发表机构
University of California, Merced; Johns Hopkins University（加州大学默塞德分校; 约翰斯·霍普金斯大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过对比两种连续介质模型，验证了增强向列相锁定（BENL）模型能更准确重现基于微管的活性向列相流体中由拓扑缺陷编织运动引起的混沌平流自混合度量。
AI 中文摘要
活性向列相是由杆状自驱动单元组成的非平衡流体，这些单元集体产生大尺度相干流动。这里，我们聚焦于一个典型的实验系统：由ATP驱动的二维活性向列相流体，由致密堆积的、延伸的微管（MT）束组成，并通过驱动蛋白马达交联。该系统的一个引人注目的特征是拓扑缺陷的产生与湮灭，其拓扑电荷为$\pm1/2$，这是由于材料的断裂所致。实验证实，正（$+1/2$）缺陷充当“虚拟搅拌棒”，它们以复杂的编织模式相互运动。正缺陷的这种集体编织运动拉伸并折叠流体本身，即产生宏观尺度的混沌平流。由混沌平流引起的自混合程度可通过拓扑熵和李雅普诺夫指数来度量。我们的目标是确定基于MT的活性向列相流体的连续介质模型能否重现这些测量结果。为此，我们使用两种连续介质模型：传统的Beris-Edwards（BE）模型和最近开发的具有增强向列相锁定（BENL）的Beris-Edwards模型。两种模型之间的区别在于采用了“向列相锁定原理”，该原理指出，由于细长致密MT束之间的空间相互作用，单个MT束不能独立于其邻居旋转。该原理在BENL模型中成立，但在材料域中材料断裂的小局部区域除外，特别是在拓扑缺陷的产生和湮灭附近。我们采用多种数值方法来估计两种模型下混沌平流的度量。我们的研究表明，BENL模型更准确地重现了基于MT的活性向列相流体中由混沌平流驱动的自混合的实验度量。
英文摘要
Active nematics are non-equilibrium fluids composed of rod-like self-propelled units that collectively generate large-scale coherent flows. Here, we focus on a canonical experimental system: an active nematic fluid in $2D$ driven by ATP, composed of densely packed, extended microtubule (MT) bundles cross-linked by kinesin motors. An intriguing feature of this system is the creation and annihilation of topological defects with topological charge $\pm1/2$, due to the fracturing of the material. Experiments confirm that the positive ($+1/2$) defects serve as "virtual stirring rods" that move around each other in a complex braiding pattern. This collective braiding motion of positive defects stretches and folds the fluid itself, i.e., produces macroscale chaotic advection. The degree of self-mixing due to the chaotic advection can be measured using topological entropy and the Lyapunov exponent. Our goal is to determine whether continuum models of MT-based active nematic fluid can reproduce these measurements. To this end, we use two continuum models: the traditional Beris-Edwards (BE) model and the more recently developed Beris-Edwards model with enhanced nematic locking (BENL). The difference between the two models is the adoption of the "nematic locking principle", which states that an individual MT bundle cannot rotate independently of its neighbors due to steric interactions among elongated dense MT bundles. This principle holds in the BENL model except in small localized areas of the material domain where the material fractures, specifically near the creation and annihilation of topological defects. We employ several numerical methods to estimate measures of chaotic advection using both models. Our study shows that the BENL model more accurately reproduces experimental measures of self-mixing driven by chaotic advection in MT-based active nematic fluids.
