Inverted Sedimentation of Active Particles in Unbiased ac Fields

TL;DR

This paper demonstrates that ac fields with zero average can steer active particles, causing them to move against gravity and invert sedimentation profiles, offering a new method for controlling active matter.

Contribution

It introduces a novel control principle using unbiased ac fields to steer inertial active particles, enabling inversion of sedimentation without bias.

Findings

Active particles can be directed upward using ac fields.

Inverted sedimentation profiles observed at the container's top wall.

The control method is generic and applicable to various active matter systems.

Abstract

Gaining control over the motion of active particles is crucial for applications ranging from targeted cargo delivery to nanomedicine. While much progress has been made recently to control active motion based on external forces, flows or gradients in concentration or light intensity, which all have a well-defined direction or bias, little is known about how to steer active particles in situations where no permanent bias can be realized. Here, we show that ac fields with a vanishing time average provide an alternative route to steering active particles. We exemplify this route for inertial active particles in a gravitational field, observing that a substantial fraction of them persistently travels in the upward direction upon switching on the ac field, resulting in an inverted sedimentation profile at the top wall of a confining container. Our results offer a generic control principle which could be used in the future to steer active motion, to direct collective behaviors and to purify mixtures.