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1.
The laser light scattering technique for non-invasivein situ simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals is introduced. By measuring
the autocorrelation function of the scattered light from nematic liquid crystals at different scattering angles, the splay
and twist elastic constantsK
11 andK
22 are obtained from the amplitudes of the autocorrelation function, and the viscosity coefficients ofη
Splay andη
Twist are determined using the viscoelastic ratiosK
11/η
Splay andK
22
η
Twist from the telaxation parameters of the two modes. 相似文献
2.
Summary The rotational diffusion of a rodlike molecule in a nematic and smecticC liquid crystal is considered in the molecular-field approximation. The microscopic friction constant, which determines the
molecular rotation drag, possesses an exponential temperature dependence with the activation energy determined by the isotropic
part of the intermolecular interaction energy. The rotational viscous coefficients,γ
1
andγ
⊥
are obtained by averaging of the corresponding microscopic stress tensor with the nonequilibrium distribution function. The
additional activation energy, proportional to the corresponding order parameter, appears in the expressions for the rotational
viscosity coefficients both in nematics andC smectics.
Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988. 相似文献
3.
We show how to simulate the equatorial section of the Schwarzschild metric through a flowing liquid crystal in its nematic
phase. Inside a liquid crystal in the nematic phase, a traveling light ray feels an effective metric, whose properties are
linked to perpendicular and parallel refractive indexes, n
o
and n
e
respectively, of the rod-like molecule of the liquid crystal. As these indexes depend on the scalar order parameter of the
liquid crystal, the Beris-Edwards hydrodynamic theory is used to connect the order parameter with the velocity of a liquid
crystal flow at each point. This way we calculate a radial velocity profile that simulates the equatorial section of the Schwarzschild
metric, in the region outside of Schwarzschild’s radius, in the nematic phase of the liquid crystal. In our model, the higher
flow velocity can be on the order of some meters per second. 相似文献
4.
Ananthaiah J Rajeswari M Sastry VS Dabrowski R Dhara S 《The European physical journal. E, Soft matter》2011,34(8):74
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. 相似文献
5.
A. M. Parshin V. A. Gunyakov V. Ya. Zyryanov V. F. Shabanov 《Bulletin of the Russian Academy of Sciences: Physics》2011,75(8):1045-1048
The change in the domain texture of the 5CB nematic liquid crystal on the surface of a polyvinyl butyral gelatinous polymer
solution cured in a magnetic field is studied optically. The alignment of the nematic is described by the effective order
parameter S* of the polydomain cells. The anchoring energy of a liquid crystal with gelatinous polymer W
ϕ = 1 × 10−3 erg/cm2 is determined. 相似文献
6.
A nematic liquid crystal confined between two identical flat solid substrates, with an alternating stripe pattern of planar
and homeotropic anchoring, is studied in the framework of the Frank-Oseen theory. By means of numerical minimization of the
free energy functional we study the effect of the sample thickness D on the location of the phase transition between a uniform alignment, either planar or homeotropic, and a distorted nematic
texture. The solvation force f due to distortions of the nematic director is also studied. It is found that f is always attractive, and for D small compared to the periodicity of the surface structure it exhibits two distinct asymptotic behaviors: f ∼ - D
-1/2 or f ∼ - D
-1, depending on the relation between D and the extrapolation lengths.
Received 12 November 2002 Published online: 16 April 2003 相似文献
7.
The molecular dynamics (MD) simulation, based on a realistic atom-atom interaction potential, was performed on 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic phase. The rotational viscosity coefficients (RVCs) γ
i, (i = 1, 2) and the ratio of the RVCs λ = - γ
2/γ
1 were investigated. Furthermore, static and frequency-dependent dielectric constants and ε were calculated using parameters obtained from the MD simulation. Time correlation functions were computed and used to determine
the rotational diffusion coefficient, D
⊥. The RVCs and λ were evaluated using the existing statistical-mechanical approach (SMA), based on a rotational diffusion
model. The SMA rests on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian
motion in a certain potential of mean torque. According to the SMA, γ
i are dependent on the orientational order and rotational diffusion coefficients. The former was characterized using: i) orientational
distribution function (ODF), and ii) a set of order parameters, both derived from analyses of the MD trajectory. A reasonable
agreement between the calculated and experimental values of γ
i and λ was obtained.
Received 22 March 2000 and Received in final form 8 October 2000 相似文献
8.
Xing-Bin Pan 《Communications in Mathematical Physics》2008,280(1):77-121
P. G. de Gennes predicted the analogies between the effect of the elastic coefficients to liquid crystals and the effect of
applied magnetic fields to superconductors, and predicted that all elastic coefficients diverge to infinity at smectic-C to
nematic transition. One would expect quantitative comparison in the analogies. In the case of equal elastic coefficients (K
1 = K
2 = K
3 = K), we define the critical value K
c
of the elastic coefficients and make comparison of it with the upper critical magnetic field H
C
3 for type II superconductors. We classify the smectic liquid crystals into subcritical, critical and supercritical cases according
to the Ginzburg-Landau parameter κ, the wave number q and the boundary value of the director at the surface. We show that in the subcritical case the liquid crystal does not undergo
phase transition; and in the supercritical case both phase transition and hysteresis occur. The prediction of de Gennes is
true in the critical case where μ
π
(u
0, q) = κ
2 and K
c
= + ∞. 相似文献
9.
