共查询到20条相似文献,搜索用时 9 毫秒
1.
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
2.
L. M. Blinov D. Z. Radzhabov S. V. Yablonsky S. S. Yakovenko 《Il Nuovo Cimento D》1990,12(9):1353-1361
Summary The flexo-electric polarization of hybrid nematic cells filled with 5 CB was measured as a function of cell thickness. To
this aim the pyroelectric response of a nematic layer with small amount of light absorbing dye dissolved was detected in a
nematic temperature range. From the experimental data the angle of the director deviation at the homeotropic boundary was
calculated as a function of cell thickness. This dependence allows the qualitative determination of the shape of the potential
well for the director deviation which is inconsistent with Rapini’s sinesquared form. The ?apparent? Rapini’s anchoring energy
varies with a surface director angle (at the homeotropic boundary) from 5·10−3 to 3.5·10−2 erg·cm−2.
Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988. 相似文献
3.
理论分析和数值模拟相结合,研究了表面序电极化、挠曲电极化对非对称弱锚定向列液晶盒在饱和点双稳态的影响,得到了两种极化共存时液晶指向矢满足的方程和边界条件.通过引入新的状态参量,从能量的角度对两种极化的物理效应进行了分析.结果表明,在一定条件下,两种极化耦合,在饱和点通过改变锚定能影响边界条件,从而诱导双稳态.文中给出了判断是否存在双稳态的一般条件,此条件与数值计算结果符合.
关键词:
表面序电极化
挠曲电极化
饱和点
双稳态 相似文献
4.
A new structural transition occurs at the free surface of some nematic liquid crystals when the temperature reaches a critical value T0. In this work we study the temperature dependence of the anchoring energy of the director at the free surface close to the critical point. We find that the anchoring energy tends to zero with the critical exponent δ = 1 when the temperature approaches the critical value T0. The experimental results are interpreted in terms of the Parsons and Mada theories. 相似文献
5.
We have measured the angular dependence of the surface torque associated with a relatively weak anchoring of a nematic liquid crystal, for large deviations θ (near π/2) from the orientation of minimum energy. Analysing the torque in a Fourier series, we find that in addition to the usual first term, viz. sin 2θ, an important contribution from a sin 6θ term is needed to adequately represent the experimental variation. 相似文献
6.
A. L. Alexe-Ionescu 《Il Nuovo Cimento D》1993,15(12):1489-1506
Summary The elastic deformations in nematic liquid crystals are analysed in one and two dimensions. The influence of the flexoelectric
polarization on the elastic properties of the medium is studied. It is shown that for one-dimensional deformations, the presence
of the flexoelectric polarization introduces a renormalization of the elastic constants. On the contrary in the bidimensional
case, the effect of the flexoelectricity is not a simple renormalization of the elastic constants. The profiles of the electric
potential and of the tilt angle are evaluated for the cases in which the substrate is an insulating or conducting material.
The validity of the calculations is discussed by considering the value of Debye's screening length of the nematic liquid crystal. 相似文献
7.
Summary A new reflectometric method to measure the azimuthal anchoring energy at the interface between a nematic liquid crystal and
an isotropic method is proposed. The spurious contribution to the reflectivity coefficients due to the presence of a director-field
distortion near the interface is calculated by using the numerical Berreman algorithm and is shown to be negligible for magnetic
fields small enough (H<10 kG). Preliminary measurements of the azimuthal anchoring energy at the interface between the NLC 5CB and a rubbed glass
plate are given.
Research supported in part by MPI and in part by CNR. 相似文献
8.
Deformations of homeotropically aligned flexoelectric nematic layers induced by dc electric fields were simulated numerically. Two different anchoring strengths on the limiting surfaces were assumed. Nematic material was characterised by negative dielectric anisotropy. Both signs of the sum of flexoelectric coefficients were taken into account. The electric properties of the layer were described in terms of a weak electrolyte model. Mobility of cations was assumed to be one order of magnitude lower than that of anions. Quasi-blocking electrode contacts were assumed. The threshold voltages for deformations were determined by means of calculations of the phase difference Φ between ordinary and extraordinary light rays passing through a layer placed between crossed polarisers. The threshold values depended on the polarity of the bias voltage U. When the threshold value was exceeded, the phase difference increased with the voltage. Two different Φ(U/Uthreshold) dependencies for the two polarities of the voltage were found for each layer if the nematic possessed the flexoelectric properties. The possibility of using this effect to detect the flexoelectricity in the nematic was explored by simulated experiments. The effectiveness of the proposed method is discussed. 相似文献
9.
