Theory of capillary-induced interactions beyond the superposition approximation

TL;DR

This paper develops a theoretical framework to analyze capillary interactions between colloids at fluid interfaces beyond the common superposition approximation, revealing that such interactions are attractive under certain conditions and differ from previous assumptions.

Contribution

It provides the first calculation of the leading correction to the superposition approximation for capillary interactions at large distances.

Findings

Superposition approximation is invalid in mechanical isolation.

Effective interaction is attractive beyond the superposition approximation.

At large separations, capillary attraction does not surpass electrostatic repulsion.

Abstract

Within a general theoretical framework we study the effective, deformation-induced interaction between two colloidal particles trapped at a fluid interface in the regime of small deformations. In many studies, this interaction has been computed with the ansatz that the actual interface configuration for the pair is given by the linear superposition of the interface deformations around the single particles. Here we assess the validity of this approach and compute the leading term of the effective interaction for large interparticle separation beyond this so-called superposition approximation. As an application, we consider the experimentally relevant case of interface deformations owing to the electrostatic field emanating from charged colloidal particles. In mechanical isolation, i.e., if the net force acting on the total system consisting of the particles plus the interface vanishes, the superposition approximation is actually invalid. The effective capillary interaction is governed by contributions beyond this approximation and turns out to be attractive. For sufficiently small surface charges on the colloids, such that linearization is strictly valid, and at asymptotically large separations, the effective interaction does not overcome the direct electrostatic repulsion between the colloidal particles.