共查询到19条相似文献,搜索用时 713 毫秒
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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 相似文献
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针对向列相液晶,从自由能的角度出发,分弹性形变和电场作用两个部分,在理论上分析了其在电场作用下的第一类弗雷德里克兹转变,得到了阈值电场与液晶材料物理性质之间的关系,并与磁场作用下的第一类弗雷德里克兹转变进行了比较。阈值电压的结论可用于测定液晶材料的展曲弹性系数。 相似文献
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提出一种基于双折射晶体劈的偏振态实时测量方法,并将该方法用于扭曲向列型液晶透射光的偏振态分析,实现了液晶分子平均指向矢的动态测量.利用基于晶体劈的偏光干涉法将待测光的偏振参量编码为两组干涉条纹,通过对干涉条纹定位实现偏振态的实时测量;再根据液晶指向矢的倾角、扭转角与透射光偏振态之间的关系,推算出液晶分子平均指向矢的动态信息.本实验测量液晶指向矢的速度为每秒5次,液晶平均指向矢的倾角、扭转角的测量准确度达到0.2°.本文研究为液晶分子平均指向矢的动态测量提供了一种有效途径. 相似文献
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Lin Ziyang Jue Shi Xiang Ying L. P. Shi Wang Jian Z. X. Liang Z. R. Qiu 《Physics letters. A》2000,270(6):207-330
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. 相似文献
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E. Nowinowski-Kruszelnicki J. K?dzierski Z. Raszewski L. Jaroszewicz M. A. Kojdecki W. Piecek P. Perkowski M. Olifierczuk E. Miszczyk K. Ogrodnik P. Morawiak 《Opto-Electronics Review》2012,20(3):255-259
A new method for quick and pretty accurate measurements of splay, twist and bend elastic constants of nematic liquid crystals is experimentally verified. The main concept relies on exploiting only the electric field and determining magnitudes of nematic elastic constants from threshold fields for Freedericksz transitions in only one hybrid in-plane-switched cell. In such cell the deformations of an investigated liquid crystal are controlled by three separated pairs of electrodes confining measurement domains. In two of them inter-digital electrodes are mounted on one cell cover. Splay, twist and bend elastic constants can be measured by a proper choice of electrodes?? configuration together with orienting cover coatings (without applying magnetic fields). In this paper, we describe layout of our cells and results of experimental tests by using different liquid crystals: 5CB and 6CHBT (with positive dielectric anisotropy), Demus?? esters (with negative dielectric anisotropy) and new liquid crystals mixtures produced in our university. 相似文献
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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. 相似文献
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P. Schiller 《Phase Transitions》2013,86(2):59-83
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. 相似文献
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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. 相似文献
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The optical Freedericksz transition in a homeotropic nematic liquid crystal cell is modeled using a solver which combines direct solution of Maxwell’s equations with a relaxation algorithm for the liquid crystal director. We find that even in the equal elastic constant case the continuous optical Freedericksz transition can be driven first order. For films in which the optical retardation of the extraordinary wave is sufficiently large, a whole set of discontinuous jumps in transmission coefficient can occur. These jumps correspond to the existence of optical resonances in the liquid crystal film. Our results agree in the short wavelength limit with paraxial approximation calculations, and provide a strong test of the FDTD method for anisotropic materials such as liquid crystals. 相似文献
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I. A. Budagovsky A. S. Zolot’ko T. E. Koval’skaya M. P. Smayev S. A. Shvetsov N. I. Boiko M. I. Barnik 《Bulletin of the Lebedev Physics Institute》2013,40(1):6-11
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. 相似文献
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Fonseca JG Galerne Y 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》2000,61(2):1550-1558
We present an electro-optic method for measuring the zenithal anchoring strength of nematic liquid crystals, based on the determination of the distortion produced by a small electric field. This method yields the zenithal anchoring strength at small applied torques, and remarkably, only needs local measurements (optical path difference versus applied voltage, sample thickness), in contrast to the classical methods that use measurements integrated over the entire sample. We determine the zenithal anchoring strength for two nematic liquid crystals (5 CB and 5 OCB) with positive dielectric anisotropy, onto poly(tetrafluoroethylene) (PTFE) treated surfaces, that yield planar liquid crystal cells. We find that the anchoring at the PTFE-5 CB interface is strong, with an extrapolation length approximately 30 nm, and independent of temperature far enough from the isotropic transition. We observe a pretransitional weakening of the anchoring strength near the nematic-isotropic transition, due to the reduction of the orientational order parameter at the interface. With 5 OCB, we measure a stronger anchoring, with an extrapolation length approximately 15 nm. This result may be explained by the increase of the van der Waals interactions between the liquid crystal molecules and the surface, due to the presence of the oxygen atom. 相似文献