Discontinuity walls terminated by twist disclinations in smectic a liquid crystals

A discontinuity wall in smectic A liquid crystals was discussed in M. Kléman:Points, Lines and Walls in Liquid Crystals, Magnetic Systems and Various Ordered Media (J. Wiley, Chichester, 1983). Possible solutions describing a singularity — twist disclination which terminates the discontinuity wall in an infinite model smectic A liquid crystal are discussed. One of solutions proposed can be used to describe the so-called zigzag defect line in chiral smectic C liquid crystals on which opposite oriented chevrons are terminated. This work was supported by Grant No.: 202/96/1687 of the Grant Agency of the Czech Republic.     

Theory of the incommensurate smectic-A phases in dimesogenic liquid crystals

A mechanism assuming a mismatch between the basic smectic periodicity and the amplitude and phase modulations periods of the mass density wave order parameter is proposed, for describing the incommensurate smectic-A structure found in dimesogenic liquid crystals. The different sequences of phases found in this family of compounds are described theoretically.PACS: 61.30.Cz Molecular and microscopic models and theories of liquid crystal structure - 64.70.Md Transitions in liquid crystals     

Optical phenomena in liquid crystals and their application in aerophysical studies

The present paper is devoted to optical phenomena in thermotropic liquid crystals and their application to temperature and shear-stress visualization. The main properties of liquid crystals (structure, texture, viscoelastic and optical characteristics) that are of interest for experimental aerophysics are discussed. The results of temperature and shear-stress visualization studies on a wing and corrugated channel wall are presented.     

Natural optical activity and liquid crystals

Optical activity of matter is related to the chirality of its constitutive molecules. In liquid crystals, chiral molecules can give rise to superstructures in which the local dielectric tensor rotates in space describing a helix, a fact which greatly enhances the optical activity of the medium. The structures and the optical properties of some helical phases are well-known, as for instance the cholesteric and some chiral smectic phases. For short enough helix pitches, the periodic medium can be considered optically as homogeneous and described by the same constitutive equations used to define the optical properties of solid crystals. Such liquid crystal phases represent an ideal tool to apply the methods, used since a long time in optics, to define homogeneous models for non homogeneous media and to discuss their limits of validity. A brief account is given of the main results recently found in this research area.     

Twist-edge dispiration system in a finite sample of antiferroelectric liquid crystals

One solution describing an antiferroelectric liquid crystal in a finite sample having the helical structure in the centre and unwound at surfaces is proposed. This solution is composed of twist-edge dispirations. Their elastic interaction energy is estimated and used to determine static equilibrium dispiration positions. Another solution is the system of the 2π-twist disclinations in antiferroelectric liquid crystals. Both solutions can play the same role as 2π-twist disclinations in ferroelectric liquid crystals. Which solution occurs depends on material constants.     

Random lasing from dye-doped chiral nematic liquid crystals in oriented and non-oriented cells

These are the experimental results describing random lasing in dye-doped chiral nematic liquid crystals. A novel random lasing emission is studied in this article based on the helical domains of dye-doped chiral nematic liquid crystals in oriented and non-oriented cells. Under frequency doubled 532 nm Nd:YAG (yttrium aluminum garnet) laser-pumped optical excitation, we carefully observed and analysed random lasing from dye-doped chiral nematic liquid crystals with wavelength ranges from 600 nm to 620 nm. In addition, the line-width of multi-mode peaks is less than 0.2 nm. The difference between the two random lasing behaviours in the oriented and non-oriented cells arises from the fact that random lasing appearing in the oriented cell results from stronger multiple scattering of light generated by the spiral domains of the liquid crystal molecules. Furthermore, chiral nematic liquid crystal micro-domains with different orientations can induce variation of the diffusion constant, thereby resulting in a decrease or increase in the lasing intensity of the random lasers, and an increase or decrease in their energy thresholds. In addition, a detailed comparison of the two experimental results is also presented in the article, showing the dependence of the lasing threshold and the number of lasing modes on the transport mean free path, the excited area, and the sample size. This process allows us to obtain a random laser by changing the structure of the sample, realising tunable random lasers at low cost.     

Thermal orientation effect in a nematic liquid crystal

The system of equations describing the thermal orientation effect in nonchiral liquid crystals, which was observed earlier, is proposed and substantiated. The effect of the director reorientation under the action of the temperature gradient is in many ways analogous to the Freedericksz transition in electric or magnetic fields. The angle of deviation of the director from the initial position is determined by the square of the temperature gradient. The effect is observed for a nematic liquid crystal, initially uniformly oriented, and is not accompanied by macroscopic fluxes of the medium under stationary conditions.     

