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1.
We study the nature of molecular deformations in a nematic liquid crystal film with elastic energy under homeotropic boundary conditions. The deformation in terms of splay, twist and bend fields of the director axis is found to be governed by the completely integrable Davey-Stewartson-I (DS-I) equation in (2+1) dimensions. Using the line soliton and breather solutions of the DS-I equation, the director axis is constructed, the components of which exhibit damped spatial oscillations. However, the splay and bend fields of the director axis exhibit localized structures of deformation. 相似文献
2.
The Frank elasticity constants which describe splay (K
1), twist (K
2), and bend (K
3) distortion modes are investigated for 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic liquid crystal. The calculations
rest on statistical-mechanical approaches where the absolute values of K
i
(i=1,2,3) are dependent on the direct correlation function (DCF) of the corresponding nematic state. The DCF was determined
using the pair correlation function by solving the Ornstein-Zernike equation. The pair correlation function, in turn, was
obtained from molecular dynamics (MD) trajectory. Three different approaches for calculations of the elasticity constants
were employed based on different level of approximation about the orientational order and molecular correlations. The best
agreement with experimental values of elasticity constants was obtained in a model where the full orientational distribution
function was used. In addition we have investigated the approximation about spherical distribution of the intermolecular vectors
in the nematic phase, often used in derivation of various mean-field theories and employed here for the construction of the
DCF. We found that this assumption is not strictly valid, in particular a strong deviation from the isotropic distribution
is observed for short intermolecular distances.
Received 22 March 2000 and Received in final form 9 June 2000 相似文献
3.
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
4.
We have described a novel reorientation mechanism in the form of the traveling waves, under influence of an external electric field, directed parallel to both glass plates, which occur in the twisted nematic cell (TNC). It is found that the slowest velocity of the traveling front is proportional to the field strength, and, approximately, in three times higher than the front velocity corresponding to the non-traveling solution. The value of the critical electric field Ecr which may excite the traveling waves in the TNC in π times less than the value of the threshold electric field Eth corresponding to the untwisted geometry. 相似文献
5.
We studied nonlinear molecular deformations in a nematic liquid crystal with homeotropically aligned molecules and hard boundaries. As the basic dynamical equation for the director axis of the liquid crystal resembles the Landau-Lifshitz equation representing spin dynamics in a one dimensional classical continuum isotropic Heisenberg ferromagnetic spin chain, we invoke here the space curve formalism and the stereographic projection technique used in the case of the Heisenberg spin chain. Under space curve mapping, the director dynamics with elastic deformation is found to be governed by a perturbed nonlinear Schrödinger equation. A multiple-scale perturbation analysis brings out perturbed solitons to represent molecular deformations in the nematic liquid crystal. However, when a constant electric field is applied, the director dynamics is expressed under stereographic projection and the molecular deformations are found to be governed by periodic and localized static planar director configurations. A linear stability analysis on the static planar configurations shows that the system exhibits stable deformations. 相似文献
6.
We investigated SmC* films sandwiched between silane coated glass plates and observed formation of textures exhibiting a uniform tilt of the smectic
layers with respect to the boundary plates. The layer tilt angle increases from zero to as the sample is cooled from the smectic A phase to room temperature. These films show linear electro-optical effects because
the permanent polarization can be aligned so that it has a component parallel to the applied field without changing the layer
structure. Our analysis indicates that mainly two effects determine the layer tilt. On the one hand, the surface tension tends
to minimise the layer tilt. On the other hand, the surface energy promotes the director to be normal to the boundary plates.
Received 17 July 1998 相似文献
7.
We have studied molecular director reorientation in a twisted nematic liquid crystal induced by a two-dimensional temperature gradient. We studied the effect of rate change between the temperature gradients in two directions. Our obtained director reorientations are in the range that can be observed experimentally very easily. 相似文献
8.
