External magnetic field-dependent tricritical points of uniaxial-to-biaxial nematic transition

External magnetic field-dependent behavior of tricritical points of uniaxial–biaxial nematic phase transition in a mixture of prolate and oblate molecules has been studied using the Landau phenomenological theory. Topological classification of phase diagrams in the field-temperature coordinates is performed using a sixth-order term in order parameter tensor. For both prolate and oblate molecules, the tricritical order parameters corresponding to the uniaxial–biaxial increase with the intensity of the external magnetic field. Field-induced paranematic phase has only been reported for prolate molecules, but for oblate molecules, field-induced splitting of the order parameter has been reported.     

Effects of fields and anchoring on biaxial nematic ordering

Symmetry of a nematic liquid crystal phase is broken by an anchoring wall and also by an external field. Nematic system sandwiched between biaxial anchoring walls is introduced as a correspondent to a bulk nematic system exposed to a couple of fields, an electric field and a magnetic one in directions perpendicular to each other, and thermal behaviours of the system are studied. The crossover between a homeotropic structure and homogeneous one occurs, similarly to the bulk system in the fields, in which the anchoring condition of coexistence is shown to have the same expression as the one at the bulk. As to a characteristic phenomenon at the sandwiched system, it is proved that an appearance of a biaxial nematic order suppresses a uniaxial nematic order. A surface transition, i.e., a wetting phenomenon is shown to occur also in the biaxial nematics, even though the uniaxial order is suppressed therein.     

Temperature dependence of ratio between dielectric anisotropy and order parameter in fluorinated nematic liquid crystals

Temperature dependence of ratio between dielectric anisotropy and order parameter of fluorinated nematic liquid crystal is investigated by using a semi-empirical molecular orbital package that can accurately calculate an angle between molecular dipole moment and long axis. We optimize the molecular conformations with three semi-empirical Hamiltonians AM1, PM3 and PM5, and then make a comparison between computational results and experimental measurements. It is shown that the results obtained from AM1 method are in good agreement with the measurements. The present study offers an applicable method to predict the dielectric properties of liquid crystal material.     

向列相液晶中光学相位共轭技术的研究进展

对向列相液晶中光学相位共轭技术的研究进展进行了介绍,分析总结了向列相液晶中光学相位共轭过程的机理与实验方法,并对向列相液晶中的光学相位共轭技术的应用前景进行了展望。     

Comparison of optical compensations based on discotic liquid crystals with linear and non-linear orientation for wide viewing angle twisted nematic liquid crystal displays

Optical compensation film using discotic liquid crystals has been commercialized to suppress light leakage of twisted nematic (TN) liquid crystal (LC) cells in the dark state when the viewing directions are off-axes. In the conventional film, the tilt angle variation of the discotic LC layers is linear, whereas that of the rod-like LC layers in the TN cell is non-linear with twist alignment. We investigated the optical compensation effects in terms of iso-contrast ratio, gray scale inversion and color shift with linear and non-linear orientation in a tilt angle or twist and non-twist orientation on TN-LC cells. The results showed that the optical compensation with non-linear orientation exhibited an improved performance in suppressing light leakage in the dark state.     

Orientational order parameters of a binary mixture showing both induced smectic and re-entrant nematic phases

The phase diagram of a new binary system of two polar nematic compounds, 4-cyanophenyl [4′ (4″-n-heptylphenyl)] benzoate(7CPB) and 4-cyanophenyl 4-nonylbenzoate (9.CN), shows the presence of both the induced smectic and re-entrant nematic phases in a certain concentration range 0.4 < x _9.CN < 0.87. The results of X-ray diffraction and density measurements are reported here. Orientational order parameters have been determined in the induced smectic, normal nematic and re-entrant nematic phases from X-ray diffraction measurements. The order parameters have been fitted to those calculated from modified McMillan's theory as proposed by Luckhurst and Timimi. The agreement is found to be closer to the modified model. The experimental variation of the layer thickness with mole fraction of 9.CN has also been explained fairly well by assuming the presence of two types of homo dimers (7CPB + 7CPB, 9.CN + 9.CN) and one type of hetero dimer (7CPB + 9.CN).     

Exposure and temperature dependences of contact angle of liquid crystals on photoaligning surface

We report on first studies of wetting of liquid crystal on photoaligning surface. We observed strong light-induced variations of a contact angle of a liquid LC 5CB on a photoaligning surface of fluoro-polyvinyl-cinnamate (PVCN-F) and we connected these variations with changes of PVCN-F polarity. We also present drastic changes of the contact angle of a nematic liquid crystal (MLC-6080) on the PVCN-F surface in a vicinity of temperature anchoring transition of liquid crystal from homeotropic orientation to planar one. We did not find any peculiarities in temperature dependence of the contact angle of isotropic liquid (glycerol) on PVCN-F and of nematic liquid crystal on pure glass in the same temperature range. It allows us to suggest that rearrangement of LC molecules and flexible fragments in the LC-polymer interface are responsible for the change of surface tensions both of LC and polymer and the observed jump of the contact angle.     

