共查询到20条相似文献,搜索用时 15 毫秒
1.
Y. Galerne P. Hubert 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(2):245-249
The active oxygen gas arising from a plasma reactor is used to realize progressive chemical modifications onto silane coatings
that could be particularly interesting as alignment layers for liquid crystal display applications. Depending on the oxygen
density grafted onto the substrate, these alignment layers provide different zenithal anchoring angles, or pretilt angles,
with anchoring transitions, for polar and non-polar nematic liquid crystals as 5CB and MBBA, respectively. The anchoring transitions
are found to be smoother with the polar nematics. Such a behavior is discussed in terms of the differential wetting model
by adding a cosine term to the interaction energy between the nematic and the substrate. A local justification is proposed
for this symmetry breaking term.
Received: 18 May 1998 相似文献
2.
M. Petit M. Nobili P. Barois 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(3):341-345
We report the first observation of an electroclinical effect at the TGB-TGB transition induced by an external DC electric field applied perpendicular to the pitch direction. Upon increasing the field,
the smectic layers rather than the director field tilt over relative to the helical axis, allowing to detect the effect by
X-ray scattering from well aligned samples. The observations are qualitatively interpreted in the frame of a mean field phenomenological
model of a helically modulated electroclinical effect.
Received: 24 April 1998 / Revised: 22 June 1998 / Accepted: 31 July 1998 相似文献
3.
R. Barberi I. Dozov M. Giocondo M. Iovane P. Martinot-Lagarde D. Stoenescu S. Tonchev L.V. Tsonev 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(1):83-91
We measure the azimuthal anchoring energy of the nematic 5CB on sinusoidal holographic unidimensional and bidimensional gratings.
We find that the anchoring strength is almost the same for all of the gratings and up to one order of magnitude stronger than
expected by the topographic mechanism proposed by Berreman. We observe strong memory effects which dominate both the elastic
anchoring and the easy axis gliding. We show that these memory effects limit from below the anchoring energy and are the main
obstacle in the realization of substrates with weak anchoring conditions.
Received: 29 April 1998 / Accepted: 23 June 1998 相似文献
4.
D. Schlauf C. Bahr V.K. Dolganov J.W. Goodby 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(3):461-469
We report a detailed ellipsometric study of freely suspended films of chiral liquid-crystal compounds possessing smectic-A
and smectic-C phases. In the temperature region between the smectic-A - smectic-C bulk and surface transitions, a discontinuous
reconstruction of the tilt profile across the film is observed in the presence of a constant d.c. electric field. Comparison
of the measured ellipsometric quantities with values calculated from model tilt profiles reveals a competition between a structure
possessing a homogeneous tilt direction and large ferroelectric polarization and a structure with opposite tilt direction
in the two film halfs and large flexoelectric polarization.
Received 21 October 1998 相似文献
5.
Optical reflectivity studies have been conducted on freely suspended films with synclinic and anticlinic structures. For the
first time quantitative data were obtained on orientational ordering in films with anticlinic structure. In the same wide
temperature range of more than 30 K we observed both transverse and longitudinal ferroelectricity in the films of the same
thickness. Equilibrium transition temperatures between structures with transverse and longitudinal polarization were above
the bulk transition temperature to the SmC phase. The molecule tilt was determined in superthin films with synclinic and anticlinic ordering.
Received 29 October 1999 and Received in final form 17 February 2000 相似文献
6.
A.J. Jin M.R. Fisch M.P. Mahajan K.A. Crandall P. Chu C.-Y. Huang V. Percec R.G. Petschek C. Rosenblatt 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):251-255
Magnetic and electrical Freedericksz measurements were performed on the second generation monodendritic liquid crystal G2(OH). The deduced elastic constants were found to be exceptionally anisotropic: the splay elastic constant K11 is more than an order of magnitude larger than the bend elastic constant K33, and the twist constant K22 is approximately twice K33. The results are discussed in terms of molecular conformations.