We discuss the symmetry properties and the macroscopic behavior of a nematic liquid crystal phase with D2d symmetry. Such a phase is a prime candidate for nematic phases made from banana-shaped molecules where the usual quadrupolar
order coexists with octupolar (tetrahedratic) order. The resulting nematic phase is nonpolar. While this phase could resemble
the classic D
∞h nematic in the polarizing microscope, it has many static as well as reversible and irreversible properties unknown to nonpolar
nematics without octupolar order. In particular, there is a linear gradient term in the free energy that selects parity leading
to ambidextrously helical ground states when the molecules are achiral. In addition, there are static and irreversible coupling
terms of a type only met otherwise in macroscopically chiral liquid crystals, e.g. the ambidextrous analogues of Lehmann-type effects known from cholesteric liquid crystals. We also discuss the role of hydrodynamic
rotations about the nematic director. For example, we show how strong external fields could alter the D2d symmetry, and describe the non-hydrodynamic aspects of the dynamics, if the two order structures, the nematic and the tetrahedratic
one, rotate relative to each other. Finally, we discuss certain nonlinear aspects of the dynamics related to the non-commutativity
of three-dimensional finite rotations as well as other structural nonlinear hydrodynamic effects. 相似文献
10.
L.M. Blinov M.I. Barnik H. Ohoka M. Ozaki N.M. Shtykov K. Yoshino 《The European physical journal. E, Soft matter》2001,4(2):183-192
The temperature dependence of the surface polarization has been measured for both the planar and homeotropic orientation of
a nematic liquid crystal at a solid substrate. A conventional liquid crystal 5CB, pure and doped with an azo-dye, was used
in cells with controlled asymmetry for light absorption. The measurements have been made by a pyroelectric technique using
short pulses of a YAG laser to create a temperature increment. The latter, in turn, was measured independently by a novel
time-resolved “optical thermometer” technique monitoring temperature-dependent birefringence by a He- Ne laser beam. In accordance
with the symmetry of the order parameter, the surface polarization has different sign for the two orientations, its magnitude
ranges from -4 to +2pC/m. The same technique has been used for the measurement of the flexoelectric polarization in hybrid
cells. The sum of the flexoelectric coefficients is e
1 + e
3 = - 13pC/m at 25°C.
Received 28 February 2000 and Received in final form 5 September 2000 相似文献
11.
Byoungchoo Park Sang Young Eom Sin-Doo Lee Dong Hoon Choi Soo Young Park Nakjoong Kim 《Optical and Quantum Electronics》1995,27(5):337-345
Second-harmonic generation (SHG) has been studied for understanding the enhancement mechanism for the second-order optical nonlinearity by the nematic (or axial) ordering in a liquid crystal doped with one-dimensional nonlinear optical (NLO) organic molecules. An extended version of the Maier-Saupe mean-field theory for nematic liquid crystals was developed to obtain analytical expressions for the second-order NLO coefficients in terms of the axial order, the polar order and the effective nematic potential. From the SHG data in a guest-host system composed ofN,N'-dimethylaminonitrostilbene molecules (0.5% by weight) and a liquid crystal, the enhancement of the second-order NLO coefficient,d
33, by nematic ordering becomes almost 3, which agrees well with our theoretical predictions. 相似文献
12.
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) 相似文献
13.
Lenart VM Gómez SL Bechtold IH Figueiredo Neto AM Salinas SR 《The European physical journal. E, Soft matter》2012,35(1):4
We use Z-scan technique to investigate the nonlinear optical response of the thermotropic liquid crystal E7 in the neighborhood of
the nematic-isotropic phase transition. The analysis of the data for the nonlinear optical birefringence is compatible with
an effective critical exponent of the order parameter, β = 0.28 ± 0.03, which is close to the classical value, β = 0.25 , for a tricritical point. The nonlinear optical absorption in the nematic range depends on the geometrical configuration
of the nematic director with respect to the polarization beam, and vanishes in the isotropic phase. 相似文献
14.
I. Gvozdovskyy K. Shcherbin D. R. Evans G. Cook 《Applied physics. B, Lasers and optics》2011,104(4):883-886
Refractive index grating recording is achieved in the infrared in a nematic liquid crystal placed between semiconductor CdTe
substrates. Light-induced space-charge field created in photorefractive semiconductor substrates varies an alignment of liquid
crystal molecules inhomogeneously in space forming a spatial modulation of the refractive index in liquid crystal. Two-beam
coupling is studied in the hybrid cell, in which a gain factor Γ=16 cm−1 is achieved in the layer of liquid crystal of the sandwich. 相似文献
15.
