Stationary particle currents in sedimenting active matter wetting a wall

Area of Science:

• Physics
• Soft Matter Physics
• Statistical Mechanics

Background:

• Recent theoretical work predicted scalar active matter rising against gravity near walls.
• This prediction was based on a lattice gas model, suggesting counter-intuitive upward motion despite repulsive interactions.

Purpose of the Study:

• To numerically confirm the prediction of active matter rising against gravity.
• To elucidate the underlying mechanism of meniscus formation in sedimenting active Brownian particles (ABPs).
• To investigate the role of particle activity, confinement, and interactions on this phenomenon.

Main Methods:

• Numerical simulations of sedimenting active Brownian particles (ABPs) in a confined box.
• Analysis of particle distribution, current formation, and vortex dynamics.
• Investigation of the influence of Péclet number and inter-particle interactions.

Main Results:

• Confirmed the formation of a meniscus rising above the bulk sedimentation region.
• Demonstrated that meniscus height increases algebraically with the Péclet number (activity).
• Identified stationary circular particle currents (vortices) as the key mechanism, whose strength and size correlate with ABP activity.

Conclusions:

• Active Brownian particles exhibit directed motion against gravity, forming a distinct meniscus.
• The observed phenomenon is driven by self-generated vortices originating from confinement and activity.
• This work predicts experimental observability in active colloids under gravitational fields, showcasing active matter's ability to perform work against external fields.