How to steer active colloids up a vertical wall

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.

Abstract

It has been demonstrated theoretically earlier that a dense phase made of repulsive active particles, in phase-separating regime, exhibit wetting-like behaviors: wall-meniscus or capillary rise in channels. Here authors experimentally investigate a non-phase separating assembly of active colloids under gravity and show that a wetting-like phenomenology persists against a vertical wall, and they evidence a counter-gravity wall climbing.