The dependence of the threshold parameters and the period of the electric-field-induced spatially periodic reorientation of the director in a flexoelectric nematic liquid crystal (NLC) on the anchoring conditions at the surface of a planar NLC cell has been studied. The threshold electric field and the corresponding wave-number of the periodic structure of the director field have been numerically calculated for arbitrary values of the anchoring energy. In the case of strong anchoring, the corresponding analytical expressions are obtained in a single-constant approximation. A decrease in the azimuthal anchoring energy leads to an increase in the intervals of possible values of the flexoelectric parameter ν and the ratio K2/K1 of the Frank elastic constants. A decrease in the polar anchoring energy leads to narrowing of these intervals as compared to the case of infinitely strong anchoring at the NLC cell surface. 相似文献
10.
The azimuthal anchoring energy of the nematic liquid
crystal 4-pentyl-4'-cyanobiphenil (5CB) on a
poly(tetrafluoroethylene) (teflon, PTFE) film is measured for
the first time. The PTFE film is deposed using the Wittmann and
Smith technique which consists on rubbing a bar of this polymer
against a glass substrate at a controlled temperature and
pressure. Measurements of the azimuthal anchoring energy are
made with a reflectometric technique which provides high
accuracy and sensitivity. The dependence of the azimuthal
anchoring energy on temperature and on the rubbing pressure is
investigated. The extrapolation length remains virtually
constant in the whole temperature range of the nematic phase
except for an increase of 25% close to the clearing temperature.
The azimuthal anchoring energy is somewhat strong and increases
with increasing the deposition pressure of PTFE. The observation
of a relevant pre-transitional anisotropy of the reflection
coefficients in the isotropic phase proves that the surface
interactions favor an excess of orientational order. Ageing of
the anchoring energy and gliding of the easy axis are
experimentally observed. Both these phenomena suggest the
presence of an anisotropic adsorbed layer of nematic molecules
on the PTFE film. 相似文献
11.
N. M. Shtykov V. P. Panov M. I. Barnik 《Journal of Experimental and Theoretical Physics》2000,91(1):126-131
Measurements of the temperature dependence of the pyroelectric coefficients in cells with homeotropic and planar boundary conditions were performed in order to study the surface polarization and determine its nature in the isotropic phase of a nematic liquid crystal. Analysis of these data established that the ordering polarization arising as a result of the nonuniformity of the order parameter in the near-boundary region of the liquid crystal and the polar monolayer due to the bifilar properties of the molecules of the liquid crystal at the boundary with the solid wall of the cell are responsible for the surface polarization in the isotropic phase of a nematic liquid crystal. The values of the pyroelectric coefficients of monolayers and the coefficients of the ordering polarization for planar and homeotropic orientations are estimated. 相似文献
12.
Deformations of homeotropic layers of nematic liquid crystal possessing flexoelectric properties and subjected to external
dc voltage were studied numerically. The influence of the anchoring strengthWwas investigated. The calculations were performed
for two types of layers, one with positive dielectric anisotropy and positive sum of flexoelectric coefficients and the other
with opposite signs of these parameters. Strongly blocking electrodes were assumed. The threshold voltages for deformations
were computed for several anchoring strength values and for low, moderate, and high ion contents. The director distributions
were also determined. They were influenced by joined effects of anchoring strength and space charge of ions. For increasing
ion contents and increasing anchoring strength, remarkable change of deformation character was noticed. In the case of negative
dielectric and flexoelectric parameters, the threshold was nearly proportional to W for low anchoring strength and saturated
at some high value of W. In the case of positive dielectric and flexoelectric parameters, the threshold voltage increased
strongly with W. In the range of enhanced ion concentrations, the deformation arose in two voltage ranges. The increasing
anchoring strength damped the deformations in the whole layer, however, this effect was minor at high ion content. 相似文献
13.
Azimuthal anchoring of a nematic liquid crystal on a grooved interface with anisotropic polar anchoring 下载免费PDF全文
Zhang Y J et al.[Zhang Y J,Zhang Z D,Zhu L Z and Xuan L 2011 Liquid Cryst.38 355] investigated the effects of finite polar anchoring on the azimuthal anchoring energy at a grooved interface,in which polar anchoring was isotropic in the local tangent plane of the surface.In this paper,we investigate the effects of both isotropic and anisotropic polar anchoring on the surface anchoring energy in the frame of Fukuda et al.’s theory.The results show that anisotropic polar anchoring strengthens the azimuthal anchoring of grooved surfaces.In the one-elastic-constant approximation(K11 = K22 = K33 = K),the surface-groove-induced azimuthal anchoring energy is entirely consistent with the result of Faetti,and it reduces to the original result of Berreman with an increase in polar anchoring.Moreover,the contribution of the surface-like elastic term to the Rapini-Papoular anchoring energy is zero. 相似文献
14.
