Active colloidal propulsion over a crystalline surface

TL;DR

This study investigates the movement of chemically self-propelled Janus colloids on a crystalline surface, revealing how propulsion strength influences the transition from hindered to enhanced diffusion through experimental and theoretical analysis.

Contribution

It introduces a combined experimental and Langevin model approach to understand active colloid dynamics on a periodic surface, highlighting the effects of propulsion strength and surface interactions.

Findings

Enhanced diffusion observed at long times

Langevin model accurately describes particle motion

Crossover from hindered to active diffusion

Abstract

We study both experimentally and theoretically the dynamics of chemically self-propelled Janus colloids moving atop a two-dimensional crystalline surface. The surface is a hexagonally close-packed monolayer of colloidal particles of the same size as the mobile one. The dynamics of the self-propelled colloid reflects the competition between hindered diffusion due to the periodic surface and enhanced diffusion due to active motion. Which contribution dominates depends on the propulsion strength, which can be systematically tuned by changing the concentration of a chemical fuel. The mean-square displacements obtained from the experiment exhibit enhanced diffusion at long lag times. Our experimental data are consistent with a Langevin model for the effectively two-dimensional translational motion of an active Brownian particle in a periodic potential, combining the confining effects of gravity and the crystalline surface with the free rotational diffusion of the colloid. Approximate analytical predictions are made for the mean-square displacement describing the crossover from free Brownian motion at short times to active diffusion at long times. The results are in semi-quantitative agreement with numerical results of a refined Langevin model that treats translational and rotational degrees of freedom on the same footing.