Peculiarities of light propagation in chiral liquid crystal cells in an external electric field

We investigate the propagation of the extraordinary ray in a cell with a chiral liquid crystal at oblique incidence. For a 180° twist cell, we study the dependence of the minimum incidence angle at which the light does not yet pass through the cell on the applied voltage. The orientational structure of a liquid crystal in an external electric field has been calculated by directly minimizing the free energy. This has allowed the ray trajectories and the limiting refraction angles to be determined. The results of our calculations are consistent with the experiment. We show that allowance for the electric field nonuniformity in a chiral system is important for agreement between theory and experiment.     

Effect of an external electric field on liquid crystalline polymers

The effect of an external electric field on the order parameter and on the isotropic-anisotropic phase transition temperature for semi-flexible liquid crystalline polymers is studied by a mean-field approximation. For the polymers whose electric dipole moments are parallel to the chain backbone, the critical transition temperature T_c is extensively changed by gDT ∼ ∥E∥², where E is the external electric field.     

Anomalous orientation of ferroelectric liquid crystal films in an electric field

An anomalous orientation of ferroelectric liquid crystals in an electric field is observed. The tilt planes of the molecules are oriented in a direction parallel to the electric field. The anomalous orientation is observed in freely suspended films above the smectic-C*-smectic-A bulk phase transition temperature. The effect is explained by the appearance of giant flexoelectric polarization, 10⁴ times greater than that in bulk liquid-crystal samples, in the film. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 414–418 (25 September 1997)     

Deformation of a pretilted nematic liquid crystal layer in an electric field

The deformation of the optical axis of a nematic liquid crystal layer under the influence of an electric field is caused by the dielectric torque. This momentum counteracts with an elastic torque generated by interfacial forces between the surface of the electrode and the liquid crystal. By means of a variational analysis, the deformation profiles within the liquid crystal layer are calculated, assuming large interfacial energies and various angles of pretilt of the liquid crystal directors with respect to the electrodes. Both the extreme case of a homeotropic (vertical) alignment on one electrode and a homogeneous (parallel) alignment on the opposite electrode, as well as the general case of arbitrary and different alignments on both electrodes, lead to heavily pretilted liquid crystal layers, resulting in definite deformation profiles without disclinations. Liquid crystal cells prepared in this way neither show threshold voltages nor delay times when electrical fields are applied. Measured and calculated characteristics of such liquid crystal cells are presented, they show good agreement.     

Strain-induced matrix and droplets anisotropic deformation in liquid crystalline cellulose dispersed liquid crystal films

The present work deals with the preparation of new liquid crystalline cellulose dispersed liquid crystal films (100 μm) using a shearing casting technique. The matrix of the films presents the so-called band texture perpendicular to the shear direction. The nematic low molecular weight liquid crystal is encapsulated in micron and submicron size ellipsoids. The ratio between the lengths of the main axis and the short axis is around 1.23. The main axis is oriented, on average, 28° away from the shear direction. The evolution of the band texture and of the ellipsoidal liquid crystal droplets is investigated by polarizing optical microscopy and light scattering techniques as a function of the strain imposed along and perpendicular to the shear direction. Stretch along shear with strain equal to 0.8 seems to have no effect on the banded structure of the matrix, while the nematic liquid crystal ellipsoids slightly orient the main axis to the stress direction and their shape anisotropy increases by a factor of 2. Deformation in the direction transverse to the shear direction induces a deep change of the polymeric matrix and, at the end, a fibrillar structure is found. The nematic ellipsoids rotate their main axis and align along the stretch direction. Their shape anisotropy evolves from a value of 1.23 to 1 for intermediate deformations with strain equal to 0.5, to a final value of 5 with strain equal to 0.8, in the perpendicular direction.     

Threshold effects in homeotropically oriented nematic liquid crystals in an external electric field

The electric field-induced orientational transition in a homeotropically oriented nematic liquid crystal cell is investigated. The interaction with the field as a result of anisotropy of the permittivity and flexoelectric polarization is taken into account. For an arbitrary energy of interaction of the nematic with the substrate simple relations are derived for determining the threshold characteristics of the phase transition. It is shown that, in contrast with planar orientation, in fields above a critical value a periodic structure can occur only by virtue of the flexoelectric effect. The resulting dependences for the threshold parameters in the given geometry are exceptionally useful for determining experimentally the surface energy and the difference in the flexoelectric coefficients. Zh. éksp. Teor. Fiz. 116, 543–550 (August 1999)     

Formation and structure of a soliton in an antiferroelectric liquid crystal in an electric field

The soliton structure in an antiferroelectric liquid crystal in an electric field is calculated in the discrete phenomenological Landau model for phase transitions. The anticlinic structure of the antiferroelectric crystal gives rise to nontrivial features of the soliton structure and the stability of the soliton. The soliton is topologically stable in a metastable state if the electric field is much higher than the field of antiferroelectric-helix unwinding. A structural transition with a step-like change in the orientation of molecules occurs as the soliton field varies. A peculiar soliton (synclinic pair) can be formed if the barrier between the anti- and ferroelectric structures is large. The calculation shows the possibility of a large electroclinic effect in the soliton, i.e., a variation in the molecular tilt angle in an electric field in the vicinity of the transition of the structure to the synclinic state.     

