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Command of active matter by topological defects and patterns

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Science  18 Nov 2016:
Vol. 354, Issue 6314, pp. 882-885
DOI: 10.1126/science.aah6936

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Directing traffic with patterns

Biological entities, such as bacteria, may direct their motion in response to their environment, but this usually does not lead to large-scale patterns or collective behavior. Peng et al. found that the orientational ordering of a liquid crystal could direct the flow of self-propelling bacteria, which in turn influenced the patterning of the liquid crystal molecules. Patterns on a substrate caused surface anchoring of the liquid crystals that transmitted to the ordering of the bacteria, thus imparting control on what would otherwise be chaotic out-of-equilibrium behavior.

Science, this issue p. 882

Abstract

Self-propelled bacteria are marvels of nature with a potential to power dynamic materials and microsystems of the future. The challenge lies in commanding their chaotic behavior. By dispersing swimming Bacillus subtilis in a liquid crystalline environment with spatially varying orientation of the anisotropy axis, we demonstrate control over the distribution of bacterial concentration, as well as the 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 toward defects of positive topological charge and avoiding negative charges. Sensitivity of bacteria to preimposed orientational patterns represents a previously unknown facet of the interplay between hydrodynamics and topology of active matter.

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