Stranski-Laboratorium für Physikalische und Theoretische Chemie, Fakultät für Mathematik und Naturwissenschaften, Technische Universität Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany.
Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom.
Phys Rev E. 2019 Aug;100(2-1):022702. doi: 10.1103/PhysRevE.100.022702.
We employ a version of classical density functional theory to study the phase behavior of a simple model liquid crystal in an external field. The uniaxially symmetric molecules have a spherically symmetric core with superimposed orientation-dependent attractions. The interaction between the cores consists of a hard-sphere repulsion plus an isotropic square-well attraction. The anisotropic part of the interaction potential allows for the formation of a uniaxially symmetric nematic phase. The orientation of the molecules couples to an external polar field. The external field is capable of rotating the nematic director n[over ̂] in the x-z plane. The field is also capable of changing the topology of the phase diagram in that it suppresses the phase coexistence between an isotropic liquid and a nematic phase observed in the absence of the field. We study the transition from an unpolar to a polar nematic phase in terms of the orientation-distribution function (odf), nematic and polar order parameters, and components of n[over ̂]. If represented suitably the odf allows us to study orientational changes during the switching process between nonpolar and polar nematic phases. We also give a simple argument that explains why nematic order is lost whereas polar order persists up to the gas-liquid critical point along the coexistence curve. We also discuss the relevance of our theory for future experimental studies.
我们采用经典密度泛函理论来研究外场中简单模型液晶的相行为。各向异性分子具有球形对称核，其上叠加有各向异性的取向相关吸引力。核之间的相互作用由硬球排斥和各向同性方阱吸引组成。相互作用势能的各向异性部分允许形成单轴对称向列相。分子的取向与外极场耦合。外场能够在 x-z 平面内旋转向列指向矢 n[over ̂]。该场还能够改变相图的拓扑结构，因为它抑制了在没有场的情况下观察到的各向同性液体和向列相之间的相共存。我们根据取向分布函数（odf）、向列和极序参数以及 n[over ̂]的分量来研究从非极性到极性向列相的转变。odf 如果表示得当，它允许我们在非极性和极性向列相之间的转换过程中研究取向变化。我们还提出了一个简单的论点，解释了为什么向列序在沿着共存曲线的气液临界点失去，而极序仍然存在。我们还讨论了我们的理论对未来实验研究的相关性。
