Uniaxial and biaxial nematic liquid crystals

The unusual exhibition of a biaxial nematic phase in nonlinear thermotropic mesogens derived from the 2,5-oxadiazole biphenol (ODBP) core is placed in a general context; the uniaxial nematic phase of the prototypical rod-like mesogen para-quinquephenyl does not follow the classical mean-field behaviour of nematics, thus questioning the utility of such theories for quantitative predictions about biaxial nematics. The nuclear magnetic resonance spectra of labelled probe molecules dissolved in ODBP biaxial nematic phases suggest that a second critical rotation frequency, related to the differences in the transverse diamagnetic susceptibilities of the biaxial nematic, must be exceeded in order to create an aligned two-dimensional powder sample. Efforts to find higher viscosity and lower temperature biaxial nematics (with lower critical rotation rates) to confirm the above conjecture are described. Several chemical modifications of the ODBP mesogenic core are presented.     

Some observations on defects in nematic liquid crystals

After briefly describing the usually observed defects in nematic liquid crystals, we give a summary of our observations on high strength line defects and a regular network of point disclinations on the nematic-isotropic interface.     

Thermomicropolar theory of heat-conducting nematic liquid crystals

Balance laws governing the flow of heat-conducting nematic liquid crystals are presented. Appropriate constitutive equations are formulated taking into account thermal gradients on the basis of the micropolar theory of Eringen [1–4]. These constitutive equations are then simplified taking advantage of the full material symmetry of the nematic phase. A generalized form of the Clausius-Duhem inequality is derived featuring the effects of heat-conduction. A complete set of thermodynamic restrictions governing the material coefficients are obtained. The resulting constitutive theory is then specialized to the linear case with a view to adopt it for practical applications.     

Reciprocal and related theorems for nematic liquid crystals

Reciprocal and related theorems are considered for uniaxial and biaxial nematic liquid crystals. In the general case, where the directors of real and reciprocal materials are assumed to coincide, integral relations between velocity and stress fields of the real and reciprocal materials are obtained. Simplified expressions are also obtained for the high viscosity and elasticity limits.     

Studying the orientation of bio-objects by nematic liquid crystals

We have studied the ability of a liquid-crystal (LC) matrix to visualize and orient DNA molecules. It is established that the relief of the interface between the LC mesophase and conducting contact can be improved without using an additional high-ohmic polymer layer. Spectroscopic and ellipsometric techniques revealed changes in the refractive properties and structure of composites. The obtained results can be used in creating devices for rapid DNA testing with retained form of biostructures.     

An experimental study of dielectric properties of nematic liquid crystals

Dielectric properties of three successive members of the azomethine homologous series are considered in the liquid crystal and isotropic phases. Experimental values are presented for the relaxation time and the dipole relaxation activation entropy and enthalpy.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 4, pp. 656–659, April, 1978.     

Temperature dependence of magnetic susceptibility in three nematic liquid crystals

Results are presented of an experimental study of temperature dependence of magnetic susceptibility in the isotropic state and mesophase of nematic liquid crystals by Gui's relative method.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 35, No. 6, pp. 994–997, December, 1978.     

Carbon nanotube dispersion in nematic liquid crystals: An overview

The aim of present review article is to present a comprehensive and current overview of scientific advancement in liquid crystal and carbon nanotube suspension. Particular attention has been paid to the recent developments and fundamental understanding of carbon nanotube dispersion in nematic liquid crystals. The dispersion and interaction of carbon nanotube in liquid crystal matrices and more elaborately the effect of dispersion on the properties of liquid crystalline materials has been extensively discussed. Recent progress has shown that even a very minute concentration of carbon nanotube in liquid crystals can have a reflective impact on the electrical and optical properties of liquid crystals. Liquid crystals provide a distinctive environment for controlling the alignment of carbon nanotubes whereas carbon nanotubes are important for the enhancement and fine-tuning of liquid crystalline properties. Potential applications of liquid crystal and carbon nanotube suspension are briefly discussed. Conclusion and future perspectives of this rapidly emerging field is provided at the end.     

Self-diffusion and molecular motion in nematic and smectic liquid crystals

A molecular-dynamic simulation of nematic and smectic liquid crystals yields information about the characteristics of rotational and translational motion of molecules in these mesophases.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 4, pp. 685–691, April, 1980.     

Experimental determination of the thermodynamic properties of three nematic liquid crystals

The results of measurements of the density, the magnetic susceptibility, the molar specific heat, and the heat of the nematic-isotropic transition are reported.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 3, pp. 467–472, September, 1985.     

