共查询到20条相似文献,搜索用时 15 毫秒
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We study the topological properties of the manifolds which describe uniaxial and biaxial nematics in order to compare and
classify all types of bulk and surface defects that can arise in a nematic liquid crystal.
We explain how topological charges can be assigned both to isotropic defects in uniaxial nematics and to uniaxial defects
in biaxial nematics; we further see how it is possible to distinguish between isolated surface defects and surface defects
which extend in the bulk, and between surface defects which can relax in the bulk and defects forced to stay on the surface.
Received: 28 November 1994 / Accepted: 30 September 1996 相似文献
3.
M. Yu. Kovalevskii L. V. Logvinova V. T. Matskevich 《Physics of Particles and Nuclei》2009,40(3):368-390
The generalization of dynamic equations in uniaxial and biaxial nematic liquid crystals is obtained yielding an approach accounting for the deformation of structural elements of these media. Collective excitation spectra are calculated and their angular dependence is established. The possibility of propagation of one, two, and three acoustic waves in nematics is predicted, and extreme angular characteristics of these waves are elucidated. Results of investigation are compared with available experimental data and satisfactory agreement between them is demonstrated. Low-frequency asymptotics of two-time Green’s functions of uniaxial nematics are calculated. It is found that their structure depends essentially on the character of spatial anisotropy of the considered condensed medium and the shape of molecules. Polarization specific features of acoustic waves in uniaxial nematics are studied. It is shown that the first and second sounds are the superposition of the longitudinal and transverse components. The ratio of amplitudes of these components as a function of angle between the wave vector and the anisotropy axis is established. 相似文献
4.
M. I. Monastyrskiĭ P. V. Sasorov 《Journal of Experimental and Theoretical Physics》2006,102(1):149-154
It is shown that linear defects in biaxial nematics may form dynamically stable bound states if these defects are linked, almost parallel, and repel one another. Such a situation is possible for pairs of linear defects with a certain topology. This effect can also be observed for nearly parallel vortices in superfluid liquids and superconductors. 相似文献
5.
Results are reported on light transmission versus the temperature near the phase transitions between lyotropic nematics. Strong pretransitional effects are observed when the phase transitions occur between biaxial nematic and uniaxial phases. The light is strongly depolarized both at the calamitic-biaxial and at biaxial-discotic phase transitions. The results are qualitatively explained. 相似文献
6.
本文根据Freiser关于向列相液晶分子相互作用模型,利用平均场理论,建立了梳型高分子聚合物向列相液晶分子的哈密顿量,导出序参数自洽方程,得到各向同性相至单轴向列相以及单轴至双轴向列相的相变,结果表明,当分子主、侧链耦合项中排斥作用及主、侧链中较强向列相场的场强增大时,单轴至双轴向列相相变温度升高,并从一级相变逐渐变为二级相变。
关键词: 相似文献
7.
O. Pelletier C. Bourgaux O. Diat P. Davidson J. Livage 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(4):541-546
Aqueous suspensions of V2O5 ribbons are one of the very few examples of mineral liquid crystals. In the concentrated regime, we show that these ribbons
organize in a biaxial nematic gel phase. A Couette shear cell was used to produce a well oriented sample for in situ synchrotron X-ray scattering studies. We observed two perpendicular anisotropic sections of reciprocal space, which proves
the biaxial symmetry of the nematic order. The thermodynamic and flow properties of the biaxial nematic are well described
by hard-core theories. We suggest the use of a shear geometry to produce and investigate single domains of other biaxial nematics,
reported but still questioned in the literature.
Received 26 May 1999 相似文献
8.
The elastic anisotropy of nematic elastomers 总被引:2,自引:0,他引:2
We examine the robustness of order in nematic elastomers under mechanical strains imposed along and perpendicularly to the
director when director rotation is prohibited. In contrast to electric and magnetic fields applied to conventional nematics,
mechanical fields are shown theoretically and experimentally to greatly affect the degree of nematic order and related quantities.
Unlike in liquid nematics, one can impose fields perpendicular to the director, thereby inducing biaxial order which should
be susceptible to experimental detection. Nematic elastomers with unchanging director and degree of order should theoretically
have the same elastic moduli for longitudinal and transverse extensions. This is violated when nematic order is permitted
to relax in response to strains. Near the transition we predict the longitudinal modulus to be smaller than the transverse
modulus; at lower temperatures the converse is true, with a crossover a few degrees below the transition. The differences
are ascribed to the different temperature dependence of the stiffness of uniaxial and biaxial order. We synthesised side chain
single-crystal nematic polymer networks, performed DSC, X-ray, birefringence, and thermo-mechanical characterisations, and
then obtained linear moduli from stress-strain measurements.
Received 29 September 2000 相似文献
9.
应用Saupe形式的弹性自由能,研究了简单的扭曲,展曲和弯曲形变对相变的影响.结果表明,弹性形变可以产生从各向同性相至双轴丝状相的一级相变,对螺状相液晶,当系统手征性增加并超过某一临界值时,一级相变可过渡到二级相变.
关键词: 相似文献
10.
The phase diagram of a polydisperse mixture of uniaxial rodlike and platelike hard parallelepipeds is determined for aspect ratios kappa=5 and 15. All particles have equal volume, and polydispersity is introduced in a highly symmetric way. The corresponding binary mixture is known to have a biaxial phase for kappa=15, but to be unstable against demixing into two uniaxial nematics for kappa=5. The phase diagram for kappa=15 is qualitatively similar to that of the binary mixture, regardless of the amount of polydispersity, while for kappa=5 a sufficient amount of polydispersity stabilizes the biaxial phase. This provides clues for designing an experiment to observe this long searched biaxial phase. 相似文献
11.