Dynamic behaviors of ferroelectric liquid crystal molecules under an applied electric field

The dynamic changes in ferroelectric liquid crystal (FLC) molecular alignments under an applied electric field are examined by observing the formation of conoscopic figures with a time resolution of 0.1 ms. Close agreements between observed and simulated conoscopic figures under low voltage (30 V) were obtained. Under high voltage (120 V), however, the observed conoscopic figures became blurred between 0.8 ms and 1.1 ms after reversal of the electric field. The light scattering producing the blurriness occurred due to the development of fast transient molecular alignments during the switching transition above the applied voltage 70 V.     

Electric field induced deformation of hemispherical sessile droplets of ionic liquid

Sessile droplets of an ionic liquid with contact angles close to 90° were subjected to an electric field E = V/w inside a capacitor with plate separation w and potential difference V. For small field induced deformations of the droplet shape the change in maximum droplet height, Δh = h(E) – h(0), was found to be virtually independent of the plate separation provided that w > 3h(0). In this regime a scaling law obtains Δh α E²r², where r is the constant droplet radius, in agreement with the asymptotic predictions of Basaran and Scriven (J. Coll. Int. Sci. 140, 10, 1990).     

Diffraction behavior of an azo-dye-doped nematic liquid crystal without applied electric field

We investigate the dependence of the two-beam coupling diffraction efficiency on the experimental parameters, and hence achieve the optimum experimental conditions for holographic recording in planar Methyl Red-doped nematic liquid crystal film without external electric field. Under the optimum experimental conditions, the diffraction efficiency of a holographic grating is improved to 21.4%, and holographic image with long-term stability is stored in the film. These results provide an experimental basis for holographic data storage in this liquid crystal film.     

Surface states in an external electric field

In an electric field typical for field ion microscopy, true surface states can exist. Their shift towards higher energies can be quite significant, and moreover, additional surface levels at still higher energies can appear.     

Paramagnetic resonance study of a nematic liquid crystal oriented by an electric field

We perform experiments similar to the previously unexplained experiments of Schara and Sentjurc in which we observe the paramagnetic resonance spectra of a probe dissolved in p-azoxyanisole. Our experiments employ a nitroxide probe rather than vanadyl acetylacetonate. The numatic is aligned by an electric field and the resonance spectra are observed as a function of orientation of the magnetic field. The results obtained, though different from those of Schara and Sentjurc, are explained in a conventional manner. A result of dielectric measurements made by others is also shown to be in agreement with our results.     

Self-focusing in photorefractive BaTiO3 crystal under external DC electric field

Experimental observations of 1D and 2D self-focusing in a BaTiO3 crystal under externally DC applied electric field are reported. Stationary self-focusing behaviour can be obtained with and without additional uniform illumination.     

Controllable dissipative tunneling in an external electric field

We have studied the problem of controllable dissipative tunneling in the system of tunnel-binding quantum dots (quantum molecules) and in the “AFM/STM cantilever tip-quantum dot” system, which was simulated by a double-well oscillator potential interacting with a heat-bath in an external electric field. We show that theoretical results qualitatively describe some experimental I–V curves for “the AFM/STM cantilever tip-zirconium quantum dot” system. These experimental curves were obtained in the Research Institute of Physics and Technologies at the State University of Nizhniy Novgorod.     

Uniform rotation of smectic layers of a ferroelectric liquid crystal in an asymmetric electric field

An asymmetric ac electric field with amplitudeE=10 V/μm gives rise to a matched rotation of the normal to the smectic layers, while a sinusoidal fieldE=1 V/μm is used to study the rotation in the method of modulation total internal reflection ellipsometry, which makes it possible to probe the region of a ferroelectric liquid crystal (FLC) next to the electrode (h≈0.7 μm). It is shown that the angle of rotation of the normal to the smectic layers near the surface of the electrode varies reversibly as a function of the polarity and number of electric pulses applied, just as in the interior region. The characteristic dynamic properties of thin layers of nematic liquid crystals, such as an anomalously long relaxation time and a high-frequency relaxation process, are observed in thin FLC layers.     

Nematic order in nanoscopic liquid crystal droplets

We have used atomistic molecular-dynamics simulations to model the detailed molecular configuration of 5CB (4-n-pentyl-4'-cyanobiphenyl) molecules in the form of a nanoscopic liquid crystal droplet in a vacuum microgravity environment. We find the equilibrium state of droplets consisting of as few as 26 or 50 molecules to exhibit significant nematic ordering. The shape of the droplets is also anisotropic, but there is little angular correlation between the nematic director and the long axis of the droplet. Some tendency to micelle formation is observed in droplets of 50 molecules.     

Surface flow of liquid in an electric field

An analytic solution is proposed for a model problem which demonstrates the occurrence of a surface flow of weakly conducting liquid in an electric field which was previously observed experimentally by the author. Zh. Tekh. Fiz. 69, 127–128 (June 1999)     

Transformation of the polariton spectrum of a one-dimensional magnetic photonic crystal in an external dc electric field

The conditions are determined for a one-dimensional magnetic superlattice composed of an easy-axis antiferromagnet and a nonmagnetic dielectric, whose axis is collinear with the easy magnetization axis, under which quadratic magneto-optical interaction in an external dc electric field leads to a number of specific features upon localization and propagation of magnetic TM and TE polaritons.     

Driving liquid crystal lens without LC molecular orientation defects induced by an electric field

Optical Review - Using the finite element method, we numerically investigated the transient behavior of molecular orientations of a liquid crystal (LC) lens with a circular electrode beside a...