Thermotaxis of Janus Particles

TL;DR

This study investigates how Janus particles respond to temperature gradients, revealing how their orientation and movement are influenced by surface properties and external thermal fields, with implications for active matter control.

Contribution

It provides a detailed analysis of thermotactic behavior of Janus particles, introducing a theoretical model linking temperature profiles to particle orientation and motion.

Findings

Orientation-dependent repulsion and alignment observed.

Theoretical model explains angular velocity based on temperature profile.

Heterogeneous heat conductivity breaks symmetry in temperature field.

Abstract

The interactions of autonomous microswimmers play an important role for the formation of collective states of motile active matter. We study them in detail for the common microswimmer-design of two-faced Janus spheres with hemispheres made from different materials. Their chemical and physical surface properties may be tailored to fine-tune their mutual attractive, repulsive or aligning behavior. To investigate these effects systematically, we monitor the dynamics of a single gold-capped Janus particle in the external temperature field created by an optically heated metal nanoparticle. We quantify the orientation-dependent repulsion and alignment of the Janus particle and explain it in terms of a simple theoretical model for the induced thermoosmotic surface fluxes. The model reveals that the particle's angular velocity is solely determined by the temperature profile on the equator between the Janus particle's hemispehres and their phoretic mobility contrast. The distortion of the external temperature field by their heterogeneous heat conductivity is moreover shown to break the apparent symmetry of the problem.