Analysis of dynamic dislocation-assisted penetration of helium into tin and cadmium deformed by stretching in liquid helium

Curves describing the liberation of helium from Sn single crystals deformed by stretching in liquid ³He and ⁴He and from Cd polycrystals in ⁴He are analyzed. It is found that the stress-strain diagrams for Sn are in qualitative agreement with the concentration of helium in the samples. The peaks in the amount of He liberated from Cd and Sn at temperatures both below and above the melting point are found to be of different types. The reasons for this difference are investigated, and the assumption concerning the existence of a chemical bond between helium atoms and structural defects of the metals under investigation is formulated.     

Surface-induced order and diffusion in 5CB liquid crystal confined to porous glass.

Liquid crystals confined into small cavities are known to have a weak orientational order even above the nematic-isotropic transition temperature. The surface-induced order and molecular dynamics in this temperature range are studied with the aid of deuteron NMR spectra, spin relaxation times T(1) and T(2,) proton dipolar-correlation effect, and direct measurements of the effective diffusion coefficient for the liquid crystal 5CB confined to controlled-pore glasses. Our results show that an arrangement of molecules parallel to the wall is induced by local molecular interactions between the liquid crystal and solid, resulting in a weak and temperature independent surface order parameter, S(0) approximately 0.02 +/- 0.01. There is no indication of a significant slowing-down of molecular diffusion at the wall, neither rotational nor translational. In cavities of nanometer size, where the nematic order evolves gradually upon cooling, a broadening of the NMR linewidths due to dynamic effects should be taken into account.     

Bistability of a “chevron” smectic C* liquid crystal in an external electric field

The system of nonlinear equations describing a surface-stabilized ferroelectric smectic C* liquid crystal in the chevron geometry has been investigated by numerical methods in the framework of the continuum model of liquid crystals. Stable orientational and structural configurations have been studied, and the results obtained have been compared with those derived using simplified models. The height of the potential barrier separating two stable configurations has been determined, and the transition between them under the action of the external electric field has been examined. It has been demonstrated that this effect has a threshold character. The dependence of the threshold field on the film thickness has been analyzed.     

Magnetic and electric birefringence in the isotropic phase of nematic liquid crystals

A theory is developed of the anomalous magnetic and electric birefringence in the isotropic phase of nematic liquid crystals taking into account orientational correlations between neighbouring molecules. Use is made of a modification of Bethe’s method due to Krieger and James, and the properties of the system are derived in terms of a single parameter, viz., the two-particle interaction constant. The expressions for the magnetic and electric birefringence are similar in form to those given by the phenomenological model of de Gennes. Theoretical curves forp-azoxyanisole reproduce the trends in the observed data. A calculation of the nematic-isotropic transition point confirms that this treatment is an improvement over the mean field approximation in describing pre-transition phenomena in the isotropic phase.     

Interaction between disinclinations in nematic liquid crystals

The forces of interaction between disinclinations in nematic liquid crystals as well as between a disinclination and a wall are calculated on the basis of the elastic theory of Oseen and Frank under the assumption K₁₁ = K₃₃.     

Threshold orientation transition in nematic liquid crystals under ultrasonic action

The adequacy of the new theoretical model based on the ideas of nonequilibrium thermodynamics and nonlinear hydrodynamics of liquid crystals for describing the mechanism of threshold reorientation of molecules in a homeotropically oriented layer of a nematic liquid crystal under the effect of a cross-homogeneous ultrasonic wave is experimentally substantiated. It is demonstrated that (i) the critical particle velocity is inversely proportional to the layer thickness, (ii) its relation to the frequency of oscillations is ambiguous and depends on the relation between the ultrasonic frequency and the inverse relaxation time of the orientation order parameter, and (iii) the stationary distortion of microstructure is homogeneous along the layer.     

Fluctuations in bounded cells of liquid crystals in an external field

Spatial correlation functions of orientation fluctuations in bounded cells of smectic and nematic liquid crystals are calculated taking into account the effect of external fields and finiteness of the energy of anchoring to the surface. The cases of positive and negative anisotropies of magnetic susceptibility or permittivity are considered. The calculations are based on the division of degrees of freedom into bulk and surface ones and on the reduction of the computation of the continual integral determining the correlation function to the solution of the Euler equation with corresponding boundary conditions of the first or third kind. The obtained correlation functions are used for describing the intensity of light scattered in nematics for the planar and homeotropic orientations. It is shown, in particular, that the measurements of the angular dependence of the scattered light intensity may serve as a reliable method for determining the energy of anchoring of a liquid crystal to a substrate for different values of the external field.     

