杆状细菌菌落中的拓扑缺陷和集体运动

Active liquid crystals are composed of rod-shaped self-propelled units, such as microtubule bundles driven by ATP and motile spindle cells. Recent continuum theories have predicted the following picture for active liquid crystals: while rod-shaped units at high densities lead to nematic regions with local orientation order, self-propulsion destabilizes the orientation order, generates topological defects and large-scale collective motion. Experimental studies have provided qualitative supports for the predictions but a quantitative verification is still lacking. To address this problem, we have carried out preliminary experiments and used rod-shaped motile bacteria to construct a model system for active liquid crystal, which allows simultaneous measurements of orientation and velocity fields. With our bacterial system, we plan to characterize the structure and dynamics of single defects, measure the interactions between defects, and quantify bacteria collective motion. Experimental parameters, including bacteria length, motility, and density, will be systematically varied. We will also investigate the effects of spatial confinement on system properties. Experimental results will be used to both quantitatively check the predictions of the continuum theories and develop a discrete model based on self-propelled units. Beyond their theoretical significance, our results may provide new methods to control a range of biological phenomena, including cell migration and bacterial colony growth.

活性液晶系统由杆状的自驱动个体组成，如ATP驱动的微管束和纺锤形的运动细胞等。相关理论预言：高密度的杆状个体会形成取向有序的向列相，而个体的自驱动特性可导致向列相发生动力学失稳，激发拓扑缺陷，驱动多尺度的集体运动。实验工作为上述预言提供了定性支持，但定量的检验依然缺失。针对这一现状，申请人开展了前期实验研究，成功构建了一个基于杆状运动细菌的活性液晶系统，并能精确测量细菌取向和速度。我们计划利用该系统定量研究单个缺陷的结构和动力学、缺陷间的相互作用及其引发的细菌集体运动；改变细菌长度、速度和密度等参数，量化这些参数对拓扑缺陷和集体运动的影响；还将关注空间约束的效应，研究两种约束方法（微流腔体和气液界面）对拓扑缺陷的调控作用。利用上述实验结果，我们将定量检验现有的连续介质理论，并发展一个基于自驱动个体的新模型；同时实验结果还可能为理解和调控细胞迁徙、菌落生长等生物现象提供新思路。

本项目关注活性物质中的集体动力学，以运动细菌、运动藻类、人造活性颗粒为研究对象，通过实验、理论和模拟相结合的方法研究了系列问题，取得如下主要成果：1. 测量了杆状细菌菌落中的拓扑缺陷和集体运动，并构造了一个新的理论模型；2. 通过偏振光场的几何与拓扑特性精确调控细胞运动及其产生的输运流场；3. 发现虫黄藻可以自组织形成无序超均匀态，并构造了一个基于有效长程排斥作用和随机细胞运动的理论模型；4. 发现自驱动转子形成的边界流，证明边界流的鲁棒性，测量强度受转子密度以及边界曲率的影响。项目负责人已发表论文8篇，包括1篇Physical Review Letters 和 2篇Proceedings of the National Academy of Sciences等。本项目的成果推动了对活性物质动力学的理解，为进一步研究奠定了基础。