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.     

Thermo‐morphological properties of the smectic C* mesophase in new chiral salicyaldimine liquid crystals

Specific textures and their temperature transformations in large temperature interval for the chiral smectic C* mesophase of new liquid crystalline materials, synthesized by our group, have been investigated in detail. Temperature dependences of the pitch of helical structure of the smectic C* mesophase have been determined for these materials. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Schlieren textures in smectic liquid crystals

The details of well developed schlieren textures have been observed at smectic C and B modifications. In these textures whole numbered point singularities occur and inversion walls with strong similarities to nematic schlieren textures. It is shown that the occurrence of smectic schlieren textures is due to suppositions derived theoretically by Nehring and Saupe. These suppositions are: tilted structure and plane layers with the layers parallel to the supporting glass slides. In smectic B inversion walls occur which allow to conclude on variable tilt angles of the structure.     

A special shear method of alignment for smectic liquid crystals

A special shear method is presented for the planar alignment of smectic liquid crystals. The shear was applied at the isotropic-smectic A or cholesteric-smectic A phase transition temperature, when the “batonnets” floated freely in the isotropic or cholesteric fluid. This type of shearing provided good alignment in the broad range of sample thickness (up to 100 μm) with monodomain areas of about one square centimeter.     

Structure and thermal fluctuations in thin films of smectic liquid crystals

Thermal fluctuations of the smectic layers in freely suspended thin liquid crystalline films have been studied by the methods of X-ray diffraction and reflectometry. The fluctuation dynamics is studied by the methods of photon correlation spectroscopy with the use of a source of coherent synchrotron radiation. In freely suspended smectic films, the typical relaxation times are of the order of several microseconds. In thin liquid crystalline films, the simultaneous damped and oscillatory behavior was observed for the layer undulations.     

Phenomenological model of phase transitions between isotropic, nematic, and smectic phases in liquid crystals

The existence of the biaxial smectic phase SmA _b has been proved in the model of the Landau potential with two (nematic and smectic) interacting order parameters.     

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.     

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.     

X-ray crystal orientation determination for semiconductor single crystals

An improved method to measu reorientation angles of flat single crystals was developed taking Buerger precession technique. The method is described and compared with Laue- and goniometer techniques. Advantages are low costs and short measuring time. An easy evaluation of the records allows to take this method as a routine angle determination for semiconductor manufacturing.     

Discontinuous Freedericksz transition in the smectic C phase

The threshold field of the Freedericksz transition in planar oriented S_C monocrystals possessing positive dielectric anisotropy is calculated for two boundary conditions. It is shown that the threshold field is relatively low when a discontinuous transition is taken into consideration. The effective elastic constant is determined from the experimental data.     

Induced smectic A phase by homeotropic orientation in 4, n-octyloxybenzoic acid (OOBA) at nematic-smectic C phase transition

Using homeotropic and quasi-homeotropic orientation in thin film of 4-n-octyloxybenzoic acid, we indicated intermediate smectic A-like state during the nematic-smectic C phase transition. Pseudoisotropic and focal conic textures are characteristic for homeotropic and quasihomeotropic orientations respectively. The smectic A-like state induced by the boundary conditions exists in a very short (about 0.5 °C) temperature region.     

Phase transition of nematic to smectic C through an intermediate boundary induced smectic A structure

An intermediate phase has been observed at the transition nematic to smectic C where a smectic A structure induced by the boundaries and a nematic phase exist simultaneously. The temperature interval of existence of the intermediate phase is of the order of one tenth of degree. Several conditions are necessary for its appearance: the latent heat L of the phase transition nematic-smectic C of the liquid crystal should be sufficiently small (L < k_BT_c); the nematic phase should undergo a bend deformation by the boundaries (possibly a twist one as well) and the transition nematic-smectic C should be carried out as slower as possible.     

Thermal and optical studies on induced smectic phases of inter molecular hydrogen bonded liquid crystals between decyloxy benzoic acid and citric acid

Hydrogen bonded liquid crystal complex (HBLC) is prepared from mesogenic 4-decyloxy benzoic acid (10OBA) and aliphatic nonmesogenic citric acid (CA). Liquid crystal (LC) phases are investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) studies. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR) validate the intermolecular complementary, cyclic type of hydrogen bond (HB) and molecular environment in the designed HBLC complex. Mesomorphic phases like nematic, smectic X (Sm X) and smectic G (Sm G) are characterized by various textures and using different techniques such as POM, DSC and optical tilt angle measurements. Thermal span width and thermal stability factor for the observed phase is calculated. The complexes are prepared in different mole ratio and their corresponding influences on the phase transitions are discussed. Also it is observed that the HB units play a vital role in stabilizing the new Sm X phase. The variation in thermal stability of smectic phases due to the influence of aromatic cores and length of end chain in the different mole ratio of the present HBLC complexes are also discussed. The variation of tilt angle with respect to temperature in the smectic phase has been experimentally calculated and analyzed. The lowered melting and clearing transition temperatures and extended thermal span width in the Sm X phase are also reported.     

Monte carlo simulations for blue-phase structure in liquid crystals

Blue-phase liquid crystals form three-dimensional structures in a self-organizing manner and are similar to living tissue structures such as the teeth of mice and collagen tissues. This study presents numerical results regarding the conditions under which blue-phase liquid crystals occur. The Monte Carlo simulations are performed by employing an improved Lennard–Jones potential that considers anisotropy and chirality. The conditions for the formation of the blue phase, which vary with respect to the chirality, are examined first. The relationship between the anisotropic parameters and the chiral parameter for the formation of the blue phase is discussed. Identical blue-phase structures are obtained, even when the cell size and molecular number are varied drastically. This discussion is useful for considering the scale-up problem, which is almost always a difficult issue for molecular-scale simulations.     

High-frequency acoustic domains in liquid crystals

A mechanism for the formation of one-and two-dimensional domains in planar nematic and cholesteric liquid crystal samples in an ultrasonic field is discussed. A new approach to the analysis of this phenomenon based on the concepts of nonequilibrium hydrodynamics and taking into account the relaxation properties of the mesophase is experimentally substantiated.