On orientational fluctuations in nematic liquid crystals

The factors determining the spectral density of intrinsic optical noise of a nematic liquid crystal (NLC), caused by fluctuations of director orientation in a frequency range of 0.01–1 Hz, have been experimentally established for the first time for (10–100)-μm layers, characterized by rigid anchoring at the boundaries and a quasi-homeotropic macrostructure. A model developed for estimating the spectral density of this noise for an NLC layer with a previously deformed macrostructure and finite molecular anchoring energy has been approved.     

Dual nature of the orientational effect of ultrasound on liquid crystals

The new model of thresholdless distortion of the orientational structure in a homeotropic layer of nematic liquid crystal with free ends in ultrasonic field has been experimentally substantiated for the first time. The model is constructed within the concepts of nonequilibrium thermodynamics and statistical hydrodynamics of liquid crystals for the frequency range in which the elastic and viscous wavelengths are, respectively, longer and shorter than the layer thickness. The main regularities of the phenomenon, which relate the conditional effect threshold to the ultrasonic frequency and layer thickness, have been established based on the experimental data for (20–150)-μm-thick layers in the frequency range of 0.1–9 MHz. These data are compared with the results of numerical calculations, performed taking into account two mechanisms of liquid crystal structure distortion (convective and nonlinear relaxation ones).     

On the nature of the orientational effect of ultrasound on nematic liquid crystal

Experimental substantiation of the validity of the model of orientational distortion in a homeotropic layer of nematic liquid crystal under an ultrasonic beam with a sharp boundary is presented for the first time. The model is constructed within the concepts of nonequilibrium thermodynamics and statistical hydrodynamics, taking into account the processes of structural relaxation of the mesophase. It establishes the relationship between the characteristics specifying the homeotropic structure deformation (layer thickness, ultrasound frequency, parameters of the molecular micromodel of liquid crystal, and its material constants) and the layer transparency for a linearly polarized light beam. The calculation results are compared with the experimental data in the frequency range of 0.1–3 MHz.     

Functionalisation of cholesteric liquid crystals by direct laser writing

Cholesteric liquid crystals selectively reflect circularly polarised light with the same handedness as the helix. Because of their sensitivity to external stimuli, such as heat and electrical fields, various applications utilising their tunability have been proposed. Tuning is usually performed in the bulk, meaning that cholesteric liquid crystals usually possess a single pitch throughout the medium. However, when the helical structure is locally modulated, different optical properties arise, such as tunable photonic defect modes and multiple reflection bands. Here we show a technique to locally modulate the helical structure of cholesteric liquid crystals on a submicron scale, based on two-photon excitation direct laser lithography. Two examples of cholesteric liquid crystal structures with modulated helical structures will be presented.     

On the nature of instability of the hardening and structural states in crystals with forest dislocations

The branch of studies of work-hardening and the related structural states as yet has not been developed sufficiently. This paper touches upon the problem of work-hardened state formation by a dislocation forest and its stability against a range of infiuences.     

The investigation of the kinking process with help of cholesteric liquid crystals

Process of kinking in CsI crystals has been studied with help of CLC (cholesteric liquid crystals) sensitive to deformation and temperature. It is shown that kinking formation is accompanied by local heat in the kink band. The velocity of deformation is the more the larger the quantity of heat in the kink band.     

Investigations of surface of glasses by nematic and cholesteric liquid crystals

The relatively strong interaction between the surface of the glass and the nematic liquid crystals is demonstrated. The same texture of liquid crystals is observed after every renewed cycle of warming and cooling. Defects, which are invisible without an electrical field, become visible by the dynamic scattering effect. The thermografic results, which are received by cholesteric liquid crystals, are able to demonstrate only strong defects.     

Topological defects of cholesteric liquid crystals for volumes with spherical shape

The existence of the monopoles both with one x (+2) line and two x (+1) lines in cholesteric volumes with spherical shape is shown experimentally by microscopic observations with polarized light. The effect of disapperarnce of x lines near the cholesteric-smectic A transition is discovered.     

Effects of electrohydrodynamic instability in smectic C liquid crystals

The effects of electrohydrodynamic instability (EHDI) in smectic C liquid crystals in an electric field are studied. The objects of study are oriented layers of para-hydroxybenzoic acid possessing a smectic phase. The observed effects are divided into three groups according to the character of the motion of molecular centers of mass and the orientation of the director and the smectic C liquid crystal layers. The instabilities of the azimuthal and Kapustin-Williams domains are experimentally found. The experimental cell consists of two glass plates with tin dioxide electrodes separated by Mylar spacers of a specified thickness. Various schemes of domain rotation are considered. The EHDI is found to depend on the layer geometry.     

Chiral SmC* liquid crystals obtained by mixing of SmC and cholesteric phase

The binary mixtures of the substance providing cholesteric phase exhibit a chiral SmC* phase for certain concentration and temperature range. This phase was verified to be ferroelectric. The temperature dependences of the spontaneous polarization and the coercive field were determined for three concentrations. The pitch of the helical structure is approximately indirectly proportional to the molar concentration of the cholesteric substance and varies from 3 μm to about 25 μm in the concentration range from 20 mol% to 5 mol% of the cholesteric substance. For the lower concentrations the sample is unwound in planar samples due to the surface anchoring. Two possible unwound planar configurations can be switched by electric field.     

