An azo-isomerization-induced Freedericksz transition of a nematic liquid crystal

We observed an azo-isomerization-induced Freedericksz transition of the 5CB liquid crystal doped with 1% double-azo. In experiment, we found that the Freedericksz transition threshold occurred at optical intensity level of 0.3 mW/mm² for the 12.5 μm-thickness planar alignment doped 5CB, which is four orders of magnitude lower as that for the pure 5CB. Using optical phase retardation, we measured the relations between the Freedericksz transition threshold and the pump beam polarization. The maximum twist angle of liquid crystal molecules under different pump beam intensity is given. The experiment indicated that trans–cis isomerization plays an important role in the decrease of the Freedericksz transition threshold.     

掺杂对液晶磁场弗里德里克斯转变影响的探讨

采用相位延迟法，通过对分别掺入微量卟啉类物质、纤维蛋白质和双偶氮苯沿面排列5CB液晶磁场弗里德里克斯转变的实验和数值计算，从实验和原理上探讨了微量掺杂对沿面排列5CB液晶磁场弗里德里克斯转变的影响.数值计算结果与实验相符，呈现两种转变过程.研究表明，掺杂物的磁学性质不同、分子的形状和大小不同对液晶弗里德里克斯转变和弗里德里克斯转变阈值的影响不同.进一步的掺杂2-benzothiazolethiol-linked porphyrinatozinc(Ⅱ)实验说明了这种情况. 关键词： 液晶 掺杂 弗里德里克斯转变     

一种测量微量有机材料磁化率各向异性的光学方法

提出了一种测量有机材料的磁化率各向异性的方法。原理是利用掺杂对丝状液晶磁场扭曲费雷德里克兹转变影响的现象,将待测有机材料作为掺杂物,采用光学相位延迟法测量掺杂丝状液晶磁场扭曲费雷德里克兹转变阈值和过程,从而获得待测有机材料的磁化率各向异性。通过实验测量,从原理和方法上验证了测量方法的可行性。测得有机材料磁化率各向异性的同时也为有机材料的表征提供了一种方法。测量方法适用于无固有磁矩的有机材料。     

Influence of the segregation effect on the magnetic and optical properties of a compensated ferronematic liquid crystal

The Freedericksz transition in splay and bend geometry in a ferronematic (a suspension of single-domain magnetic particles in a nematic liquid crystal) is considered in the framework of the continuum theory. In zero magnetic field, the ferronematic was assumed to be compensated (i.e., having equal fractions of an impurity with magnetic moments directed parallel and antiparallel to the local director). Spatial distortions of the director and the concentration redistribution of the magnetic impurity in the ferronematic layer are studied as functions of the applied magnetic field and the segregation parameter. It is shown that the magnetic-field induced Freedericksz transition from the homogeneous to the inhomogeneous state has a threshold nature. The transition field as a function of the material parameters of the ferronematic is determined analytically. Magnetization of the ferronematic and its optical properties for a strong and weak segregation as functions of the magnetic field are studied.     

Study of light-induced reorientation of the nematic liquid crystal director by birefringence dynamics

It is proposed to apply the birefringence method to measure the threshold of the light-induced Freedericksz transition and the nonlinearity enhancement factor of nematic liquid crystals by director relaxation dynamics in the light field. A birefringence change is recorded by interference of ordinary and extraordinary components of the light beamaffecting the liquid crystal director orientation. The Freedericksz transition thresholds and nonlinearity enhancement factors are measured for the samples doped with comb-shaped polymers with various degrees of polymerization. The determined thresholds are in agreement with the results obtained by the aberrational self-action method.     

Decrease in the threshold of electric Freedericksz transition in nematic liquid crystals doped with ferroelectric nanoparticles

A phenomenological theory of electric Freedericksz transition in planar-oriented nematic liquid crystals doped with spherical ferroelectric nanoparticles is considered. It is shown that even spherical nanoparticles can critically reduce the threshold of Freedericksz transition, due to their ability to strongly increase the electric field in the medium. At higher values of polarization and concentration of nanoparticles a Freedericksz transition without threshold can occur. In this case only an insignificant electric field needs for orienting the nanoparticles.     

