近日，美国加州大学圣地亚哥分校的Mattia Serra与哈佛大学的L.Mahadevan等人合作并取得一项新进展。经过不懈努力，他们成功提出了缺陷介导动力学理论，用于描述活性向列中的相干结构。相关研究成果已于2023年5月25日在国际知名学术期刊《自然—物理学》上发表。

该研究团队运用拉格朗日相干结构的概念，通过分析二维微管和肌动蛋白混合物的实验数据以及活性向列动力学方程模拟的数值数据，对活性向列流体中的流体运动组织性进行了研究。相干结构由移动的吸引子和斥力子组成，协调着复杂的运动。通过分析实验和模拟数据，研究人员发现+1/2缺陷引起吸引子的移动和变形，成为集体运动的控制中心，进一步揭示了位置一致性和定向一致性的相互作用。此外，他们发现孤立+1/2缺陷周围的区域经历高度弯曲和低度拉伸/剪切变形，与局部应力分布一致。缺陷处的应力最小，而沿缺陷方向的高差异应力导致了折叠现象。这项研究为描述和控制活性流体中的自组织现象提供了新的视角，并具有在多细胞系统中应用的潜力。

据系，活性流体，如细胞骨架纤维、细菌群体和上皮细胞层，表现出独特的方向一致性，通常以向列序和其破缺的存在为特征，这些破缺由拓扑缺陷定义。然而，关于位置一致性的了解仍然有限，即底层流体运动中是否存在组织性，尽管这是一个突出的特征并可在实验中观察到。

附：英文原文

Title: Defect-mediated dynamics of coherent structures in active nematics

Author: Serra, Mattia, Lemma, Linnea, Giomi, Luca, Dogic, Zvonimir, Mahadevan, L.

Issue&Volume: 2023-05-25

Abstract: Active fluids, such as cytoskeletal filaments, bacterial colonies and epithelial cell layers, exhibit distinctive orientational coherence, often characterized by nematic order and its breakdown, defined by the presence of topological defects. In contrast, little is known about positional coherence, that is, whether there is an organization in the underlying fluid motion—despite this being both a prominent and an experimentally accessible feature. Here we characterize the organization of fluid motion in active nematics using the notion of Lagrangian coherent structures by analyzing experimental data of two-dimensional mixtures of microtubules and kinesin, as well as numerical data obtained from the simulation of the active nematodynamic equations. Coherent structures consist of moving attractors and repellers, which orchestrate complex motion. To understand the interaction of positional and orientational coherence, we analyse experiments and simulations and find that +1/2 defects move and deform the attractors, functioning as control centres for collective motion. Additionally, we find that regions around isolated +1/2 defects undergo high bending and low stretching/shearing deformations, consistent with the local stress distribution. The stress is the minimum at the defect, whereas high differential stress along the defect orientation induces folding. Our work offers a new perspective to describe and control self-organization in active fluids, with potential applications to multicellular systems.

DOI: 10.1038/s41567-023-02062-y

Source: https://www.nature.com/articles/s41567-023-02062-y