Command of active matter by topological defects and patterns

Abstract

Self-propelled bacteria are marvels of nature with a potential to power dynamic materials and microsystems of the future. The challenge is in commanding their chaotic behavior. By dispersing swimming Bacillus subtilis in a liquid-crystalline environment with spatiallyvarying orientation of the anisotropy axis, we demonstrate control over the distribution of bacteria, geometry and polarity of their trajectories. Bacteria recognize subtle differences in liquid crystal deformations, engaging in bipolar swimming in regions of pure splay and bend but switching to unipolar swimming in mixed splay-bend regions. They differentiate topological defects, heading towards defects of positive topological charge and avoiding negative charges. Sensitivity of bacteria to pre-imposed orientational patterns represents a new facet of the interplay between hydrodynamics and topology of active matter.One Sentence Summary: Patterned orientational order controls concentration, trajectories and flows of swimming bacteria in a liquid crystal.

Topological Defects in a Living Nematic Ensnare Swimming Bacteria

“…Recent experiments [56] on dilute suspensions of bacteria in patterned chromonic liquid crystals with preimposed director field show that B. subtilis can distinguish between defects of different topological charge and of different geometry. In particular, bacteria can accumulate at the cores of radial defects of strength þ1, form unidirectionally rotating vortices around defects of strength þ1 with a spiraling director of a bend-splay type, and avoid defects of strength −1.…”

“…In particular, bacteria can accumulate at the cores of radial defects of strength þ1, form unidirectionally rotating vortices around defects of strength þ1 with a spiraling director of a bend-splay type, and avoid defects of strength −1. Furthermore, the bacteria prefer to move away from the −1=2 defects towards the 1=2 defects [56]. One should, however, bear in mind that the experimental setting of Ref.…”

Geometry of thresholdless active flow in nematic microfluidics

“…We consider this regime mainly to derive analytical solutions and provide qualitative insights into the nature of the flow, but it has in fact been realized experimentally in living liquid crystals [30]. We also demonstrate that this approximation is asymptotically exact in the experimentally relevant limit of weak orientational order.…”

“…We show in Appendix B that there is a very natural, generic, and well-defined limit in which γ 1 will always be much less than η, namely, the limit of weak nematic order. Although we initially conceived the frozen director regime for calculational convenience to derive analytical expressions for the flow, this regime has in fact been realized experimentally in living liquid crystal systems in which bacteria are subjected to a preimposed director field that is effectively frozen in the experiment [30].…”

Formation of Photo-Responsive Liquid Crystalline Emulsion by Using Microfluidics Device

“…Moreover, the LC flow was successfully controlled by a photo-patterned alignment layer. When the LC is made into a droplet or capsule form, and mobile molecules are included into it, the LC droplet starts motion [6], the motion can be controlled by a couple of defects formed on the LC capsule [7]. In some studies, the motion can be controlled by such topological defects.…”