Structures, thermodynamics and dynamics of topological defects in Gay–Berne nematic liquid crystals
Structures, thermodynamics and dynamics of topological defects in Gay–Berne nematic liquid crystals
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Gay Berne 向列液晶拓扑缺陷的结构、热力学和动力学
DOI：
10.1039/d2sm01178f
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发表时间：
2023
期刊：
影响因子：
3.4
通讯作者：
Zhang, Rui
中科院分区：
文献类型：
--
作者：
Huang, Yulu;Wang, Weiqiang;Whitmer, Jonathan K.;Zhang, Rui
拓扑缺陷是不同物理系统中普遍存在的现象。更好地理解缺陷有助于阐明许多真实材料系统的物理行为。在向列液晶中，缺陷表现出独特的光学特征，并且可以分离杂质，显示出它们作为分子载体和纳米反应器的前景。连续介质理论和模拟已成功应用于将拓扑缺陷的静态和动态行为与底层向列的材料常数联系起来。然而，仍然缺乏进一步的证据和分子细节。在这里，我们对 Gay-Berne 粒子（向列模型）进行分子动力学模拟，以检查薄膜向列中 +1/2 缺陷的分子结构和动力学。具体来说，我们使用两个独立的间接测量来测量弯曲与张开比 K3/K1，结果显示出良好的一致性。接下来，我们研究了一对±1/2缺陷的湮灭事件，其轨迹与实验和流体动力学模拟一致。我们进一步研究了 NVE 系综中缺陷湮灭的热力学，使我们能够利用能量守恒定律正确估计弹性模量。最后，我们探讨了这些粗粒度分子模型中温度不均匀区域中缺陷消灭的影响，这些影响无法通过现有的连续体水平模拟进行分析。我们发现+1/2缺陷倾向于向较热的区域移动，并且它们在温度梯度中的速度变化可以通过从弹性模量的温度依赖性导出的术语来定量地理解。因此，我们的工作提供了对拓扑缺陷的结构和动力学的分子见解，提出了通过检查缺陷来测量弹性常数的独特且易于使用的方法，并提出了使用温度梯度的缺陷的替代控制参数。
Topological defects are a ubiquitous phenomenon across different physical systems. A better understanding of defects can be helpful in elucidating the physical behaviors of many real materials systems. In nematic liquid crystals, defects exhibit unique optical signatures and can segregate impurities, showing their promise as molecular carriers and nano-reactors. Continuum theory and simulations have been successfully applied to link static and dynamical behaviors of topological defects to the material constants of the underlying nematic. However, further evidence and molecular details are still lacking. Here we perform molecular dynamics simulations of Gay–Berne particles, a model nematic, to examine the molecular structures and dynamics of +1/2 defects in a thin-film nematic. Specifically, we measure the bend-to-splay ratio K3/K1 using two independent, indirect measurements, showing good agreement. Next, we study the annihilation event of a pair of ±1/2 defects, of which the trajectories are consistent with experiments and hydrodynamic simulations. We further examine the thermodynamics of defect annihilation in an NVE ensemble, leading us to correctly estimate the elastic modulus by using the energy conservation law. Finally, we explore effects of defect annihilation in regions of nonuniform temperature within these coarse-grained molecular models which cannot be analysed by existing continuum level simulations. We find that +1/2 defects tend to move toward hotter areas and their change of speed in a temperature gradient can be quantitatively understood through a term derived from the temperature dependence of the elastic modulus. As such, our work has provided molecular insights into structures and dynamics of topological defects, presented unique and accessible methods to measure elastic constants by inspecting defects, and proposed an alternative control parameter of defects using temperature gradient.