Leonid V. Mirantsev 《Physics letters. A》2008,372(21):3904-3908
A homeotropically oriented smectic-A film on a solid substrate with periodical microrelief is considered. Periodical distortions of the free surface of the film induced by this microrelief are theoretically investigated. The dependence of these distortions on the film thickness, the temperature, and external magnetic (electric) field is obtained. It is shown that, for a certain choice of the shape of the substrate surface microrelief, one can realize a temperature and external magnetic (electric) field control on the microrelief at the free surface of the smectic-A film. 相似文献
9.
We study the director field around a spherical particle immersed in a uniformly aligned nematic liquid crystal and assume
that the molecules prefer a homeotropic orientation at the surface of the particle. Three structures are possible: a dipole,
a Saturn-ring, and a surface-ring configuration, which we investigate by numerically minimizing the Frank free energy supplemented
by a magnetic-field and a surface term. In the dipole configuration, which is the absolutely stable structure for micron-size
particles and sufficiently strong surface anchoring, a twist transition is found and analyzed. We show that a transition from
the dipole to the Saturn ring configuration is induced by either decreasing the particle size or by applying a magnetic field.
The effect of metastability and the occurrence of hysteresis in connection with a magnetic field are discussed. The surface-ring
configuration appears when the surface-anchoring strength W is reduced. It is also favored by a large saddle-splay constant K24. A comparison with recent experiments [#!itapdb:Poulin1997!#,#!itapdb:Poulin1998!#] gives a lower bound for W, i.e., for the interface of water and pentylcyanobiphenyl (5CB) in the presence of the surfactant sodium dodecyl sulfate.
Received 2 November 1998 相似文献
10.
The interaction between particle-like sources of the
nematic director distortions (e.g., colloids, point
defects, macromolecules in nematic emulsions) allows for a useful
analogy with the electrostatic multipole interaction between
charged bodies. In this paper we develop this analogy to the level
corresponding to the charge density and consider the general
status of the pairwise approach to the nematic emulsions with
finite-size colloids. It is shown that the elastic analog of the
surface electric charge density is represented by the two
transverse director components on the surface imposing the
director distortions. The elastic multipoles of a particle are
expressed as integrals over the charge density distribution on
this surface. Because of the difference between the scalar
electrostatics and vector nematostatics, the number of elastic
multipoles of each order is doubled compared to that in the
electrostatics: there are two elastic charges, two vectors of
dipole moments, two quadrupolar tensors, and so on. The
two-component elastic charge is expressed via the vector of
external mechanical torque applied on the particle. As a result,
the elastic Coulomb-like coupling between two particles is found
to be proportional to the scalar product of the two external
torques and does not directly depend on the particles' form and
anchoring. The real-space Green function method is used to develop
the pairwise approach to nematic emulsions and determine its form
and restrictions. The pairwise potentials are obtained in the
familiar form, but, in contrast to the electrostatics, they
describe the interaction between pairs (dyads) of the elastic
multipole moments. The multipole moments are shown to be uniquely
determined by the single-particle director field, unperturbed by
other particles. The pairwise approximation is applicable only in
the leading order in the small ratio particle
size-to-interparticle distance as the next order contains
irreducible three-body terms. 相似文献
11.
We study numerically the effect of an external magnetic or electric field on the director profiles of a nematic liquid crystal
around a spherical particle. We pay particular attention to the stability of a hyperbolic hedgehog defect accompanying the
particle, which transforms into a Saturn-ring defect encircling the particle under a sufficiently strong external field. We
focus on the particle size dependence of the two important threshold field strengths: the “thermodynamic-transition” field
strength H1 at which the hedgehog and the Saturn-ring configurations have the equal free energy, and the critical field strength H2 at which the hedgehog loses its (meta)stability. Our numerical results demonstrate that while H1 is non-monotonically dependent on the particle radius R0, H2 monotonically increases with R0 and the dependence of H2 is weak for large R0. The non-monotonic dependence of H1 on R0 can be explained by comparing the energies of the two configurations and assuming the dependence of those energies on a rescaled
field. A crude argument of the energetics of a hyperbolic hedgehog defect under an external field shows that for an asymptotically
large R0 the critical field strength is independent of R0, which agrees with our numerical finding. 相似文献
12.
