Control of microswimmers by spiral nematic vortices: transition from individual to collective motion and contraction, expansion, and stable circulation of bacterial swirls

TL;DR

This paper demonstrates how spiral nematic vortices can control bacterial collective motion, inducing transitions from individual swimming to stable swirls, with the ability to expand or contract these swirls based on vortex properties.

Contribution

It introduces a method to manipulate bacterial swirls using patterned nematic liquid crystal vortices, revealing how vortex geometry influences collective bacterial behavior.

Findings

Dilute bacteria follow spiral trajectories aligned with molecular orientation.

Above a threshold concentration, bacteria form stable circular swirls.

Vortex splay or bend dominance controls swirl contraction or expansion.

Abstract

Active systems comprised of self-propelled units show fascinating transitions from Brownian-like dynamics to collective coherent motion. Swirling of swimming bacteria is a spectacular example. This study demonstrates that a nematic liquid crystal environment patterned as a spiral vortex controls individual-to-collective transition in bacterial swirls and defines whether they expand or shrink. In dilute dispersions, the bacteria swim along open spiral trajectories, following the pre-imposed molecular orientation. The trajectories are nonpolar. As their concentration exceeds some threshold, the bacteria condense into unipolar circular swirls resembling stable limit cycles. This collective circular motion is controlled by the spiral angle that defines the splay-to-bend ratio of the background director. Vortices with dominating splay shrink the swirls towards the center, while vortices with dominating bend expand them to the periphery. 45o spiraling vortices with splay-bend parity produce the most stable swirls. All the dynamic scenarios are explained by hydrodynamic interactions of bacteria mediated by the patterned passive nematic environment and by the coupling between the concentration and orientation. The acquired knowledge of how to control individual and collective motion of microswimmers by a nematic environment can help in the development of microscopic mechanical systems.