How to steer active colloids up a vertical wall

TL;DR

This study demonstrates how active Janus colloids can climb a vertical wall against gravity by forming a dynamic adsorption layer, leveraging wall interactions to generate a steady-state flux unlike passive systems.

Contribution

It provides experimental and numerical evidence of active particles climbing walls, revealing new mechanisms for harnessing work in active matter systems.

Findings

Active colloids form an adsorption layer at the wall.

Wall interactions induce polarity heterogeneity.

Active particles can climb against gravity, creating a steady flux.

Abstract

An important challenge in active matter lies in harnessing useful global work from entities that produce work locally, e.g., via self-propulsion. We investigate here the active matter version of a classical capillary rise effect, by considering a non-phase separated sediment of self-propelled Janus colloids in contact with a vertical wall. We provide experimental evidence of an unexpected and dynamic adsorption layer at the wall. Additionally, we develop a complementary numerical model that recapitulates the experimental observations. We show that an adhesive and aligning wall enhances the pre-existing polarity heterogeneity within the bulk, enabling polar active particles to climb up a wall against gravity, effectively powering a global flux. Such steady-state flux has no equivalent in a passive wetting layer.