Freedericksz transition in an anticlinic liquid crystal

It is shown experimentally that a very-long-pitch, surface-stabilized, anticlinic liquid crystal undergoes a two-step electric-field-induced transition to the synclinic phase. The liquid crystal remains undistorted below a threshold field E(th). For E>E(th), a Freedericksz transition occurs, wherein molecules in adjacent smectic layers undergo unequal azimuthal rotations about the layer normal. At higher fields a transition to the synclinic phase occurs via solitary waves.     

可调谐液晶红外滤光片的光学特性分析

可调谐红外滤光片是高分辨率红外成像仪谱段识别和分光的重要光学器件。采用琼斯矩阵方法对平行向列相和90°扭曲向列相液晶法-珀红外滤光片的本征偏振态及光学特性进行了研究。结果表明,当非偏振光垂直入射时,平行向列相液晶法-珀滤光片在任何电压下两组本征偏振态均不发生模式混合,具有偏振敏感性;90°扭曲向列相液晶法-珀滤光片在电压较低时具有偏振敏感性,当电压大于三倍Freedericksz阈值电压时,两组本征偏振态发生模式混合,偏振敏感性消除。     

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.     

A THEORETICAL AND EXPERIMENTAL STUDY OF FREQUENCY REDSHIFT IN OPTICAL-FIELD-INDUCED FREEDERICKSZ TRANSITION IN A NEMATIC LIQUID CRYSTAL FILM

The frequency redshift in the optical-field-induced Freedericksz transition in a nematic liquid crystal film is investigated, We derive a theoretical expression for the frequency shift, and then measure the temporal evolution of the frequency shift and the light-intensity de-pendence of the maximum frequency shift. The theory agrees with the measurements.     

The effect of magnetic fields and boundary conditions on the Couette flow of nematics

The paper discusses the theory of Couette flow of a nematic liquid crystal. The apparent viscosity, orientation and velocity profiles are computed forp-azoxyanisole as functions of shear rate and magnetic field for symmetric and asymmetric molecular alignments at the boundaries and for different relative radii of the cylinders. For symmetric homeotropic boundary condition an azimuthal field exhibits a threshold analogous to a Freedericksz transition. An expression is also derived for the Freedericksz threshold in the hydrostatic case.     

Thermal orientation effect in a nematic liquid crystal

The system of equations describing the thermal orientation effect in nonchiral liquid crystals, which was observed earlier, is proposed and substantiated. The effect of the director reorientation under the action of the temperature gradient is in many ways analogous to the Freedericksz transition in electric or magnetic fields. The angle of deviation of the director from the initial position is determined by the square of the temperature gradient. The effect is observed for a nematic liquid crystal, initially uniformly oriented, and is not accompanied by macroscopic fluxes of the medium under stationary conditions.     

Light-induced orientation transition in nematic liquid crystalline polymer

Optical orientation in transparent nematic liquid crystalline polymer (NLCP) was observed for the first time. In NLCP with dye dopant, the light-induced second-order orientation transition being an analogue of the Freedericksz transition in low-frequency fields is found. The transition threshold is ~10 μW which is lower than the threshold of the light-induced Freedericksz transition in low-molecular nematics by four orders ofmagnitude and is lower than the characteristic value for low-molecular nematics with dye dopant by two orders of magnitude.     

Electrical Freedericksz transitions in nematic liquid crystals containing ferroelectric nanoparticles

A new theoretical approach was elaborated to explain the contradictions reported in many papers about the electric threshold for Freedericksz transition in nematic liquid crystal (NLC) with ferroparticles additives. The free energy density of the mixture was estimated and the contributions of the interaction energy of NLC molecules with ferroparticles surface were calculated. Experimental results for 5CB-BaTiO₃ mixture are given.     

Light-induced orientation of the molecules of nematic liquid crystals doped with comb-shaped polymers with different spatial distributions of chromophores

The orienting effect of light on nematic liquid crystals (NLCs) doped with comb-shaped polymers with different spatial distributions of side absorbing azobenzene fragments, i.e., a homopolymer (containing only azofragments), a block copolymer (containing additionally a block of non-absorbing fragments), and a statistical copolymer (containing randomly arranged absorbing and non-absorbing fragments) is experimentally studied. The light-induced Freedericksz transition threshold for the block copolymer is two times lower than that for the homopolymer. For NLC with statistical copolymer dopant, the first-order orientation transition with an extremely wide optical bistability region is observed.     

