Fluctuations in liquid crystals in the presence of a flexoelectric effect

We examine director fluctuations in nematic liquid crystals in an electric field in the presence of a flexoelectric effect. For the planar and homeotropic orientations we calculate the correlation functions and analyze these functions near the Freedericksz transition and the flexoelectric instability threshold. For both geometries we calculate the angular dependence of the intensity of light scattered by director fluctuations and examine its behavior at electric field strengths close to critical. Zh. éksp. Teor. Fiz. 112, 1675–1693 (November 1997)     

Thermomechanical effect in a planar nematic induced by a quasistatic electric field

A thermomechanical flow of uniformly oriented nematic liquid crystal induced by a quasistatic electric field is observed. This flow occurs when the electric field strength exceeds the static Fréedericksz transition threshold. The effect is attributed to an electric-field-induced nonuniformity of the director orientation which is required for the onset of the thermomechanical effect. The experimental results show good agreement with the theoretical estimates. Zh. Tekh. Fiz. 69, 122–124 (April 1999)     

Local enhancement of a uniform electrostatic field near the tip of a spheroidal drop

The enhancement in a uniform electrostatic field at the tip of a spheroidal drop is shown to depend on the dielectric constant of the drop material, its initial radius, and the external electric field and to become greater as these increase. The loss of stability of a drop in an external electrostatic field that is accompanied by a very rapid growth in the magnitude of the spheroidal deformation causes a rapid, transient enhancement of the field at its tip. Zh. Tekh. Fiz. 69, 49–54 (August 1999)     

Calculation of the critical conditions of instability in an electric field of a hemispherical droplet on a hard substrate

The critical conditions of instability of a hemispherical drop of a conducting liquid lying on a hard, electrically conducting substrate in an electric field parallel to the symmetry axis of the drop are found. These critical instability conditions are found to be higher than those of an insulating drop of the same size. Zh. Tekh. Fiz. 68, 9–12 (September 1998)     

Nonlinear repolarization processes in ferroelectric liquid-crystal thin films

The dynamics of spontaneous polarization switching of the ferroelectric smectic C* in a variable electric field are examined theoretically and experimentally with the help of polarized light scattering. The observed effect of quasiresonant scattering both in freely suspended smectic films and in ordinary electro-optical cells is interpreted within the framework of the nonlinear model of isolated movable kinks in the director orientation distribution. It is shown that the maximum of the scattering intensity at the characteristic frequency of the applied electric field disappears at low temperatures and for small thicknesses of the smectic film. The dependence of the “resonant” frequency on the electric field amplitude, the proximity to the phase transition temperature, the film thickness and thickness of the ferroelectric domains, and also various material parameters is found. Estimates are made of such important characteristics as the dielectric anisotropy, viscosity, and elasticity of the smectic films. The effect of film thickness on the density distribution of the polar anisotropy energy in the film and on the corresponding shape of the moving orientation front within the film are discussed. Zh. éksp. Teor. Fiz. 111, 919–937 (March 1997)     

Role of different polarization mechanisms in the self-organization of the director of a thin layer of nematic liquid crystal

A theoretical analysis of the processes occurring in a symmetric metal-liquid-crystal-metal structure is carried out to obtain the dependence of the anisotropic component of the surface free energy on the angle of deviation of the director from the direction of easy orientation. The differences between the angular dependence of the surface energy obtained in this paper from the form described by the Rapini potential are discussed. The coordinate dependence of the ionic and dipolar polarization are calculated, as is the order parameter characterizing the ordering of the dipole moments of the liquid-crystal molecules. The interrelationships of the the boundary polarization with the equilibrium orientation of the director near the surface bounding the liquid crystal and with the value of the internal electric field in the volume of the mesophase are discussed. Zh. Tekh. Fiz. 67, 23–28 (May 1997)     

Accurate method for determining tilt bias angles in thin films of nematic liquid crystals

We have developed a new method for measuring tilt bias angles in spatially uniform and nonuniform thin films of nematic liquid crystals. The method employs modulation ellipsometry, based on the use of an exponentially decaying light wave to probe the boundary layer. Oscillations of the director of the liquid crystal, which are induced by the flexoelectric torque, are excited with an external periodic field. A periodic variation of the ellipticity of the light wave reflected from the interface is detected at both the first and second harmonics of the exciting electric field. When these two Fourier components of the electrooptic response are known, it is possible to calculate both the tilt bias angle θ₀ of the director and the dynamic deviation δ₀ of the tilt bias angle. The angles θ₀ and δ₀ measured by this method on the surface of an electrode (ITO) and on the surface of a ferroelectric film (a copolymer of vinylidene fluoride and trifluoroethylene), oriented in a corona discharge, were equal to θ ₀=5.1°, δθ=0.5° and θ₀=89°, δθ=0.06°, respectively. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 387–392 (25 March 1998)     

