Topographical pathways guide chemical microswimmers

TL;DR

This paper demonstrates that sub-micron topographical features can reliably guide and dock chemical microswimmers, offering a magnetic-field-free method for controlling active colloids in microfluidic applications.

Contribution

It introduces a novel approach using topographical features to guide chemically active colloids, bypassing the need for magnetic guidance systems.

Findings

Colloids reliably dock at edges of topographical features.

Colloids move along edges for extended periods, increasing with fuel concentration.

A continuum model captures the guiding behavior qualitatively.

Abstract

Achieving control over the directionality of active colloids is essential for their use in practical applications such as cargo carriers in microfluidic devices. So far, guidance of spherical Janus colloids was mainly realized using specially engineered magnetic multilayer coatings combined with external magnetic fields. Here, we demonstrate that step-like sub-micron topographical features can be used as reliable docking and guiding devices for chemically active spherical Janus colloids. For various topographic features (stripes, squares or circular posts) docking of the colloid at the feature edge is robust and reliable. Furthermore, the colloids move along the edges for significantly long times, which systematically increase with fuel concentration. The observed phenomenology is qualitatively captured by a simple continuum model of self-diffusiophoresis near confining boundaries, indicating that the chemical activity and associated hydrodynamic interactions with the nearby topography are the main physical ingredients behind the observed behaviour.