Ultrasonic velocity, density and viscosity were measured in two ternary liquid systems namely,n-pentane +n-hexane + benzene(I) andn-hexane + cyclohexane + benzene(II) and one quaternary liquid system,n-pentane +n-hexane + benzene + toluene (III). The experimental as well as literature values of thermal expansion coefficient and iso-thermal
compressibility of pure liquid components were utilized to deduce the ideal value of internal pressure and excess internal
pressure for the above liquid systems at 298·15K using two different approaches. In the conventional approach one needs the
experimental values ofα andβ
T of mixtures for computing internal pressure, which was not possible. The second method which is proposed here utilizes only
the density, ultrasonic velocity and viscosity data of the mixture. This method is used in computing internal pressure and
its excess value for multicomponent liquid systems. A satisfactory agreement has been observed. 相似文献
16.
Arkady I. Leonov 《Mathematical Physics, Analysis and Geometry》2008,11(1):87-116
This paper consists of two parts. The first one develops algebraic theory of linear anisotropic nematic “N-operators” build
up on the additive group of traceless second rank 3D tensors. These operators have been implicitly used in continual theories
of nematic liquid crystals and weakly elastic nematic elastomers. It is shown that there exists a non-commutative, multiplicative
group N6 of N-operators build up on a manifold in 6D space of parameters. Positive N-operators, which in physical applications hold
thermodynamic stability constraints, do not generally form a subgroup of group N6. A three-parametric, commutative transversal-isotropic subgroup of positive symmetric nematic operators is also briefly discussed. The special case of singular, non-negative symmetric N-operators
reveals the algebraic structure of nematic soft deformation modes. The second part of the paper develops a theory of linear
viscoelastic nematodynamics applicable to liquid crystalline polymer. The viscous and elastic nematic components in theory
are described by using the Leslie–Ericksen–Parodi (LEP) approach for viscous nematics and de Gennes free energy for weakly
elastic nematic elastomers. The case of applied external magnetic field exemplifies the occurrence of non-symmetric stresses.
In spite of multi-(10) parametric character of the theory, the use of nematic operators presents it in a transparent form.
When the magnetic field is absent, the theory is simplified for symmetric case with six parameters, and takes an extremely
simple, two-parametric form for viscoelastic nematodynamics with possible soft deformation modes. It is shown that the linear
nematodynamics is always reducible to the LEP-like equations where the coefficients are changed for linear memory functionals
whose parameters are calculated from original viscosities and moduli.
相似文献
17.
A statistical approach including direct correlation functions is applied to study the influence of the surface confining the
nematic liquid phase on the Franck elastic coefficients. Specific calculations are made for a model system composed of ellipsoidal
molecules interacting by means of the Gay-Berne potential near the interacting surface.
Fiz. Tverd. Tela (St. Petersburg) 40, 1356–1359 (July 1998) 相似文献
18.
TU Mei & HUANG Yaoxiong Institute of Biomedical Engineering Jinan University Guangzhou China Correspondence should be addressed to Huang Yaoxiong 《中国科学G辑(英文版)》2004,47(1)
Theelasticconstantsandtheviscositycoefficientsarethekeyparametersforthephysicalpropertiesofnematicliquidcrystals(LCs)[1,2],fortheyarecorrespondingtothethresholdvoltageandtherespondingtimeofliquidcrystaldisplaydevices[3,4].Sofarnotechniquethatcansimultaneouslymeasurethetwoparametersinsituhasbeenreported.TheelasticconstantsandviscositycoefficientsofnematicLCsatthepresentaremeasuredusingdifferentmethodsindependently.TheviscositycoefficientsisgenerallymeasuredwithOstwaldviscosimeter,andtheelasti… 相似文献
19.
ABSTRACTA molecular-statistical theory describing the nematic liquid crystals (LCs) with spherical inclusions (or point defects) is proposed. At given size of inclusions and nematic order parameters at the surfaces of inclusions (zero in the case of point defects) and far from inclusions (where the nematic LC is almost uniform), the distribution of nematic order parameters in the bulk of LC with inclusions was found to be fully determined by the elastic constants of LC. We have found and explained the two-step heat-driven transformation from the nematic phase into the isotropic phase, with the intermediate phase in between. The nematic order parameters and the elastic constants are evaluated in the framework of a unified approach based on the features of pair interaction potentials of the individual LC molecules. It is shown that, in the case of K33 < K11, the point defects should destroy the conventional nematic phase. 相似文献
20.
R. Somashekar H. Somashekarappa A. P. Divya D. Revannasiddaiah M. S. Madhava 《Pramana》1999,52(1):67-74
The existing methods for the determination of the orientational distribution functionf(β) in the nematic liquid crystals using X-rays have been reviewed. A simple Fourier method which givesf(β) in terms of the measured intensity is analysed. Using this distribution function, the accuracy with which the order parameters
could be evaluated is discussed and the results show the elegance of the Fourier method used here. 相似文献