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Active systems comprised of self-propelled units show fascinating transitionsfrom Brownian-like dynamics to collective coherent motion. Swirling of swimmingbacteria is a spectacular example. This study demonstrates that a nematicliquid crystal environment patterned as a spiral vortex controlsindividual-to-collective transition in bacterial swirls and defines whetherthey expand or shrink. In dilute dispersions, the bacteria swim along openspiral trajectories, following the pre-imposed molecular orientation. Thetrajectories are nonpolar. As their concentration exceeds some threshold, thebacteria condense into unipolar circular swirls resembling stable limit cycles.This collective circular motion is controlled by the spiral angle that definesthe splay-to-bend ratio of the background director. Vortices with dominatingsplay shrink the swirls towards the center, while vortices with dominating bendexpand them to the periphery. 45o spiraling vortices with splay-bend parityproduce the most stable swirls. All the dynamic scenarios are explained byhydrodynamic interactions of bacteria mediated by the patterned passive nematicenvironment and by the coupling between the concentration and orientation. Theacquired knowledge of how to control individual and collective motion ofmicroswimmers by a nematic environment can help in the development ofmicroscopic mechanical systems.

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