Stability of a bubble in a dielectric liquid in an external electrostatic field

Critical instability conditions are found for a gas bubble in a liquid dielectric in a uniform external electrostatic field E ₀. It is shown that they depend both on the magnitude of E ₀ and on the properties of the liquid, as well as on the gas pressure in the bubble. In a linear approximation with respect to the square of the eccentricity of an equilibrium spheroidal form, the equilibrium eccentricity of the bubble exceeds the equilibrium eccentricity of a drop in the field E ₀. The gas pressure in the bubble lowers the critical electric field E ₀ for development of an instability in the bubble. Zh. Tekh. Fiz. 69, 43–48 (August 1999)     

The influence of temperature fields on the orientational relaxation in hybrid liquid-crystal cells

The relaxation of the director field $ \hat n The relaxation of the director field , the velocity field v, and the shear and normal components of the stress tensor in a hybrid liquid-crystal cell with differently heated bounding surfaces is investigated theoretically. The system of nonlinear hydrodynamic equations that describe the director reorientation with allowance made for the field of velocities v induced by the director reorientation, on the one hand, and by the temperature gradient, on the other hand, is solved numerically. The relaxation time and the influence of the velocity field on the relaxation processes are analyzed for a number of hydrodynamic regimes arising in the hybrid liquid-crystal cell under the effect of the temperature field. Original Russian Text ? A.V. Zakharov, A.A. Vakulenko, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 3, pp. 557–564.     

Influence of the structure of the director field on the optical properties of a nematic liquid-crystal droplet

The changes in the optical properties of a spherical, bipolar, liquid-crystal droplet due to the change in structure of its director field under the influence of an applied external field are investigated on the basis of calculations using the nested-ellipsoids model in the discretedipole approximation. Graphs are plotted for the attenuation efficiency factor and the angular structure of the scattered radiation for the two typical cases of strong and weak dependence of the optical properties of the droplet on the change in structure of its director field. Zh. Tekh. Fiz. 69, 86–90 (November 1999)     

Effect of electric field on the rheological and dielectric properties of a liquid crystal exhibiting nematic-to-smectic-A phase transition

We report simultaneous measurements of shear viscosity (η) and dielectric constant (ε) of octyloxy cyanobiphenyl (8OCB) in the nematic (N) and smectic-A (SmA) phases as functions of temperature and electric field. With increasing electric field η increases in the N phase whereas it decreases in the SmA phase and saturates beyond a particular field in both the phases. The flow curves in the intermediate-field range show two Newtonian regimes in the N phase. The temperature-dependent behavior of η and ε at zero or at small electric field suggests the occurrence of several structures that results from precessional motion of the director along the neutral direction as reported in similar other system. We show that the precessional motions are gradually suppressed with increasing electric field and the effective viscosity resembles with the Miesowicz viscosity η ₁ at high enough electric field. In the intermediate field range the temperature-dependent η exhibits anomalous behavior across the N-SmA phase transition which is attributed to the large contribution of Leslie coefficient α ₁.     

Determination of the surface potential of a dielectric layer on a target bombarded by an ion beam

The electric field at the surface of a charge spot created by an ion beam on a dielectric coating of a target is calculated. An expression is obtained which relates the surface potential of the insulator to the potential of the collector corresponding to saturation of the collector secondary-electron current. It permits determination of the potential drop across the oxide layer of a cold cathode without introducing complications in the construction of the experimental apparatus. Zh. Tekh. Fiz. 68, 126–128 (September 1998)     

Surface controlled nematic bistability

One discusses phenomenologically the statics and dynamics of surface anchoring of nematic liquid crystals. In statics, for symmetrical situations, one can induce, under the action of an external field, a surface breaking with a bifurcation between two opposite orientations of the director. In dynamics, one shows that the surface director relaxation is intrinsically fast, and accompanied by a transient surface shear flow. In thin cells with planar anchorings, filled by cyanobiphenyl materials which orient normal to the plates under a strong electric field, there are now two effects which couple the relaxation of the surface director: a static one, which favors parallel orientation and a uniform cell texture, and a transient dynamical coupling from the surface flow, which forces π-twisted texture. With suitable electric signals, one can control the surface bifurcations to give at will one of these two bistable textures. Results are presented which show that these mechanisms could lead to a new class of surface controlled, fast nematic bistable displays. Reproduced from ‘Liquid Crystals Today’ (Vol. 8, p. 1–7, 1998) by permission from Taylor and Francis.     