利用双轴向列相液晶表面能对指向矢和序参量矩阵的旋转不变性,构造了与指向矢^a,^b,^c对易取向轴^a0,^b0,^c0的偏离有关的表面能Fs(a,b,c),以及与序参量矩阵有关的表面能Fs(Q),并得到两种表达式的等价形式.在Fs(a,b,c)和Fs(Q)中分别有四个和两个独立参量.为讨论上述独立参量的物理意义,假设一液晶分子对表面相互作用的简单模型,将液晶分子沿易取向轴^b0,^c0的双轴性取向看成是受到沿^b0,^c0两相互垂直的平均场作用的结果.为测量表面锚泊系数,考虑了三种简单形变位型,即在外磁
关键词:
液晶
弹性能
表面能 相似文献
15.
G. Barbero L.R. Evangelista N.V Madhusudana 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):327-331
We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties
of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director
with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric
coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed
ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We
also discuss the director distortion when the anchoring energy of the surface is finite.
Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997 相似文献
16.
17.
An analytical expression for the relaxation time of a nematic liquid crystal is obtained for the first time by considering the influence of surface viscosity, anchoring energy strength and cell gap, validated numerically by using the so-called relaxation method. This general equation for the molecular response time (τ0) was derived for a vertical aligned cell and by solving an eigenvalue equation coming from the usual balance of torque equation in the Derzhanskii and Petrov formulation, recovering the usual equations in the appropriate limit. The results show that τ∼db, where b=2 is observed only for strongly anchored cells, while for moderate to weak anchored cells, the exponent lies between 1 and 2, depending on both, surface viscosity and anchoring strength. We found that the surface viscosity is important when calculating the response time, specially for thin cells, critical for liquid crystal devices. The surface viscosity’s effect on the optical response time with pretilt is also explored. Our results bring new insights about the role of surface viscosity and its effects in applied physics. 相似文献
18.
We investigated the alignment induced on a nematic liquid crystal (LC) by a photo-aligned polymer film with azo-dye side groups. The orientation of the LC molecules can be manipulated in a reversible manner by irradiating the film with polarized light. We analyzed the competition between the orientation induced by the main chain, through rubbing of the film and that induced by the photo-aligned polymer. Anchoring strength for the different processing conditions are reported. The changes in film morphology caused by rubbing or photo-alignment could be captured by atomic force microscopy. The reversibility of the photo-induced alignment and the competition between the two anchoring mechanisms may allow recording and erasing of information in a LC display. PACS 61.30.-v; 61.30.Gd 相似文献
19.
To account for azimuthal surface anchoring of a nematic liquid crystal, Berreman [Phys. Rev. Lett. 28, 1683 (1972)10.1103/PhysRevLett.28.1683] proposed a simple model attributing the surface anchoring to the elastic distortion of the liquid crystal induced by the grooves of a surface. He showed that the surface anchoring energy is proportional to sin(2)varphi, with varphi being the angle between the director at infinity and the direction of the surface grooves. We argue that his assumption of negligibly small azimuthal distortion of the nematic is not valid. Proper treatment of the azimuthal distortion reveals that the Berreman's model should yield a surface anchoring energy proportional to sin(4)varphi. This implies that surface grooves alone cannot contribute to the surface anchoring coefficient in the usual Rapini-Papoular sense. 相似文献
20.
Deformations of nematic layers caused by magnetic field allow determination of the elastic constants of liquid crystal. In
this paper, we simulated numerically the deformations of planar and homeotropic nematic layers. The flexoelectric properties
of the nematic and presence of ions were taken into account. Our aim was to show the influence of flexoelectricity on the
results of the real measurement of the elastic constants k33 and k11. In these simulations, we calculated the optical phase difference ΔΦ between the ordinary and extraordinary rays of light
passing through the layer placed between crossed polarizers as a function of the magnetic field induction B. One of the elastic
constants can be calculated from the magnetic field threshold for deformation. The ratio k33/k11 can be found by means of fitting theoretical ΔΦ(B) dependence to the experimental results. The calculations reveal that the
flexoelectric properties influence the deformations induced by the external magnetic field. In the case of highly pure samples,
this may lead to false results of measurement of the elastic constants ratio k33/k11. This influence can be reduced if the nematic material contains ions of sufficiently high concentration. These results show
that the flexoelectric properties may play an important role, especially in well purified samples. 相似文献