A new era for liquid crystal research: Applications of liquid crystals in soft matter nano-, bio- and microtechnology

Liquid crystals constitute a fascinating class of soft condensed matter characterized by the counterintuitive combination of fluidity and long-range order. Today they are best known for their exceptionally successful application in flat panel displays, but they actually exhibit a plethora of unique and attractive properties that offer tremendous potential for fundamental science as well as innovative applications well beyond the realm of displays. Today this full breadth of the liquid crystalline state of matter is becoming increasingly recognized and numerous new and exciting lines of research are being opened up. We review this exciting development, focusing primarily on the physics aspects of the new research thrusts, in which liquid crystals – thermotropic as well as lyotropic – often meet other types of soft matter, such as polymers and colloidal nano- or microparticle dispersions. Because the field is of large interest also for researchers without a liquid crystal background we begin with a concise introduction to the liquid crystalline state of matter and the key concepts of the research field. We then discuss a selection of promising new directions, starting with liquid crystals for organic electronics, followed by nanotemplating and nanoparticle organization using liquid crystals, liquid crystal colloids (where the liquid crystal can constitute either the continuous phase or the disperse phase, as droplets or shells) and their potential in e.g. photonics and metamaterials, liquid crystal-functionalized polymer fibers, liquid crystal elastomer actuators, ending with a brief overview of activities focusing on liquid crystals in biology, food science and pharmacology.     

横向电场对平面织构胆甾相液晶螺距及反射特性的影响

阐述一种理论模型,讨论共面转换（IPS）模式下横向电场对平面织构胆甾相液晶螺距及扭曲角的影响．利用琼斯矩阵法计算了光正入射情况下未加电场时胆甾相液晶的反射谱．在上述模型基础上,计算了施加电场后胆甾相液晶的反射峰值波长以及反射带宽随场强的变化关系．探讨了忽略锚定与强锚定两种边界条件下横向电场对平面织构胆甾相液晶螺距及反射特性的影响．所得结论在理论上证实：共面转换模式下电场可以调谐胆甾相液晶的反射光颜色,从而为基于电控螺距原理的胆甾相液晶反射式彩色显示方案提供了理论上的依据． 关键词： 胆甾相液晶 电控螺距 共面转换 琼斯矩阵法     

Laser beam manipulation by composite material electro-optic devices

Liquid crystals are anisotropic materials whose optical properties can be easily controlled applying an electric field. For this reason, they are widely used in electro-optical devices. Besides other applications, liquid crystals electro-optical devices are suitable for laser beam manipulation. In this paper, we present some devices developed or studied in our laboratory: a twisted nematic liquid crystal (TN-LC) polarization rotator, a TN-LC protected light sensor, a polymer dispersed liquid crystal (PDLC) light modulator, a PDLC optoelectronic polarizer and a polymer liquid crystal (PoLiCryst) light modulator.     

Phase maps of polycrystalline human biological fluids networks: statistical,correlation, and fractal analysis

The complex statistical and fractal analysis of phase properties, inherent to birefringence networks of liquid crystals consisting of optically-thin layers, prepared from synovial fluid taken from human joints, is performed in this work. Within the framework of a statistical approach, the authors have investigated values and ranges for changes of statistical moments of the 1-st to the 4-th orders that characterize coordinate distributions for phase shifts between orthogonal components of amplitudes inherent to laser radiation, transformed by synovial fluid layers, for human joints with various pathologies. The correlation criteria for differentiation of phase maps, describing pathologically changed liquid-crystal networks, have been ascertained. In the framework of the fractal approach, dimensions of self-similar coordinate phase distributions as well as features of transformation of logarithmic dependences for power spectra of these distributions for various types of human joint pathologies are determined.     

Photorefractive effect in ferroelectric liquid crystals

In this paper, we review recent progress of research on the photorefractive effect of ferroelectric liquid crystals. The photorefractive effect is a phenomenon that forms a dynamic hologram in a material. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon is applicable to a wide range of devices related to diffraction optics including 3D displays, optical amplification, optical tomography, novelty filters, and phase conjugate wave generators. Ferroelectric liquid crystals are considered as a candidate for practical photorefractive materials. A refractive index grating formation time of 8–10 ms and a large gain coefficient are easily obtained in photorefractive ferroelectric liquid crystals.     

Use of an open photoacoustic cell for the thermal characterisation of liquid crystals

In this paper, we describe the use of an open cell photoacoustic configuration for the evaluation of the thermal effusivity of liquid crystals. The feasibility, precision and reliability of the method are initially established by measuring the thermal effusivities of water and glycerol, for which the effusivity values are known accurately. In order to demonstrate the use of the present method in the thermal characterization of liquid crystals, we have measured the thermal effusivity values in various mesophases of 4-cyano-4^′-octyloxybiphenyl (8OCB) and 4-cyano-4^′-heptyloxybiphenyl (7OCB) liquid crystals using a variable temperature open photoacoustic cell. A comparison of the measured values for the two liquid crystals shows that the thermal effusivities of 7OCB in the nematic and isotropic phases are slightly less than those of 8OCB in the corresponding phases. Received: 28 March 2001 / Revised version: 8 June 2001 / Published online: 18 July 2001