Linear and angular motion of self-diffusiophoretic Janus particles

TL;DR

This paper provides a theoretical framework for understanding the linear and angular motion of self-diffusiophoretic Janus particles, highlighting the roles of chemical activity, symmetry, and solute interactions in their active motion.

Contribution

It introduces a reciprocal relations-based model that accounts for both diffusio- and electrophoretic effects, extending beyond boundary-layer approximations.

Findings

Anisotropic chemical activity induces linear motion.

Non-axisymmetric Janus particles can rotate actively.

Active velocity relates to solute friction coefficients.

Abstract

We theoretically study the active motion of self-diffusiophoretic Janus particles (JPs) using the Onsager-Casimir reciprocal relations. The linear and angular velocity of a single JP are shown to respectively result from a coupling of electrochemical forces to the fluid flow fields induced by a force and torque on the JP. A model calculation is provided for half-capped JPs catalysing a chemical reaction of solutes at their surface, by reducing the continuity equations of the reacting solutes to Poisson equations for the corresponding electrochemical fields. We find that an anisotropic chemical activity alone is enough to give rise to active linear motion of a JP, whereas active rotation only occurs if the JP is not axisymmetric. In the absence of specific interactions with the solutes, the active linear velocity of the JP is shown to be related to the stoichiometrically weighted sum of the friction coefficients (or hydrodynamic radii) of the reacting solutes. Our reciprocal treatment further suggests that a specific interaction with the solutes is required to observe far-field diffusiophoretic interactions between JPs, which rely on an interfacial solute excess at the JP surface. Most notably, our approach applies beyond the boundary-layer approximation and accounts for both the diffusio- and electrophoretic nature of active motion.