Self-diffusiophoresis of Janus particles in near-critical mixtures

TL;DR

This paper provides a theoretical analysis of how Janus particles self-propel in near-critical water-lutidine mixtures, revealing complex dependencies of velocity and squirmer parameter on heating power and surface properties.

Contribution

It introduces a theoretical framework linking particle propulsion and wetting properties in near-critical mixtures, including effects of ion drift and charge.

Findings

Particle velocity and squirmer parameter depend non-monotonically on heating power.

Wetting properties influence the direction of particle propulsion.

Inclusion of ion drift explains experimental observations.

Abstract

We theoretically study the self-propulsion of a laser-heated Janus particle in a near critical waterlutidine mixture, and relate its velocity vp and squirmer parameter \b{eta} to the wetting properties of its two hemispheres. For non-ionic surface forces, the particle moves the active cap ahead, except at strong driving, where a more complex behavior occurs if one of the hemispheres attracts water and the other lutidine. Both vp and \b{eta} show non-monotonic dependencies on the heating power, and may even change sign. Including the drift of the counterions due to the composition gradient, we find that a charged cap with lutidine affinity drives the particle the cap ahead, and with water affinity in the opposite direction, in agreement with experimental findings.