Dispersion of the dielectric permittivity in mixtures of nematic liquid crystals

The spectrum of relaxation time for the low-frequency dispersion has been found to become wider in mixtures of nematic liquid crystals, which can be interpreted as a superposition of two Debye relaxation mechanisms.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 1, pp. 104–109, July, 1979.     

Refractive index of nematic liquid crystals in the submillimeter wave region

Large electro-optic effects of liquid-crystal materials are attractive in applications to various optical devices in a wider wavelength region. Fundamental optical properties in the submillimeter wave region, such as refractive indices and transmission losses for some cyanobiphenyl nematic liquid crystals, have been investigated for the first time, to our knowledge, with a submillimeter laser. Refractive indices of the liquid crystal materials for ordinary and extraordinary rays are a little larger than those in the visible region, and a larger birefringence comparable with the visible region can also be obtained. Although the loss level is larger by ~2 orders of magnitude than that of quartz plate, which is an excellent window in the submillimeter wave region, the transmission of the liquid crystal cell is high enough.     

Weak anchoring of nematic liquid crystals on photo-induced surface relief gratings of organic polysilane

Surface relief gratings on organic polysilane thin films are fabricated by holographic exposure of ultra-violet light, and Au gratings are subsequently prepared on polysilane gratings by vapor deposition of Au. The anchoring energies of 4-pentyl-4′-cyanobiphenyl (5CB) nematic liquid crystal on the fabricated gratings are determined with a saturation voltage method. The anchoring energies of Au gratings are weaker than those of organic polysilane gratings because of suppression of π–π interaction between the liquid crystal and the alignment layer. The polar anchoring energies of Au gratings are also weaker than those reported in literature.     

Measurement of viscosities from the transient current of nematic liquid crystals with negative dielectric anisotropy

A method for measuring the complete set of the Leslie viscosity coefficients of nematic liquid crystals (NLCs) with negative dielectric anisotropy (Δε < 0) is proposed. The method is based on the analysis of the transient current, induced by step voltage excitation, in homeotropic NLC cells. It has been shown that the transient current in homeotropic NLC cells with Δε < 0 can be explained by the theory of NLCs in which the flow effects and the free-slip boundary condition are taken into account [Y. Iwata et al., Jpn. J. Appl. Phys. 43 (2004) L 1588]. It is demonstrated that the complete set of the Leslie viscosity coefficients of NLCs with Δε < 0 can be simultaneously and accurately measured by fitting the transient current calculated by the theory to the experimental data with a differential correction method.     

Optimization of the spatial light modulation with twisted nematic liquid crystals by a genetic algorithm

An optimization technique determining the configuration of optical components in a spatial light modulator is proposed. We study a spatial light modulator composed of a twisted nematic liquid crystal, a polarization state generator, and a polarization state detector. To obtain the desired phase and amplitude modulations, four parameters of the polarization state generator and detector should be optimized. A genetic algorithm is applied in searching the configurations suitable to a given twisted nematic liquid crystal. By embodying the proposed technique, the evolution of the designed cost functions is proved.     

Composite systems for display applications from cellulose elastomers and nematic liquid crystals

Composite systems made from cellulose derivatives solid films impregnated by a nematic liquid crystal can be used as electro-optical devices [H.V. Craighead, J. Cheng, S. Hackwood, Appl. Phys. Lett., 40 (1982) 22; M.H. Godinho, J.L. Figueirinhas, A.F. Martins, Liquid Cryst., 20(3) (1996) 373]. The electro-optical behavior of such cells depends on the nature of the cellulose derivative used, the nematic liquid crystal, the preparation conditions of the cells and of the cellulosic films, and the rugosity of the films' surface [M.H. Godinho, J.L. Figueirinhas, A.F. Martins, Liquid Cryst., 20(3) (1996) 373; M.H. Godinho, A.F. Martins, A. Almeida, J.L. Figueirinhas, 16th International Liquid Crystal Conference, Kent USA, June 24–28, 1996]. In this work we study the influence of preparation techniques, number of polymer layers (1–3 layers eac9 μm thick), thickness of the liquid crystal layer (8– μm) and ageing (9 months), upon the electro-optical response of the cells. The cells used in this work were shown to exhibit high transmission coefficients (up to 82%) in the activated state and high contrast (up to 266). They also show good temporal stability and the cells' optical behaviour is strongly dependent on the number of films' layers.