flow induced by single-particle motion.