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

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. 45 o 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.

Pattern-induced local symmetry breaking in active-matter systems

“…In addition to actomyosin motility assays, patterns with polar and nematic symmetries were commonly suggested in systems of rodlike particles ( 19 – 22 ) and observed in experiments with motile bacteria in liquid crystals ( 51 – 53 ). Moreover, patterns of intertwined symmetries play a prominent role even in equilibrium systems, such as in high-temperature superconductors ( 54 , 55 ).…”

“…This implies a possible relevance of this phenomenon for a broad class of experimental systems beyond the actomyosin motility assay. We hypothesize that pattern-induced symmetry breaking could serve as a useful and general design principle with broad applications to synthetic and living active matter ( 51 – 53 ) as well as other pattern-forming systems with intertwined symmetries ( 54 , 55 ).…”

“…to synthetic and living active matter (51)(52)(53) as well as other pattern-forming systems with intertwined symmetries (54,55).…”

Reconstruction of Agents’ Corrupted Trajectories of Collective Motion Using Low-rank Matrix Completion

“…Since spiral collective motion is often observed in the nature ranging from biology, such as bacteria colonies [31], to astronomy, such as spiral galaxies [32], we simulate and analyze trajectories representing a spiral collective motion scenario. First, we formulate an anticlockwise Archimedean spiral that rotates an angle of 3π and then compute the agent-wise temporal rotational matrix based on that spiral.…”

Dynamic Control of Speed and Trajectories of Active Droplets in a Nematic Environment by Electric Field and Focused Laser Beam

“…Liquid crystals, used as a medium for active colloids, offer a much higher control level over the microscale dynamics thanks to their longrange orientational order [4,8,9]. In particular, by designing patterns of the nematic director n (n ≡ − n, n2 1) that specifies the preferred direction of molecular orientation [10], one can command the polarity and geometry of propulsion trajectories [11][12][13][14][15][16][17][18], mediate transitions from individual to collective modes of propulsion [17] and control the spatial distribution of microswimmers [14,15,17].…”