A small-angle X-ray scattering study of the isotropic and nematic phases of V2O5 suspensions

We report small-angle X-ray scattering experiments performed in both the isotropic and nematic phases of aqueous V₂O₅ suspensions. We show that the scattering in the isotropic phase can be well described in the whole accessible q-range by only considering the form factor of non-interacting ribbons. We investigate the influence of concentration and pH on the dimensions of V₂O₅ ribbons and show that these parameters do not have any significant effect, as long as the system stays well within the chemical stability domain of the ribbons. We then show that nematic single domains display an anisotropic small-angle scattering pattern, even at scattering vectors small compared to that at which a characteristic correlation peak is observed. This feature is expected for a nematic phase, but was rarely observed. We finally try to describe this scattering within the framework of theories developed for the structure factor of a nematic polymer, and we reach the conclusion that chain ends are certainly important to understand this pattern. Received 21 July 1999 and Received in final form 17 December 1999     

Electro-optic characteristics as a function of pretilt angle in the optically compensated splay liquid crystal cell

Optically compensated splay (OCS) mode, where the liquid crystals (LCs) are almost homeotropically aligned with rubbed surfaces at parallel directions and a mid-director orients parallel to the substrate, was known to show a wide-viewing-angle due to self-compensation effect. The device requires a setting voltage and phase transition time in order for LC to have a transition from almost vertical alignment to the splay state. The setting voltage, transition time, and response time of the device as a function of surface pretilt angle were studied. The results indicate that with decreasing surface pretilt angle in the OCS cell, both phase transition time and setting voltage decrease and response time becomes faster.     

Localized bifurcations and defect instabilities in the convection of a nematic liquid crystal

The stationary and the time-dependent homogeneous ordered states in convection may both become unstable against localized perturbations. Defects are then created and they may contribute to the disorganization of the homogeneous state. We present an experimental study of defects in some homogeneous stationary structures as well as in the traveling-wave states of convection of a nematic liquid crystal. We show that the core of the defects is a germ of the unstable state and it can become unstable under the external stress. Then, either fully homogeneous states with the symmetry of the core, or complex disordered states can develop from the local instability of defects in processes quite similar to displacive transitions in solids. Some of the main features are qualitatively similar to numerical simulations of an appropriate Landau-Ginzburg equation.     

Temperature dependence of the critical magnetic field of the structural transition in MBBA-based ferronematics

We studied the structural transitions in ferronematics based on the thermotropic nematic liquid crystal MBBA (4^′ -methoxybenzylidene-4-n-butylaniline) having a nematic-to-isotropic transition temperature T _N–I?=?48.0^○C and in MBBA-based ferronematics doped with a magnetic suspension consisting of Fe₃O₄ particles (10?nm in diameter) coated with oleic acid as a surfactant. The ferronematic samples were prepared with different volume concentrations of magnetic particles φ?=,1× 10^?4, 2× 10^?4 and 5×10^?4. The temperature dependences of the critical magnetic fields in a bias electric field under strong applied magnetic fields are presented. We calculated the surface density of anchoring energy W at the nematic–magnetic particle boundary. Scaling of the structural transition in the MBBA and MBBA-based ferronematics with the temperature of the nematic-to-isotropic transition was observed.     

Temperature effect on threshold voltage and optical property of twisted nematic liquid crystal with applied different voltages

In this paper, temperature-dependent birefringence theory of liquid crystal is used to investigate the temperature effect on the threshold voltage. An expression for describing the non-linear relationship between the threshold voltage and temperature is deduced. In addition, we theoretically discuss the temperature effect on the transmitted ratio of the namatic twisted liquid crystal without the applied voltage and with the applied voltage. It is found that the transmitted ratio is decreased linearly as temperature is increased when the liquid crystal is not applied with voltage, but the transmitted ratio is increased linearly as temperature is increased when the liquid crystal is applied with voltage. The threshold voltages and the transmitted ratio are measured at different temperatures. The experimental data are consistent with the theoretical calculated results.     

Equilibrium structures of planar nematic and cholesteric films in electric fields

Liquid crystal layers subjected to an electric or a magnetic field can have several types of instabilities. This paper reviews recent theoretical studies concerning equilibrium structures of planar layers.

Using the Oseen-Frank elasticity theory, the Freedericksz transition and the transition to static periodic domains of planar nematic and cholesteric films are reconsidered. A perturbation treatment of nonlinear torque balance equations for the director is suitable to derive amplitude equations for the film distortions under the action of a field and to predict the topological features of phase diagrams for equilibrium states. The competition between the Freedericksz effect and the formation of periodic distortions is studied varying material and geometrical parameters. Some results are useful to optimize cholesteric mixtures for application in display devices in such a manner, that the occurrence of periodic domains is avoided.     

