Abstract
Living liquid crystals, a realization of active nematics where a lyotropic liquid crystal is combined with active bacteria, exhibit a plethora of out-of-equilibrium phenomena that range from active turbulence and dynamic spatiotemporal patterns to the creation and annihilation of motile topological defects. Experiments and hydrodynamic simulations are used here to report on the emergence of bend stripes, which arise as spontaneous undulations of the director field in circularly aligned lyotropic liquid crystals doped with bacteria. The interplay between bacterial-induced hydrodynamic flows and elastic forces in the material induces remarkable deformation patterns consisting of branched, radially elongated bands of a high curvature of the director field. The average number of such branches increases with the distance from the center of the circular alignment, leading to the formation of a radial tree of bands that is reminiscent of a snowflake structure. Hydrodynamic simulations, which are in agreement with the experiments, are used to explain the origin of such structures and to provide additional insights into regimes that are beyond the limit of experimental measurements. In particular, it is found that when activity is switched off in the early stages of pattern formation, a pronounced decay of bend-distortion energy ensues, with little change of the splay energy, serving to confirm that the bend stripes are an outcome of activity-driven bend-instability phenomena. Taken together, experiments and simulations demonstrate a system in which strain and geometry can be combined to dynamically manipulate pattern formation in active matter, paving the way to a deeper understanding and finer control of active colloidal systems.
- Received 4 December 2018
- Revised 17 May 2019
DOI:https://doi.org/10.1103/PhysRevX.9.031014
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Active materials, which can self-assemble into a variety of patterns, could lead to new structures and behaviors that are more complex than traditional materials. One new class of such materials, active nematic matter, combines the motility of active systems with the mechanical properties of liquid crystals. But to control such materials, researchers need to tackle how motility and crystallinity act on each other. Here, we introduce live bacteria into a liquid crystal and discover a growing snowflakelike structure that could be used to steer the motion of microscopic particles.
We demonstrate that the interplay between bacteria-induced flows and the elasticity of the liquid crystal leads to the onset of intriguing dynamic patterns. Flows induced by the bacteria disturb the molecular alignment of the liquid crystal, while elasticity tends to keep the molecules aligned. This gives rise to branched, radially elongated elastic bands accompanied by strong hydrodynamic flows. Simulations, which are in good agreement with the experiments, provide important insights into the origin of such structures and regimes that are beyond the limit of experimental measurements.
Our work provides new insights into the behavior of a broad class of active systems where the collective response is caused by a fine interplay between long-range and short-range interactions. It also suggests new techniques for the control and manipulation of transport in active nematic materials.
