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.     

Frequency dependence of the threshold voltage of the Freedericksz effect in nematic and nematic reentrant phases

In four substances exhibiting reentrant nematic phases the threshold voltage of the Freedericksz transition was measured as function of the frequency in the nematic and reentrant nematic phases. The isotropy frequency and the relaxation frequency were calculated using the experimental data. We found that extrapolated to the same temperature the relaxation frequency of the reentrant nematic phase is larger than that of the nematic phase.     

Increasing the lasing efficiency in cholesteric liquid-crystal photonic structures

Two types of lasing in cholesteric liquid crystals (LCs) in the range of luminescence of laser dye molecules have been investigated. The first type belongs to the Bragg modes at the photonic band edge, which propagate along the normal to the LC layer. The second type of lasing is related to the modes leaking into the substrate and propagating at small angles to the LC layer. It is shown that the Bragg lasing efficiency can be significantly increased under wide-aperture optical pumping. The method proposed for increasing the lasing efficiency is based on suppressing the excitation of leaky laser modes using partially absorbing thin films as the coatings for LC-orienting substrates. Both experimental results and the theoretical model of the effect using the numerical simulation data are discussed.     

Effect of polarity of smectic Ad molecules on the induction of the nematic phase in binary mixtures of smectics A1 and Ad

The phase diagrams are determined of binary mixtures composed of 4-n-alkylbiphenylates 4-cyanobiphenyl (n-CBB)-smectics A₁, for n < 8 and esters of 4-(trans-4′-n-alkylcyclohexyl)benzoic acid (10CPCHB, 10NPCHB, 10FOPCHB)-smectics A_d. The effect is studied of the smectic layer spacing ratio and polarity of the components on the induction of the nematic phase in the tested series of compounds. The virtual N S_Ad phase transition points in series n-CBB are estimated and the effect is discussed of the stability of the smectic phase and polarity of the molecules of the mixture components on the width and position of the nematic gap.     

Time evolution of cholesteric fingers of the second species in an electric field

Different growth modes of isolated cholesteric fingers of the second species (CF‐2) in an electric field at voltages near a coexistence line (V₂) between cholesteric and nematic mesophases are in detail described. Videomicroscopy and computer image analysis were used for investigation of the pattern in polarized light. It is shown how a drift, a lengthening and a shape of fingers depend on the voltage at which the growth sets in and three typical scenarios are distinguished. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Light-controlled change in the helical pitch and broadband tunable cholesteric liquid-crystal lasers

This paper presents an overview of the results obtained in recent investigations of the control over the helical pitch under irradiation and the use of these data for the design of compact broadband tunable lasers based on dye-doped cholesteric liquid crystals. It is demonstrated that a reversible change in the lasing frequency can be achieved upon exposure to two low-power light-emitting diodes. Another alternative approach to the generation of tunable laser radiation in the visible and ultraviolet spectral regions (370–680 nm) in a specially designed cell with a cholesteric liquid crystal that is doped with several dyes and possesses a helical pitch gradient is considered.     

On the modelling of a pretilted nematic cell

The authors have found that the perturbation method may be successfully applied in order to get approximated analytical solutions very close to the exact ones for the tilt angle distribution inside a pretilted nematic liquid crystal cell, if the elastic ratio k₁₁/k₃₃ is «, ～, or » 1. Hence, both the cell impedance and transmittance may be easily predicted for all commonly used nematics.     

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.     

Viscous and elastic properties of the nematic in the tetrapalladium organyl-pentadecane system

Temperature dependences of the dielectric anisotropy ?_a, effective rotational viscosity \(\tilde \gamma \), and elastic modulus K ₁₁ of the nematic phase of the tetrapalladium organyl-pentadecane system containing 55 wt % pentadecane have been experimentally studied. The quantities ?_a, \(\tilde \gamma \), and K ₁₁ are shown to decrease: ?_a decreases by a factor of about 2.5; \(\tilde \gamma \), by almost two orders of magnitude; and K ₁₁, by more than an order of magnitude. It is found that an applied electric field induces growth of tetrapalladium organyl crystals at 6–9°C above the nematic-crystallosolvate phase transition temperature.     

On peculiarities of the nematic behavior in the field of ultrasonic shear waves

Crystallography Reports - The peculiarities of the orientational transition in a homeotropic layer of a nematic liquid crystal (NLC) under obliquely and normally incident shear waves have been...     

Secondary dynamics in glass formers: Relation with the structural dynamics and the glass transition

In this paper, we study secondary relaxations in a neat glass former close and below the glass transition. The pressure and temperature dependences of the characteristic relaxation frequencies were investigated to find a connection between structural and secondary relaxations and the microscopic mechanism at the basis of the latter. We found that the ratio between the relaxation time of the structural and secondary processes is almost constant when considered at different values of temperature and pressure corresponding to the glass transition (same value of structural relaxation time). This result is also related to the finding of a constant broadness of the structural peak. We propose to use such dynamic relation between the two processes to distinguish between intramolecular secondary relaxation, reflecting the local motion of molecular subgroups, and intermolecular secondary relaxation, called also Johari–Goldstein process, originating by a local motion of the whole molecule.     

Contraction of side-on nematic liquid crystalline elastomers micropillars: Influence of molecular parameters

We investigated the structure-property relationship of nematic liquid crystalline elastomer (LCE) actuators obtained from a series of nematic side-on monomers. The liquid crystalline acrylate monomers were designed and synthesized with different tail lengths. Combining a soft lithography technique with photo-polymerization/photo-crosslinking of aligned nematic liquid crystalline monomers and crosslinkers, nematic LCE micropillars were obtained, based on the new monomers. Through microscopic observation, influence of some structural parameters on the thermo-mechanical behavior of the micropillars was investigated. The thermo-mechanical behavior of the LCEs showed a close correlation with to the tail length of the monomer and the spacer length of the crosslinker.     

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.     

The dispersion of the dynamic scattering of colourless nematic liquid crystals

Using a photogoniometer the dispersion of the dynamic scattering process for visible light in colourless nematic liquid crystals was studied depending on sample voltage, observation angle and primary beam polarization. The results may be interpreted by means of a scattering model based on the dispersion effects of the birefringence and of the elliptical polarization.     

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.     

Influence of magnetic field on the director profile in nematic layer with curved surface

A nematic liquid crystal layer with a curved upper boundary exposed to a magnetic field directed along the layer plane has been studied. Strong nematic anchoring at the lower boundary surface is assumed. A phenomenological expression for the director profile is proposed to solve the problem on the director profile distribution in this system and its stability with respect to the rotational deformations in relation to the degree of the upper-surface roughness and the magnetic field value. The distribution parameters are found by optimizing the expression for the free energy of the liquid crystal system.