Temperature and orientation dependence of surface relief gratings based on dye-doped polymer film with the interface of nematic liquid crystals

The formation of surface relief grating on dye-doped polymer film with the interface of nematic liquid crystals has been investigated by means of the holographic technique. The first-order diffraction efficiency of surface relief grating depends on the temperature and the orientation of molecular director in the interface of nematic liquid crystals. The diffraction efficiency is roughly independent of thermal fluctuations of molecular director in the most part of nematic temperature range and apparently drops near the transition temperature. The morphology of surface relief grating demonstrates that the surface modulation is larger for molecular director parallel to the groove direction. The experimental result also shows that the first-order diffraction efficiency is dependent on the surface modulation of surface relief grating.     

Laser light scattering technique for non-invasive in situ simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals

Theelasticconstantsandtheviscositycoefficientsarethekeyparametersforthephysicalpropertiesofnematicliquidcrystals(LCs)[1,2],fortheyarecorrespondingtothethresholdvoltageandtherespondingtimeofliquidcrystaldisplaydevices[3,4].Sofarnotechniquethatcansimultaneouslymeasurethetwoparametersinsituhasbeenreported.TheelasticconstantsandviscositycoefficientsofnematicLCsatthepresentaremeasuredusingdifferentmethodsindependently.TheviscositycoefficientsisgenerallymeasuredwithOstwaldviscosimeter,andtheelasti…     

Transport and crystallization of colloidal particles in a thin nematic cell

In a thin planar nematic cell, the application of an AC electric field induces a macroscopic transport of micrometer-sized colloidal particles along the nematic director. We have analyzed the dependence of particle velocities on the electric-field amplitude and frequency and found that it decreases exponentially with increasing frequency. Using specially designed electrodes we have observed that colloidal particles could be pumped and accelerated across the field-no-field interface, and measured the structural force and the corresponding potential, which is of the order of 10000 k_BT for 4μm particles. We demonstrate that spatially periodic close-packed crystalline colloidal structures can be obtained, which are thermodinamically metastable for many days after turning off the electric field and slowly decay into linear chains. Above the nematic-isotropic phase transition, such crystalline structures are non-stable and decay in few minutes.     

脉冲溅射V2O5薄膜结构和性能研究

V2O5薄膜具有很好的离子注入/退出可逆性,是最有潜力的锂离子储存层的候选材料之一,它的电学特性与制备方法、化学计量比、结构和取向等有直接关系,仔细控制工艺参量是制备出在锂电池上应用的V2O5薄膜关键。研究中采用脉冲磁控反应溅射方法,通过精确地控制氧分压、基底温度等关键工艺参量,在石英玻璃和硅片上制备V2O5薄膜。利用X射线衍射和X射线光电子谱,分析了薄膜的成分、相结构、结晶和价态情况,用原子力显微镜表征了薄膜的微观结构,用分光光度计测量从200—2500nm波段V2O5薄膜纯度高、相结构单一、结晶度好。高低温电阻变化2个量级,薄膜的光学能隙为2.46eV。     

Thermo acoustic parameters in the nematic and isotropic phases of 5CB and tetraethyl methane in 5CB

The addition of non-rod like solute impurity to a liquid crystalline solvent leads to some exciting results such as depression of nematic–isotropic transition temperature and the formation of two-phase region. In view of this, specific volume and thermal expansion studies in the nematic and isotropic phases of p-n-pentyl-p¹-cyanobiphenyl (5CB) and dilute solution of the quasispherical solute tetraethyl methane were considered. From these results thermo acoustic parameters such as isochoric temperature coefficient of internal pressure (X), isochoric volume expansivity (X¹), the reduced compressibility (β), the reduced volume (V), isothermal microscopic Gruneisen parameter (Γ), Huggins parameter (F), fractional free volume (f) and Sharma parameter (S_o) are estimated both in nematic and isotropic regions. Using the data given by Oweimreen et al. [G.A. Oweimren, A.K. Shihab, K. Halhouli, S.F. Sikander, Mol. Cryst. Liq.Cryst. 138 (1986) 327] variations of these thermo acoustic parameters with temperature and two phase region are critically examined.     

Numerical investigation of influence of ionic space charge and flexoelectric polarization on measurements of elastic constants in nematic liquid crystals

Deformations of nematic layers caused by magnetic field allow determination of the elastic constants of liquid crystal. In this paper, we simulated numerically the deformations of planar and homeotropic nematic layers. The flexoelectric properties of the nematic and presence of ions were taken into account. Our aim was to show the influence of flexoelectricity on the results of the real measurement of the elastic constants k₃₃ and k₁₁. In these simulations, we calculated the optical phase difference ΔΦ between the ordinary and extraordinary rays of light passing through the layer placed between crossed polarizers as a function of the magnetic field induction B. One of the elastic constants can be calculated from the magnetic field threshold for deformation. The ratio k₃₃/k₁₁ can be found by means of fitting theoretical ΔΦ(B) dependence to the experimental results. The calculations reveal that the flexoelectric properties influence the deformations induced by the external magnetic field. In the case of highly pure samples, this may lead to false results of measurement of the elastic constants ratio k₃₃/k₁₁. This influence can be reduced if the nematic material contains ions of sufficiently high concentration. These results show that the flexoelectric properties may play an important role, especially in well purified samples.