Thermotropic biaxial nematic liquid crystals

We have synthesized liquid crystal (LC) mesogens based on a nonlinear oxadiazole unit that exhibit nematic phases near 200 degrees C. Polarized microscopy and conoscopy indicate that these LCs are biaxial nematics. Unambiguous and quantitative evidence for biaxiality is achieved using 2H NMR spectroscopy. "2D powder" spectra, obtained by rotating 2H-labeled samples about an axis perpendicular to the magnetic field at approximately 200 Hz, yield phase biaxiality parameters of approximately 0.1 when coupled with rigorous and proven simulations.     

Polydispersity stabilizes biaxial nematic liquid crystals

Inspired by the observations of a remarkably stable biaxial nematic phase [van?den?Pol et?al., Phys. Rev. Lett. 103, 258301 (2009)], we investigate the effect of size polydispersity on the phase behavior of a suspension of boardlike particles. By means of Onsager theory within the restricted orientation (Zwanzig) model we show that polydispersity induces a novel topology in the phase diagram, with two Landau tetracritical points in between which oblate uniaxial nematic order is favored over the expected prolate order. Additionally, this phenomenon causes the opening of a huge stable biaxiality regime in between uniaxial nematic and smectic states.     

A biaxial nematic liquid crystal

We report a biaxial nematic liquid crystal in a simple thermotropic system.     

双轴性向列相液晶的相变理论

假设一双轴性向列相液晶分子简单相互作用模型，利用平均场理论，得到从各向同性相至单轴向列相、单轴向列相至双轴向列相的相变. 结果表明，在一定温度范围内，分子形状的非轴对称性可导致系统双轴向列相的产生. 在核磁共振实验中，导出双轴向列相序参数与谱线分裂频率间的一般关系，并讨论序参数测量性的可能性. 关键词： 向列相液晶 相变     

Paramagnetic nematic liquid crystals

The paper reports new nematic liquid crystals that are paramagnetic.     

Electroconvection in homeotropic nematic liquid crystals

Electroconvection is a classical example of pattern-forming phenomena in liquid crystals, typically observed in nematics with negative dielectric and positive conductivity anisotropies. This article focuses on how electroconvection in the homeotropic geometry differs from that in planar alignment. The influence of an additional magnetic field on the pattern characteristics and on secondary instabilities (the normal roll–abnormal roll transition) is discussed. The homeotropic alignment offers unique possibilities also for studying defect motion. Basic characteristics of some patterns of large wavelength are presented and compared with those of the classical Carr–Helfrich structures. Finally, electroconvection in substances with negative conductivity anisotropy is addressed.     

Bistability in dual-frequency nematic liquid crystals

Different modes of bistable switching in liquid crystals with frequency inversion of the dielectric anisotropy sign are discussed. The study is performed by numerical simulation and experimentally. It is shown that dual frequency driving can be effectively used to control switching between topologically equivalent and non-equivalent director field distributions. The experimental results on temperature performance of the dual-frequency switching and possible driving methods for energy consumption and expanding the temperature range are presented.     

Photoinduced effects in nematic liquid crystals

Temperature, concentration of the solvent and pressure are the parameters that are well known to bring about phase transitions in liquid-crystalline systems. In recent years a new parameter has been added to this list: light. The principle behind these photoinduced transitions is the light-driven shape transformation of certain photoactive materials like, e.g., azobenzene. In this article, we present results of various aspects of our recent investigations on such photoinduced transitions in the nematic phase and highlight the feature that light is a new tool to study phase transitions and the associated critical phenomena.     

Thermomechanical reorientation in nematic liquid crystals

Thermomechanical mechanism of the director reorientation in twisted nematic liquid crystals caused by the one-dimensional longitudinal temperature gradient is predicted and studied theoretically. The calculated director reorientations are in the range that can be measured experimentally very easily.     

End-chain ordering in nematic liquid crystals

The ordering of hydrocarbon end-chains in nematic liquid crystals has been calculated by numerical summation over all chain configurations in the molecular field. The results, used in a self-consistent calculation of the orientational ordering for the full system, explain the variation of the isotropic-nematic transition temperatures (including the even-odd effect) and transition entropies with end-chain length.     

Orientational dynamics in nematic liquid crystals

We examine the behaviour of single-particle orientational time correlation functions in nematic liquid crystals. As well as the expected dynamics involving oscillation in a mean-field potential, and occasional jumps between orientations parallel and antiparallel to the director, we provide the first simulation evidence of long-time tails characteristic of coupling to director fluctuations.     

Hexagonal arrays in nematic liquid crystals

In this letter observations are reported which were carried out by diffraction of a laser beam, by polar diagrams of the diffracted light and by optical microscopy, being performed on a hexagonal structure which appears in nematic liquid crystals for a twist geometry.     

Pretransitional behaviour in nematic liquid crystals

A simple (four sites on the water molecule) potential for Cl^-/H₂O has been generated from six gradient calculations. The potential is in reasonable agreement with the extensive SCF calculations of Dacre.     

Electroconvective instabilities in nematic liquid crystals

Electroconvective instabilities in nematic liquid crystals are caused by the electric force acting on charge densities generated in the medium due to a distortion in the orientation field of the anisotropic medium. The conventional view is that anisotropic conductivity provides the mechanism for the charge accumulation. Several experimental results however can be accounted for only by invoking an additional mechanism involving flexoelectric polarization for the charge accumulation. In this article we discuss the recent experimental and theoretical developments in this field.     

Thermotropic biaxial nematic phase in liquid crystalline organo-siloxane tetrapodes

Infrared absorbance measurements have been carried out on two liquid crystalline organo-siloxane tetrapodes. Results unambiguously show the existence of a biaxial nematic phase below a uniaxial nematic phase. The three components of IR absorbance are used to calculate the various order parameters. On cooling, a weak first-order transition from isotropic to nematic is observed, followed by a second-order phase transition to biaxial nematic where the biaxiality parameters are found to be significantly large. Results are supported by observations from conoscopy and texture. Temperature dependences of the order parameters are well explained by the mean-field model for a biaxial phase.     

双轴向列相液晶的表面能

利用双轴向列相液晶表面能对指向矢和序参量矩阵的旋转不变性,构造了与指向矢^a,^b,^c对易取向轴^a0,^b0,^c0的偏离有关的表面能Fs(a,b,c),以及与序参量矩阵有关的表面能Fs(Q),并得到两种表达式的等价形式.在Fs(a,b,c)和Fs(Q)中分别有四个和两个独立参量.为讨论上述独立参量的物理意义,假设一液晶分子对表面相互作用的简单模型,将液晶分子沿易取向轴^b0,^c0的双轴性取向看成是受到沿^b0,^c0两相互垂直的平均场作用的结果.为测量表面锚泊系数,考虑了三种简单形变位型,即在外磁 关键词： 液晶 弹性能 表面能     

Dynamics of nematic liquid crystal disclinations: the role of the backflow

We measure the electric-field-driven annihilation of nematic disclination pairs with strength +/- 1/2 in the 4-cyano-4'-n-pentylbiphenyl (5CB) liquid crystal. The use of a very weak azimuthal anchoring ensures a two-dimensional director field. The relaxation is governed by the formation of a pi wall connecting the two opposite charge defects. The +1/2 disclinations move almost twice as fast as the -1/2 disclinations. The simple used geometry allows a quantitative comparison with numerical studies based on the hydrodynamics of the tensorial order parameter. The simulations show that in the pi wall regime the symmetry breaking is due to the backflow and not to the elastic anisotropy.