Swimming Bacteria Guided by Modulated Nematic Director*

Micro-swimmers of biological and artificial nature exhibit remarkable collective behavior. This behavior can be controlled by placing the micro-swimmers in a nematic liquid crystal (NLC) environment with orientational order. The NLC can control local concentration, trajectories and net flows of active bacteria at low activity level, in particular, trigger unidirectional circulation of swimming bacteria around topological defects^†. In this work, we explore the collective motion of motile Bacillus subtilis dispersed in a water-based lyotropic chromonic NLC with spatially-varying director. The director field represents an alternating one-dimensional system of splay and bend stripes imposed through surface photoalignment. The bacteria exhibit threshold-less unidirectional collective motion along the splay bands. If a bacterium enters the patterned field with a “wrong” direction of swimming, the patterned director realigns it by 180° and the bacterium continue to swim along the same direction as other bacteria. The demonstrated unidirectional linear motion of bacteria can be used for micro-cargo transport.
^†C. Peng, T. Turiv, Y. Guo, Q.-H. Wei, O. D. Lavrentovich, Science 354 (6314), 882-885 (2016).