The Electrohydrodynamical Transient instability region in Smectic C liquid crystals

The hydrodynamical instability transient region between conducting („initial”︁ domains) and dielectric („fundamental”︁ domains) regimes in smectic C is investigated. Two-dimensional domain structure and two-dimensional coherent light diffraction is observed in the transient region. A comparison with electrohydrodynamical instability transient region in nematics is presented.     

Effect of various external factors and pretransitional phenomena on structural transformations in cholesteric liquid crystals

The publications on low-molecular thermotropic liquid crystals with cholesteric structures have been reviewed. The effect of an applied electric field on cholesteric structures is studied. Bistability of the cholesteric-nematic transition, electric field-induced color textures of cholesteric mixtures, and electro-optics of amorphous cholesteric structures are considered as well as pretransitional (blue and TGB) phases, discotic cholesterics, PDLC films, flexoelectric electro-optics, and photostimulated switching in cholesterics.     

On the theory of normal growth of crystals in binary systems

Crystallization processes of binary systems are considered, in which solid particles consisting of components A and B grow from the melt in monolayers of a two-phase boundary zone by a mechanism of spontaneous thermal equilibrium type concentration fluctuations. The calculation is based on probability distribution functions. The region of small supercoolings of the binary system melt-crystal proves most valid for this investigation. The change of the mean crystallization rate of the two-phase boundary zone with the supercooling of the melt shows a linear behaviour. The dependence of the kinetic coefficient on the degree of the atomic roughness of the phase boundary with different numbers of monolayers in the phase boundary, the total number of particles in each monolayer, and a certain roughness are connected with the energy of positional disorder in binary systems.     

On the nature of growth hillocks on sodium chlorate crystals

Optical examination of as-grown {100} surfaces of sodium chlorate crystals grown from aqueous solution revealed the presence of elliptical growth hillocks. The hillocks were present on both enantiomorphous forms and originated from dislocations, inclusions, and microcrystals attached to the growing surface. The value of the surface entropy factor equal to 4.55 at 313 K suggests that crystals grow via/or with the participation of dislocation mechanism, and the hillocks are dislocation growth centers. Compound mechanism controlled growth of some crystals because edge nucleation and dislocation centers operated simultaneously on the same surfaces.     

On the mechanism of chemical polishing of GaAs crystals

In order to establish reproducible conditions for chemical lap polishing of GaAs with NaOCl solutions, containing also OH⁻- and CO -ions, the mechanism of dissolution was investigated using the rotating disk arrangement. In solutions with excess OCl⁻ concentrations likewise diffusion and a reaction of the first order with respect to OH⁻ and CO, complexing agents for Ga-ions, control the dissolution rate, and the surfaces of the wafers are polished. OH⁻ and CO₃^2– concentrations in excess with respect to OCl⁻ lead to rate determining OCl⁻ diffusion and to irregularly polished or film covered surfaces. Diffusion constants for OH⁻, CO and OCl⁻ are given.     

On the interrelation between relaxation and hydrodynamic phenomena in nematic liquid crystals

The theoretical prediction of the interrelation between the relaxation processes and hydrodynamic phenomena observed in nematic liquid crystals (NLCs) under binary acoustic action at MHz frequencies is experimentally substantiated. The existence of a new (relaxation) mechanism of streaming generation in thin layers of the mesophase is proven by the results of measurements of the velocity of steady-state acoustic streaming in the NLC in a planar capillary with homeotropic boundary conditions induced by the simultaneous action of coherent compressional and shear waves. A model that takes into account not only the classical convective contribution to the streaming formation but also the nonlinear relaxation one is tested for the first time. It is shown that only this model describes the aforementioned phenomenon consistently with the experimental data.     

On the morphological instability of growing crystals (I) morphological peculiarities of the transition shapes of crystals in diffusion-controlled regime of growth

The shape and size of the gross morphological defect which occurs (in the form of a depression) as a result of the diffusion inhomogeneity in the matter supply on the growing crystal face with supercritical dimensions are investigated. The transition shapes of similar skeletal crystal faces are determined by considering the configuration of the steps which are moving (and accumulating) from the edge toward the centre of the crystal face. The basic parameter which is evaluated, is the width of the macroscopically flat periphery which grows around the depression and its alteration when the crystal is enlarged. The theoretical analysis is carried out by presuming that no transportation of crystallizing material occurs parallel to the crystal face. The calculations are based on the different thicknesses of the Nernst diffusion layer, a magnitude which can be easily determined experimentally by some interferometric technique, over the various regions of the growing face.     

The nature of phase separation in binary oxide melts and glasses. II. Selective solution mechanism

A comprehensive review of structural data in binary silicate systems indicates that the tetrahedral critical radius (87.2 pm) of binary silicate melts (or glasses) is associated with the silicon tetrahedral network that defines the structure of the melt. In a binary system, most of the cages present in the melt are made of six and five-membered rings of silicon tetrahedra. Cages bounded by six or more-membered rings can host cations of all sizes. However, cations that enter in cages made of five-membered rings are discriminated by their ionic radius. Cations with ionic radii larger than about 87.2 pm (network modifiers) cannot enter in pentagonal apertures; cations with radii smaller than 87.2 pm (amphoteric cations) can. Cages bounded by pentagonal rings play a key role in phase separation by selecting which cations can fit in them, adopt a four-fold coordination, and reduce the size of miscibility gaps, i.e. the cages permit explaining why some cations are amphoteric. This result is important because it shows that a structural control is exerted by the solvent (here SiO₂) upon immiscibility which creates a selective solution mechanism that affects small (<87.2 pm) cations in binary silica-rich melts.