Zhou, Shuang - Kent State University

Living liquid crystals: single and collective bacterial motion in nematic media

As a typical pusher type microswimmer, bacillus subtilis propel their cylindrical body by rotating their helical flagella buddle. It has been widely used as standard model in studies ranging from collision with another bacterium or surface, to collective motions at high concentration. However, most of the studies were done in isotropic environment. We introduce nematic order into the system by replacing the isotropic media with lyotropic chromonic liquid crystals and observe fascinating phenomena due to the interplay between activity and orientational order. At low bacteria concentration, bacteria motion is dominated by the nematic environment and boundary conditions. In a planar cell, where the director of liquid crystal is aligned tangential to the bounding substrates, bacteria swim along the local director and are “domesticated”, showing the ability to transport cargo particles along predetermined trajectories, a feature never seen in their natural habitat (water). In a homeotropic cell with director of liquid crystal normal to the bounding substrates, some of the bacteria can swim parallel to the plane (perpendicular to the director) while others are “pinned” normal to the plane (parallel to the director). Swimming bacteria in a homeotropic cell move in linear or circular motions. They also form “trains” to facilitate efficient swimming. When the bacteria concentration is increased, the activity-triggered flow reorients the director to form first periodic “bend walls”, followed by topological turbulence with steady creation and annihilation of ±1/2 defects. A correlation length ξ characterizing the director distortions is determined by both the elasticity of the chromonic LC and the activity of bacteria. In a freely suspended sessile drop, bacteria concentration required to form collective motion is <10% of the concentration for collective motion in water. The phenomenon is rooted in the long-range elasticity-mediated interactions of the bacteria in a liquid crystal. Our study provides insight to understand hierarchy of spatial scales in active systems with orientational order, and demonstrates new microfluidic concepts with particle transportation possibilities.