Effect of a magnetic field on thermally activated tunneling ionization of impurity centers in semiconductors

An expression for the probability of thermally activated tunneling ionization in an electric field in the presence of a magnetic field is obtained. It is shown that the logarithm of the ionization probability is proportional to the squared electric field, and the coefficient of proportionality decreases with increasing magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 763–767 (25 November 1998)     

Inhomogeneous two-dimensional structures in liquid crystals

Localized axisymmetric inhomogeneous states with a continuous distribution of the director field can exist in nematics. Such structures are compressed into dense filaments under the influence of a magnetic or electric field. It is hypothesized that the given states can be achieved in filamentary nematic textures. This model is an alternative to the conventional disclination model. Two types of lattices of axial structures can exist in the entire range of existence of the modulated state. Axial structures with a kernel of finite radius can exist in cylindrical capillaries. The structure and equilibrium dimensions of the axial states are easily altered over a wide range under the influence of an applied field. The feasibility of utilizing isolated axial structures and lattices of such structures in optical data processing and imaging devices is discussed. The most promising outlook in this regard is for modulated states and axial structures in chiral liquid crystals exhibiting spontaneous polarization. Zh. éksp. Teor. Fiz. 113, 1675–1697 (May 1998)     

Ohm’s law in a chiral plasma

The motion of plasma electrons in a stochastic electromagnetic field is studied in the low-conductivity limit. It is shown that under very general conditions, in the presence of a nonzero average chirality of the small-scale electromagnetic field, the effective current depends on the curl of the applied electric field, j=§ E+§_κcurl E, just as for similar dependences for the electric displacement and magnetic induction vectors in optically active and artificial chiral media. Under certain conditions such an Ohm’s law leads to growth of the magnetic field, the structure of the growth being dependent on the conductivity of the medium. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 268–273 (25 August 1999)     

Dynamics of reorientation of a constrained nematic elastomer

A model is proposed for the reorientation dynamics of a confined nematic liquid crystal elastomer, where the effect of crosslinks is to couple the director to deformations of the elastic matrix. The model combines the (equilibrium) `neo-classical' theory of liquid crystal rubber elasticity with the simplest time evolution equations for a system described by two coupled, non-conserved order parameters. Relaxation from an orientation imposed by an electric field is studied as a function of elastic softness, starting angle, surface pretilt, and the relative mobilities of director and strain. Most importantly, the absence of a `semi-soft' elastic threshold changes the long-time behaviour of the effective refractive index of the medium from exponential to inverse power law decay. Predictions are compatible with recent experimental results by Chang, Chien and Meyer [Phys. Rev. E 56, 595 (1997)]. Received 22 June 1998     

Modeling of breakdown of a gas by electrons of the boundary layer during irradiation of metal targets by picosecond laser pulses

Results are presented from a numerical simulation of the breakdown of a dense inert gas by boundary-layer electrons upon irradiation of a metal target by high-power picosecond laser pulses. It is shown that taking the electric field of the boundary layer into account leads to a substantial increase in the concentration of the seed electrons near the target surface, which accelerates the ionization process. The dependence of the breakdown time on the electric field intensity of the incident wave and the concentration of the gas atoms is derived. Zh. Tekh. Fiz. 68, 48–53 (August 1998)     

Differential thermopower of a superlattice in a strong electric field

The effect of an electric field on the differential thermopower α(E) of a one-dimensional superlattice is investigated in the semiclassical approximation. A nonmonotonic temperature dependence of α(0) is established for a degenerate electron gas. It is shown that, in principle, an electric field can be used to control the thermoelectric properties of superlattices. Fiz. Tverd. Tela (St. Petersburg) 41, 1314–1316 (July 1999)     

Rotational reorientation of the director in twisted nematic liquid-crystal cells

The orientational relaxation of the director to its equilibrium orientation under electric, elastic, and viscous torques arising in twisted nematic liquid-crystal cells is investigated. It is shown that the relaxation time of the director depends strongly on the external electric field strength and weakly on the energy of anchoring of liquid-crystal molecules to the surfaces of the cell. The relaxation time of the director anomalously increases in electric fields close to the Fréedericksz threshold. It is established that, at specific strengths of the external electric field, the relaxation can occur in the form of traveling waves propagating from one edge to the other edge of the twisted nematic liquid-crystal cell. The calculations of the relaxation processes in the vicinity of the nematic-smectic A phase transition temperature demonstrate that the distortion of the director field is uniform over the entire cross section of the liquid-crystal cell and does not depend on the strength of anchoring of the liquid-crystal molecules to the surfaces of the cell.