Paramagnetic nematic liquid crystals

The paper reports new nematic liquid crystals that are paramagnetic.     

Thermomechanical reorientation in nematic liquid crystals

Thermomechanical mechanism of the director reorientation in twisted nematic liquid crystals caused by the one-dimensional longitudinal temperature gradient is predicted and studied theoretically. The calculated director reorientations are in the range that can be measured experimentally very easily.     

Orientational dynamics in nematic liquid crystals

We examine the behaviour of single-particle orientational time correlation functions in nematic liquid crystals. As well as the expected dynamics involving oscillation in a mean-field potential, and occasional jumps between orientations parallel and antiparallel to the director, we provide the first simulation evidence of long-time tails characteristic of coupling to director fluctuations.     

Photoinduced effects in nematic liquid crystals

Temperature, concentration of the solvent and pressure are the parameters that are well known to bring about phase transitions in liquid-crystalline systems. In recent years a new parameter has been added to this list: light. The principle behind these photoinduced transitions is the light-driven shape transformation of certain photoactive materials like, e.g., azobenzene. In this article, we present results of various aspects of our recent investigations on such photoinduced transitions in the nematic phase and highlight the feature that light is a new tool to study phase transitions and the associated critical phenomena.     

Bistability in dual-frequency nematic liquid crystals

Different modes of bistable switching in liquid crystals with frequency inversion of the dielectric anisotropy sign are discussed. The study is performed by numerical simulation and experimentally. It is shown that dual frequency driving can be effectively used to control switching between topologically equivalent and non-equivalent director field distributions. The experimental results on temperature performance of the dual-frequency switching and possible driving methods for energy consumption and expanding the temperature range are presented.     

Electroconvection in homeotropic nematic liquid crystals

Electroconvection is a classical example of pattern-forming phenomena in liquid crystals, typically observed in nematics with negative dielectric and positive conductivity anisotropies. This article focuses on how electroconvection in the homeotropic geometry differs from that in planar alignment. The influence of an additional magnetic field on the pattern characteristics and on secondary instabilities (the normal roll–abnormal roll transition) is discussed. The homeotropic alignment offers unique possibilities also for studying defect motion. Basic characteristics of some patterns of large wavelength are presented and compared with those of the classical Carr–Helfrich structures. Finally, electroconvection in substances with negative conductivity anisotropy is addressed.     

Pretransitional behaviour in nematic liquid crystals

A simple (four sites on the water molecule) potential for Cl^-/H₂O has been generated from six gradient calculations. The potential is in reasonable agreement with the extensive SCF calculations of Dacre.     

Hexagonal arrays in nematic liquid crystals

In this letter observations are reported which were carried out by diffraction of a laser beam, by polar diagrams of the diffracted light and by optical microscopy, being performed on a hexagonal structure which appears in nematic liquid crystals for a twist geometry.     

Thermotropic biaxial nematic liquid crystals

We have synthesized liquid crystal (LC) mesogens based on a nonlinear oxadiazole unit that exhibit nematic phases near 200 degrees C. Polarized microscopy and conoscopy indicate that these LCs are biaxial nematics. Unambiguous and quantitative evidence for biaxiality is achieved using 2H NMR spectroscopy. "2D powder" spectra, obtained by rotating 2H-labeled samples about an axis perpendicular to the magnetic field at approximately 200 Hz, yield phase biaxiality parameters of approximately 0.1 when coupled with rigorous and proven simulations.     

Electroconvective instabilities in nematic liquid crystals

Electroconvective instabilities in nematic liquid crystals are caused by the electric force acting on charge densities generated in the medium due to a distortion in the orientation field of the anisotropic medium. The conventional view is that anisotropic conductivity provides the mechanism for the charge accumulation. Several experimental results however can be accounted for only by invoking an additional mechanism involving flexoelectric polarization for the charge accumulation. In this article we discuss the recent experimental and theoretical developments in this field.     

Optical multisoliton generation in nematic liquid crystals

The nonlocal nonlinearity stemming from molecular reorientation in nematic liquid crystals supports the formation of multiple solitary waves, following the onset of spatial modulational instability from both wide and focused input beams. We report experimental observations of such phenomena at power levels of hundreds of milliwatts in planar cells with a voltage bias.     

Derivative-dependent surface-energy terms in nematic liquid crystals

Summary The boundary energy terms dependent on the first derivative of the director are analysed theoretically. Although these terms necessarily arise from nonlocal interactions, they are generally written as purely surface terms. We show that, in this case, the stable director configurations are described by functions with discontinuous derivatives at the boundaries. All the solutions given in literature, which are found by explicitly assuming the continuity of the derivatives, must, therefore, be revised. In a more correct formulation of the problem, the ranges of the interaction forces should be taken into account and continuous solutions are then obtained, which are generally well approximated by the discontinuous ones. The approximation, instead, is not good in the case of the energy term proposed by Dubois-Violette and Parodi, where the discontinuity has the form of a Dirac function. In this case a physically meaningful solution can be found only on the basis of a more suitable expression of the boundary energy. Work supported by CNR under contract No. 84.01186.11.     

Collective effects in doped nematic liquid crystals

We studied the collective elastic interaction in a system of many macroparticles embedded in a nematic liquid crystal. A theoretical approach to the interaction of macroparticles via deformation of the director field [1] is developed. It is found that the director field distortion induced by many particles leads to the screening of the elastic pair interaction potential. This screening strongly depends on the shape of the embedded particles: it exists for anisotropic particles and is absent for spherical ones. Our results are valid for the homeotropic and the planar anchoring on the particle surface and for different Frank constants. We apply our results to cylindrical particles in a nematic liquid crystal. In a system of magnetic cylindrical grains suspended in a nematic liquid crystal, the external magnetic field perpendicular to the grain orientation results in inclining the grains to the director and induces an elastic Yukawa-law attraction between the grains. The appearance of this elastic attraction can explain the cellular texture in magnetically doped liquid crystals in the presence of the magnetic field [2].     

Short range order in nematic liquid crystals

The effect of short range order on the nematic isotropic transition is investigated by means of a generalization of the socalled Bethe approximation. The results are compared with the Maier-Saupe theory, Monte Carlo calculations, Kimura's results and experimental data for PAA.     

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.     

Freedericksz transition in polymer-stabilized nematic liquid crystals

We have constructed polymer-stabilized nematic liquid crystals by photopolymerizing diacrylate monomers in the nematic phase. The orientation of the liquid crystal was controlled by the polymer network. We studied the Freedericksz transition in these systems. Experimentally we studied the transition by measuring the capacitance of the liquid crystal cells as a function of applied voltage. The transition was affected profoundly by the dispersed polymer network. The threshold was higher with shorter interpolymer network distance. Theoretically we studied the systems using a two-dimensional model in which the polymer networks were represented by parallel cylinders with random location. The interaction between the liquid crystal and the polymer network was described by the boundary condition imposed by the polymer network. By fitting the experimental data, we found that the polymer cylinders had diameters of a few submicrons, and a substantial amount of liquid crystal was trapped inside the cylinders.     

Polarity-dependent dielectric torque in nematic liquid crystals

The dielectric dispersion in the uniaxial nematic liquid crystals affects the switching dynamics of the director, as the dielectric torque is determined by not only the present values of the electric field and director but also by their past values. We demonstrate that this "dielectric memory" leads to an unusual contribution to the dielectric torque that is linear in the present field and thus polarity sensitive. This torque can be used to accelerate the "switch-off" phase of director dynamics.