The structural instabilities in ferronematic based on liquid crystal with negative diamagnetic susceptibility anisotropy

The studied ferronematic is a nematic liquid crystal (ZLI1695) of low negative anisotropy of the diamagnetic susceptibility (χ_a<0) doped with the magnetic particles Fe₃O₄. Structural instabilities are interpreted within Burylov and Raikher's theory. The high magnetic fields were oriented perpendicular (Freedericksz transition) or parallel to the initial director. Using capacitance measurements the Freedericksz threshold magnetic field of the ferronematic B_FN, and the critical magnetic field B_max, at which the initial parallel orientation between the director and the magnetic moment of magnetic particles breaks down, have been determined. The values of these quantities have been used to estimate the surface density of the anchoring energy W of liquid crystal molecules on the surface of the magnetic particles. The obtained values indicate a soft anchoring of the liquid crystal on the magnetic particles with a preferred parallel orientation of the magnetic moment of magnetic particles and the director.     

The effect of magnetic fields and boundary conditions on the shear flow of nematics

The flow of a nematic liquid crystal between plane parallel plates, with one plate moving with uniform velocity relative to the other, is discussed. The apparent viscosity, orientation and velocity profiles are computed forp-azoxyanisole as functions of shear rate and magnetic field for symmetric and asymmetric molecular alignment at the plates. For symmetric homeotropic boundary condition, a magnetic field applied along the flow direction exhibits a threshold reminiscent of a Freedericksz transition in the hydrostatic case. In general the apparent viscosity for the asymmetric boundary condition is less than that for the symmetric case.     

Orientational transitions in antiferromagnetic liquid crystals

The orientational phases in an antiferromagnetic liquid crystal (ferronematic) based on the nematic liquid crystal with the negative anisotropy of diamagnetic susceptibility are studied in the framework of the continuum theory. The ferronematic was assumed to be compensated; i.e., in zero field, impurity ferroparticles with the magnetic moments directed parallel and antiparallel to the director are equiprobably distributed in it. It is established that under the action of a magnetic field the ferronematic undergoes orientational transitions compensated (antiferromagnetic) phase–non-uniform phase–saturation (ferrimagnetic) phase. The analytical expressions for threshold fields of the transitions as functions of material parameters are obtained. It is shown that with increasing magnetic impurity segregation parameter, the threshold fields of the transitions significantly decrease. The bifurcation diagram of the ferronematic orientational phases is built in terms of the energy of anchoring of magnetic particles with the liquid-crystal matrix and magnetic field. It is established that the Freedericksz transition is the second-order phase transition, while the transition to the saturation state can be second- or first-order. In the latter case, the suspension exhibits orientational bistability. The orientational and magnetooptical properties of the ferronematic in different applied magnetic fields are studied.     

Equilibrium structures of planar nematic and cholesteric films in electric fields

Liquid crystal layers subjected to an electric or a magnetic field can have several types of instabilities. This paper reviews recent theoretical studies concerning equilibrium structures of planar layers.

Using the Oseen-Frank elasticity theory, the Freedericksz transition and the transition to static periodic domains of planar nematic and cholesteric films are reconsidered. A perturbation treatment of nonlinear torque balance equations for the director is suitable to derive amplitude equations for the film distortions under the action of a field and to predict the topological features of phase diagrams for equilibrium states. The competition between the Freedericksz effect and the formation of periodic distortions is studied varying material and geometrical parameters. Some results are useful to optimize cholesteric mixtures for application in display devices in such a manner, that the occurrence of periodic domains is avoided.     

Surface precrystallization of normal C24 alkane in porous glass

The behavior of the enthalpy and specific heat of normal C24 alkane in the bulk and in porous glasses is investigated using an adiabatic scanning microcalorimeter. Enthalpy jumps, which precede a phase transition in the entire volume of the pores, are found in porous glass with characteristic pore size 1000 Å at a transition from the isotropic liquid into the rotator phase RII. The enthalpy jumps are interpreted as a layerwise growth of a crystal phase on the surface of porous glass. It is also found that porous glass substantially changes the phase behavior of alkanes.