Stationary particle currents in sedimenting active matter wetting a wall

TL;DR

This study confirms that sedimenting active Brownian particles can form a rising meniscus against gravity due to stationary circular currents, with the phenomenon's extent increasing with activity and influenced by confinement and phase separation.

Contribution

It demonstrates, through numerical simulations, that active Brownian particles can sustain stationary currents and a meniscus rising against gravity, extending previous lattice gas predictions.

Findings

Meniscus height increases algebraically with Péclet number.

Stationary circular vortices drive the rising particle currents.

Motility-induced phase separation influences vortex position and strength.

Abstract

Recently it was predicted, on the basis of a lattice gas model, that scalar active matter in a gravitational field would rise against gravity up a confining wall or inside a thin capillary - in spite of repulsive particle-wall interactions [Phys. Rev. Lett. 124, 048001 (2020)]. In this paper we confirm this prediction with sedimenting active Brownian particles (ABPs) in a box numerically and elucidate the mechanism leading to the formation of a meniscus rising above the bulk of the sedimentation region. The height of the meniscus increases with the activity of the system, algebraically with the P\'eclet number. The formation of the meniscus is determined by a stationary circular particle current, a vortex, centered at the base of the meniscus, whose size and strength increase with the ABP activity. The origin of these vortices can be traced back to the confinement of the ABPs in a box: already the stationary state of ideal (non-interacting) ABPs without gravitation displays circular currents that arrange in a highly symmetric way in the eight octants of the box. Gravitation distorts this vortex configuration downward, leaving two major vortices at the two side walls, with a strong downward flow along the walls. Repulsive interactions between the ABPs change this situation only as soon as motility induced phase separation (MIPS) sets in and forms a dense, sedimented liquid region at the bottom, which pushes the center of the vortex upwards towards the liquid-gas interface. Self-propelled particles therefore represent an impressive realization of scalar active matter that forms stationary particle currents being able to perform visible work against gravity or any other external field, which we predict to be observable experimentally in active colloids under gravitation.