Mou Zeyang, Li Yuan, You Zhihong, Zhang Rui
Hong Kong University of Science and Technology, Department of Physics, Clear Water Bay, Kowloon, Hong Kong SAR.
Xiamen University, Fujian Provincial Key Laboratory for Soft Functional Materials Research, Research Institute for Biomimetics and Soft Matter, Department of Physics, Xiamen, Fujian 361005, China.
Phys Rev E. 2025 Jun;111(6-2):065410. doi: 10.1103/crj5-bhwv.
It has been shown that an anisotropic liquid crystalline (LC) environment can be used to guide the self-propulsion dynamics of dispersed microswimmers, such as bacteria. This type of composite system is named "living nematic." In the dilute limit, bacteria are found to mainly follow the local director field. Beyond the dilute limit, however, they exhibit novel dynamical behaviors, from swirling around a spiral +1 defect pattern to forming undulating waves, and to active turbulence. Our current knowledge of how these different behaviors emerge at different population densities remains limited. Here, we develop a hybrid method to simulate the dynamics of microswimmers dispersed in a nematic LC. Specifically, we model the microswimmers as active Brownian particles whose dynamics are coupled to a hydrodynamic model of nematic LCs to describe the evolution of the flow field and the LC structure. Our method is validated across a wide range of microswimmer populations by comparing to existing quasi-two-dimensional (2D) experiments, including undulated swirling around a spiral +1 defect pattern and stabilized undulated jets on a periodic C-pattern. We further extend our method to three-dimensional (3D) systems by examining loop-defect dynamics. We find that the morphodynamics and destiny of a loop defect not only depend on the activity (self-propulsion velocity), effective size, and the initial distribution of the swimmers, but also rely on its winding profile. For a wedge-twist loop defect, its dynamics are mainly determined by the position and orientation of the +1/2 wedge. For a pure-twist loop defect, radial twist windings play a similar role as the +1/2 wedge in the wedge-twist loop defect, while other windings can engender out-of-plane active flows to buckle the pure-twist loop. Finally, we consider the stochastic reversals of the self-propulsion direction of the microswimmers. By varying the characteristic reversal time, we predict that microswimmers do not necessarily tend to accumulate on splay regions. Taken together, our hybrid method provides a faithful tool to explain and guide the experiments of living nematics in both 2D and 3D, sheds light on the interplay between microswimmer distribution and defect dynamics, and unravels the design principles of using LCs to control active matter.
研究表明，各向异性液晶（LC）环境可用于引导分散的微游动体（如细菌）的自推进动力学。这种复合系统被称为“活性向列相”。在稀释极限下，发现细菌主要遵循局部指向矢场。然而，超过稀释极限后，它们会表现出新颖的动力学行为，从围绕螺旋 +1 缺陷图案旋转到形成波动波，再到出现主动湍流。我们目前对于这些不同行为如何在不同种群密度下出现的了解仍然有限。在此，我们开发了一种混合方法来模拟分散在向列相液晶中的微游动体的动力学。具体而言，我们将微游动体建模为活性布朗粒子，其动力学与向列相液晶的流体动力学模型相耦合，以描述流场和液晶结构的演化。通过与现有的准二维（2D）实验进行比较，包括围绕螺旋 +1 缺陷图案的波动旋转以及周期性 C 图案上的稳定波动射流，我们的方法在广泛的微游动体种群范围内得到了验证。我们通过研究环形缺陷动力学进一步将我们的方法扩展到三维（3D）系统。我们发现环形缺陷的形态动力学和命运不仅取决于活性（自推进速度）、有效尺寸和游动体的初始分布，还依赖于其缠绕轮廓。对于楔形扭曲环形缺陷，其动力学主要由 +1/2 楔形的位置和方向决定。对于纯扭曲环形缺陷，径向扭曲缠绕在楔形扭曲环形缺陷中与 +1/2 楔形起着类似的作用，而其他缠绕会产生平面外的主动流以使纯扭曲环形弯曲。最后，我们考虑微游动体自推进方向的随机反转。通过改变特征反转时间，我们预测微游动体不一定倾向于聚集在展布区域。综上所述，我们的混合方法提供了一个可靠的工具来解释和指导二维和三维活性向列相的实验，揭示了微游动体分布与缺陷动力学之间的相互作用，并阐明了利用液晶控制活性物质的设计原理。
