Self-propulsion mechanism of active Janus particles in near-critical binary mixtures

TL;DR

This paper investigates how gold-capped Janus particles move in near-critical binary mixtures under illumination, revealing that body forces at droplet edges drive propulsion, with velocity depending on droplet shape and temperature.

Contribution

It introduces a non-isothermal diffuse interface model showing that self-propulsion arises from body forces at droplet edges, challenging the surface velocity concept.

Findings

Self-propulsion is driven by body forces at droplet edges.

Two swimming regimes depend on droplet coverage.

Swimming velocity varies non-monotonically with temperature.

Abstract

Gold-capped Janus particles immersed in a near-critical binary mixture can be propelled using illumination. We employ a non-isothermal diffuse interface approach to investigate the self-propulsion mechanism of a single colloid. We attribute the motion to body forces at the edges of a micronsized droplet that nucleates around the particle. Thus, the often-used concept of a surface velocity cannot account for the self-propulsion. The particle's swimming velocity is related to the droplet shape and size, which is determined by a so-called critical isotherm. Two distinct swimming regimes exist, depending on whether the droplet partially or completely covers the particle. Interestingly, the dependence of the swimming velocity on temperature is non-monotonic in both regimes.