Active nematic liquid crystals have the remarkable ability to spontaneously deform and flow in the absence of any external driving force. While living materials with orientational order, such as the mitotic spindle, can self-assemble in quiescent active phases, reconstituted active systems often display chaotic, periodic, or circulating flows under confinement. Quiescent active nematics are, therefore, quite rare, despite the prediction from active hydrodynamic theory that confinement between two parallel plates can suppress flows. This spontaneous flow transition—named the active Fréedericksz transition by analogy with the conventional Fréedericksz transition in passive nematic liquid crystals under a magnetic field—has been a cornerstone of the field of active matter. Here, we report experimental evidence that confinement in spherical droplets can stabilize the otherwise chaotic dynamics of a 3D extensile active nematics, giving rise to a quiescent—yet still out-of-equilibrium—nematic liquid crystal. The active nematics spontaneously flow when confined in larger droplets. The composite nature of our model system composed of extensile bundles of microtubules and molecular motors dispersed in a passive colloidal liquid crystal allows us to demonstrate how the interplay of activity, nematic elasticity, and confinement impacts the spontaneous flow transition. The critical diameter increases when motor concentration decreases or nematic elasticity increases. Experiments and simulations also demonstrate that the critical confinement depends on the confining geometry, with the critical diameter in droplets being larger than the critical width in channels. Biochemical assays reveal that neither confinement nor nematic elasticity impacts the energy-consumption rate, confirming that the quiescent active phase is the stable out-of-equilibrium phase predicted theoretically. Further experiments in dense arrays of monodisperse droplets show that fluctuations in the droplet composition can smooth the flow transition close to the critical diameter. In conclusion, our work provides experimental validation of the active Fréedericksz transition in 3D active nematics, with potential applications in human health, ecology, and soft robotics. Published by the American Physical Society 2024

中文翻译：

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主动向列液滴中的主动 Fréedericksz 转变


活性向列液晶在没有任何外部驱动力的情况下具有自发变形和流动的非凡能力。虽然具有取向顺序的活体材料（例如有丝分裂纺锤体）可以在静止的活性阶段自组装，但重组的活性系统通常在受限下表现出混乱、周期性或循环的流动。因此，静止的主动向列是相当罕见的，尽管主动流体动力学理论预测两个平行板之间的限制可以抑制流动。这种自发流跃迁（称为主动 Fréedericksz 跃迁，类似于磁场下被动向列液晶中的常规 Fréedericksz 跃迁）一直是活性物质场的基石。在这里，我们报告了实验证据，表明限制在球形液滴中可以稳定 3D 拉伸有源向列的混沌动力学，从而产生静止但仍不平衡的向列液晶。当被限制在较大的液滴中时，活性线列会自发流动。我们的模型系统由分散在被动胶体液晶中的微管和分子马达的拉伸束组成，其复合性质使我们能够展示活性、向列弹性和约束的相互作用如何影响自发流转变。当电机浓度降低或向列弹性增加时，临界直径增加。实验和仿真还表明，临界约束取决于约束几何形状，液滴中的临界直径大于通道中的临界宽度。 生化分析表明，约束和向列弹性都不会影响能耗率，证实了静止活性相是理论上预测的稳定失衡相。在密集的单分散液滴阵列中的进一步实验表明，液滴成分的波动可以平滑接近临界直径的流动过渡。总之，我们的工作为 3D 主动向列中的主动 Fréedericksz 转变提供了实验验证，在人类健康、生态学和软机器人方面具有潜在应用。美国物理学会 2024 年出版