Swimming Bacteria Guided by Patterned Nematic Liquid Crystal*

Micro-swimmers of biological nature such as motile bacteria exhibit highly correlated collective behavior. This collective behavior can be controlled by placing such microswimmers in a nematic liquid crystal (NLC) with long-range orientational order. The NLC environment can control the local concentration, trajectories, and net flow of active bacteria, thereby triggering a circular unidirectional motion of the bacteria around topological charges (Peng, Turiv et al., Science 2016). In this work, we explore the collective motion of motile B. subtilis dispersed in an aqueous solution of lyotropic chromonic NLC. The director field is set to be a smoothly varying in space, representing an alternating system of splay and bend regions imposed through photoalignment. The bacteria exhibit unidirectional collective motion along the splay regions of the director field. If the bacteria enter the patterned director field in the opposite direction of the collective motion, its motion is realigned. The experiments present a clear evidence of a highly compressible nature of bacterial dispersions in NLC. The demonstrated unidirectional linear motion can be applied to using such micro-swimmers as potential power materials.