Symmetry of a nematic liquid crystal phase is broken by an anchoring wall and also by an external field. Nematic system sandwiched
between biaxial anchoring walls is introduced as a correspondent to a bulk nematic system exposed to a couple of fields, an
electric field and a magnetic one in directions perpendicular to each other, and thermal behaviours of the system are studied.
The crossover between a homeotropic structure and homogeneous one occurs, similarly to the bulk system in the fields, in which
the anchoring condition of coexistence is shown to have the same expression as the one at the bulk. As to a characteristic
phenomenon at the sandwiched system, it is proved that an appearance of a biaxial nematic order suppresses a uniaxial nematic
order. A surface transition, i.e., a wetting phenomenon is shown to occur also in the biaxial nematics, even though the uniaxial
order is suppressed therein. 相似文献
12.
基于Landau-de Gennes理论, 利用松弛迭代法, 研究了混合排列向列相液晶薄盒中–1/2向错引起的有序重构的扩散现象, 给出了–1/2向错的核结构、双轴性结构, 以及盒厚减小时有序重构的扩散. 当盒厚小于15ξ时, 随着盒厚的减小, 向错范围和有序重构区域沿基板方向迅速扩散; 当盒厚减小到临界厚度10ξ时, 有序重构的范围扩散到整个液晶盒中, 以向错中心所对应的平面为界, 指向矢一部分垂面排列, 另一部分沿面排列. 本文的研究对拓扑缺陷对向列相液晶中的亚微米胶体粒子的调节作用具有一定的理论指导意义.
关键词:
混合排列向列相液晶薄盒
–1/2向错
有序重构扩散
松弛迭代法 相似文献
13.
Freedericksz transition, which is usually analyzed by an elastic theory, is studied on the basis of statistical mechanical
ground, where nematics with positive dielectric anisotropy in homogeneous anchoring cell is exposed to an electric field in
the direction of wall normal. In low temperature region, an oblique axial symmetry breaking occurs, which is nothing but the
Freedericksz transition. In high temperature and high field region, biaxial nematic phase with principal axis parallel to
the field direction at interior area of the system is proved to appear. A phase diagram on the field versus temperature plane
is obtained and compared with the one at a bulk with common biaxial symmetry, where both of electric and magnetic fields are
applied in directions perpendicular to each other. In the latter, no symmetry breaking occurs, in contrast with the former
case above-mentioned, and the reason why this difference occurs is elucidated. 相似文献
14.
Thermotropic biaxial nematic liquid crystals 总被引:1,自引:0,他引:1
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. 相似文献
15.
We formulate Euler–Poincaré and Lagrange–Poincaré equations for systems with broken symmetry. We specialize the general theory to present explicit equations of motion for nematic systems, ranging from single nematic molecules to biaxial liquid crystals. The geometric construction applies to order parameter spaces consisting of either unsigned unit vectors (directors) or symmetric matrices (alignment tensors). On the Hamiltonian side, we provide the corresponding Poisson brackets in both Lie–Poisson and Hamilton–Poincaré formulations. The explicit form of the helicity invariant for uniaxial nematics is also presented, together with a whole class of invariant quantities (Casimirs) for two-dimensional incompressible flows. 相似文献
16.
The variation of the light scattered by mixtures of liquid crystals under pressure variations has been studied. The best results are obtained with a mixture of cholesterics and nematics. The application of the phenomena to blood pressure measurement is considered. 相似文献
17.
J. L. Verger-Gaugry 《Phase Transitions》2013,86(1-3):65-66
General results for transient pattern dynamics in the context of spinodal decomposition as well as domain-wall formation in the Fréedericksz transition in nematics are reviewed. The question of wavelength selection and the relevance of finite-size effects in this issue are considered in a general model of transient pattern dynamics. A finite-size scaling law for domain growth is shown to be satisfied by this model. 相似文献
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In this paper, a rotational invariant of interaction energy between two biaxial-shaped molecules is assumed and in the mean field approximation, nine elastic constants for simple distortion patterns in biaxial nematics are derived in terms of the thermal average (Dmn^(l)) (Dm'n'^(l')), where Dmn^(l) is the Wigner rotation matrix. In the lowest order terms, the elastic constants depend on coefficients Γ,Γ', λ, order parameters Q0 = Q0(D00^(2)) +Q2(D02^(2)+D0-2^(2)) and Q2 = Q0(D20^(2)) + Q2(D22^(2)+D2-2^(2)). Here Γ and Γ' depend on the function form of molecular interaction energy vj′j″j (τ12) and probability function fk′k″k (τ12), where r12 is the distance between two molecules, and λ is proportional to temperature. Q0 and Q2 are parameters related to multiple moments of molecules. Comparing these results with those obtained from Landau-de Gennes theory, we have obtained relationships between coefficients, order parameters used in both theories. In the special case of uniaxial nematics, both results are reduced to a degenerate case where K11=K33. 相似文献
20.
Valerii N. Shlyannikov 《Physical Mesomechanics》2012,15(3-4):176-184
Using the Pisarenko-Lebedev criterion, equations of ultimate failure strain under complex static and low-cycle loading were derived and were given experimental grounds for different types of biaxial loads. Experimental and calculation data are presented on the use of a generalized equivalence condition to solve problems of crack mechanics for complex stress states. Models and methods of determination of the crack direction, path, velocity and growth time under mixed biaxial loading are considered. 相似文献