Swimming bacteria swirl around nematic attractors*

Microswimmers such as bacteria exhibit collective behavior that can be controlled when placed in a nematic liquid crystal (NLC) with long-range orientational order [1]. We explore the collective motion of motile Bacillus subtilis dispersed in an aqueous solution of DSCG, a lyotropic chromonic NLC. The director field, imposed through photoalignment, contains defects of topological charge +1 which serve as attractors for bacteria. We vary the azimuthal angle of the director from 0 to 90 degrees to have predominant director distortions of either splay, bend, or a mixture of the two. The bacteria exhibit collective circular motion around +1 defects, with the radius of maximum concentration and velocity increasing as bend distortion dominates over splay. The experiment presents an example of how microswimmers interact with attractor type singular defects with different degrees of splay or bend deformation. The ability to control the collective motion of microswimmers can be used as a source of energy to power microscopic mechanical systems.

[1] C. Peng, T. Turiv et al., Science 354 (6314), 882-885 (2016).