Anomalous hydrodynamic interaction in a quasi-two-dimensional suspension

TL;DR

This study investigates the unique hydrodynamic interactions in a quasi-two-dimensional suspension, revealing negative transverse coupling, a specific decay rate, and concentration independence, driven by 2D dipolar flows.

Contribution

It uncovers three anomalies in hydrodynamic interactions in confined suspensions, highlighting the role of 2D dipolar flow in long-range correlations.

Findings

Transverse coupling is negative ('anti-drag')

Interaction decays as 1/r^2 with distance

Pair interaction is concentration-independent at large distances

Abstract

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.