Self-diffusiophoresis induced by fluid interfaces

TL;DR

This paper investigates how a fluid-fluid interface influences the self-phoretic motion of spherical colloids, revealing controllable directionality based on fluid properties through analytical and approximate methods.

Contribution

It provides the first detailed analysis of self-phoresis near fluid interfaces, showing how interface properties can control colloid motion direction.

Findings

Colloids can move even with homogeneous activity due to interface-induced anisotropy.

Motion direction can be tuned by changing fluid phase properties.

Analytical and approximate methods agree qualitatively and quantitatively.

Abstract

The influence of a fluid-fluid interface on self-phoresis of chemically active, axially symmetric, spherical colloids is analyzed. Distinct from the studies of self-phoresis for colloids trapped at fluid interfaces or in the vicinity of hard walls, here we focus on the issue of self-phoresis close to a fluid-fluid interface. In order to provide physically intuitive results highlighting the role played by the interface, the analysis is carried out for the case that the symmetry axis of the colloid is normal to the interface; moreover, thermal fluctuations are not taken into account. Similarly to what has been observed near hard walls, we find that such colloids can be set into motion even if their whole surface is homogeneously active. This is due to the anisotropy along the direction normal to the interface owing to the partitioning by diffusion, among the coexisting fluid phases, of the product of the chemical reaction taking place at the colloid surface. Different from results corresponding to hard walls, in the case of a fluid interface the direction of motion, i.e., towards the interface or away from it, can be controlled by tuning the physical properties of one of the two fluid phases. This effect is analyzed qualitatively and quantitatively, both by resorting to a far-field approximation and via an exact, analytical calculation which provides the means for a critical assessment of the approximate analysis.