Transient coarsening and the motility of optically heated Janus colloids in a binary liquid mixture

TL;DR

This study investigates the nonequilibrium dynamics and transient coarsening phenomena around optically heated Janus colloids in a near-critical binary liquid, revealing complex pattern formation and reversed propulsion during transient states.

Contribution

It combines experimental and numerical approaches to elucidate the transient coarsening and motility mechanisms of Janus colloids in near-critical binary mixtures, highlighting the role of hydrodynamic effects.

Findings

Initial diffusive composition layers travel outward from the colloid surface.

Hydrodynamic effects induce complex, evolving coarsening patterns.

Transient propulsion occurs opposite to steady-state motion.

Abstract

A gold-capped Janus particle suspended in a near-critical binary liquid mixture can self-propel under illumination. We have immobilized such a particle in a narrow channel and studied the nonequilibrium dynamics of a binary solvent around it, using experiment and numerical simulations. For the latter we consider both a purely diffusive and a hydrodynamic model. All approaches indicate that the early time dynamics is purely diffusive and characterized by composition layers traveling with a constant speed from the surface of the colloid into the bulk. Subsequently, hydrodynamic effects set in and the transient state is destroyed by strong nonequilibrium concentration fluctuations, which arise as a result of the temperature gradient and the vicinity of the critical point of the binary liquid mixture. They give rise to a complex, permanently changing coarsening patterns. For a mobile particle, the transient dynamics results in propulsion in the direction opposite to that observed after the steady state is attained.