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
13.
We study numerically the director and orientational order parameter configurations in a nematic liquid crystal around a two-dimensional
spherical particle on the basis of the tensor order parameter formalism. To properly account for the large length scale difference
between the particle and the accompanying orientational defect, we devise an adaptive grid scheme in which the lattice spacing
is automatically and locally adjusted in response to the spatial gradient of the orientational order parameter. This adaptive
grid scheme is useful in studying dynamical as well as static orientational structures. We present a simulation result which
shows how a hedgehog defect of topological charge -1 becomes unstable in two dimensions, and splits into a defect pair of
topological charge -1/2, located symmetrically around the particle.
Received 14 September 2000 and Received in final form 27 December 2000 相似文献
14.
Light traveling through a liquid crystal with disclinations perceives a geometrical background which causes lensing effects
similar to the ones predicted for cosmic objects like global monopoles and cosmic strings. In this paper we explore the effective
geometry as perceived by light in such media. The comparison between both systems suggests that experiments can be done in
the laboratory to simulate optical properties, like gravitational lensing, of cosmic objects. 相似文献
15.
J. Buey L. Díez P. Espinet H.-S. Kitzerow J.A. Miguel 《Applied physics. B, Lasers and optics》1998,66(3):355-358
Received: 24 June 1997 相似文献
16.
In this work, an extended version of the Hess-Baalss conformal approach is used to propose a relation connecting the viscosity coefficients of the nematic liquid crystals. Starting from the well-known fact that, in its usual form, the conformal transformation leads to results which are not observed experimentally, it is shown that, when the director field of an ordered nematic phase under sheared motion is taken as a three-dimensional surface with torsion, the resulting theory describes the observed experimental data efficiently. Moreover, this model predicts that the five viscosity coefficients of the Leslie ah hoc model are not independent, but connected. A comparison of the deduced relationship with experimental data is performed and an excellent agreement is obtained. 相似文献
17.
We discuss several elastic energies for nematic elastomers and their small strain expansions both in the regime of large director
rotations, and in the case that director changes are small. We propose two fully non-linear model anisotropic energies and
compare the behavior they predict with the currently available experimental evidence. 相似文献
18.
The effect of a surface electric field produced by ionic adsorption on the molecular orientation of a nematic liquid crystal
sample is analyzed. The eigenvalue problem for a semi-infinite medium is analytically solved both for strong and weak anchoring
situations. The threshold instabilities are numerically determined and it is shown that the homeotropic pattern can be destabilized
also in the situation of strong anchoring. The dependence of the threshold field on the anchoring strength and on the surface
polarization is determined by taking into account also the coupling of the quadrupolar component of the flexoelectric coefficient
with the field gradient.
Received 4 November 1999 and Received in final form 4 April 2000 相似文献
19.
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
20.
The effective rotational viscosity coefficient and flow alignment angle are investigated for polar liquid crystals (LCs), such as 4-n-octyloxy- 4′-cyanobiphenyl (8OCB), in the vicinity of a charged bounding surface. is calculated using the Ericksen-Leslie theory, both for stationary and nonstationary regimes. Calculations of , both for homeotropic and planar alignment of 8OCB molecules, at a charged indium tin oxide(ITO)-coated glass plate show
an additional contribution to up to 7.8%. The nonequilibrium flow alignment angle (τ) is also calculated for the surface region bounded by 0.1≤y≤3.0 μm. Transition from a tumbling situation to a flow aligning regime can occur near the charged boundary surface.
Received 22 November 2001 and Received in final form 31 January 2002 相似文献