Received: 23 February 1998 / Revised and Accepted: 26 May 1998 相似文献
7.
Achard MF Bedel JP Marcerou JP Nguyen HT Rouillon JC 《The European physical journal. E, Soft matter》2003,10(2):129-134
Taking advantage of the great number of bent-core or “banana" compounds synthesized and studied in the laboratory, we describe
their behaviour under the application of an external electric field. If the field were a static one, we would work within
the frame of an equilibrium phase diagram in a (field E, temperature T) space where some phases would be simple dielectrics and others ferroelectric ones with a macroscopic polarization, either
spontaneous or induced by the field. In this paper, we deal with the basic responses of “banana” liquid crystals under the
application of a low frequency (1 to 100 Hz) AC field. Firstly square-wave voltages allow us to locate the phase boundary
between dielectric (at lower field) and ferroelectric phases (higher field) at a given temperature and field threshold. Then
we apply slowly varying AC voltages with shapes like triangle or “triple-plateau” to check out the stability of the induced
ferroelectric phase versus field removal. Three behaviours are encountered, the unstable one (short lifetime of the high-field ferroelectric phase)
where the macroscopic polarization is destroyed and then rebuilt in the opposite direction during each half period and usually
called “antiferroelectric”; the stable one (long lifetime) with a polarization that rotates at constant modulus which is labeled
as “ferroelectric” and a new one where the macroscopic polarization is proportional to the applied fied, we named this behaviour
as “superparaelectric”. Let us stress that these observations apply to the ferroelectric phases of the (E, T) phase diagram not to the zero field (0,T) phases observed in the usual phase characterization experiments except for an
eventual spontaneous ferroelectric phase.
Received 18 April 2002 and Received in final form 17 January 2003 Published online: 16 April 2003
RID="a"
ID="a"e-mail: marcerou@crpp.u-bordeaux.fr
RID="b"
ID="b"URL: http://www.crpp-bordeaux.cnrs.fr 相似文献
8.
Cluzeau P Bonnand V Joly G Dolganov V Nguyen HT 《The European physical journal. E, Soft matter》2003,10(3):231-240
The behaviour of freely suspended smectic-C* ( SmC*) films at the bulk SmC*-cholesteric ( N*) phase transition has been investigated using polarized-reflected-light microscopy. Our experimental observations show that
above the bulk SmC*- N* phase transition the N* order appears in different ways according to the film thickness. In thin films, the conventional layer-by-layer thinning
occurs. In films of intermediate thickness N* inclusions nucleate inside the SmC* film. The distortions of the in-plane orientational order of the SmC* host phase induce elastic interactions between the inclusions and lead to their self-organization in chain-like structures.
Both the dynamic of the chaining and the parameters driving the equilibrium distance between the inclusions in the chain are
investigated. In thick films, N* fingers grow inside the film. The influence of the experimental conditions on the various processes is analysed.
Received 1 July 2002 / Published online: 15 April 2003
RID="a"
ID="a"e-mail: Philippe.cluzeau@univ-lille1.fr 相似文献
9.
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
10.
We report the experimental high electric field phase diagram of a nematic
liquid crystal which exhibits a large negative dielectric
anisotropy. We measure simultaneously
the birefringence (Δn) and the dielectric constant
(epsilon⊥) at various applied fields as
functions of the
local temperature of an aligned sample. We also measure the higher harmonics of
the electrical response of the
medium. The following experimental results are noted: (i) enhancement
of orientational order parameter
S in the nematic phase due to both
the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to
nematic transition temperature (TPN) with field; (iii) divergence of the order parameter
susceptibility beyond the tricritical point as measured
by third harmonic electrical signal; (iv) a small second harmonic
electrical signal which also diverges
near TPN, indicating the presence of
polarised domains. Our measurements show that ΔTPN(= TPN(E)-TNI(0))
varies linearly with |E|
whereas the Landau de Gennes theory predicts a dependence on E2. It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations,
including the thermodynamics of the transition. 相似文献
11.
We discuss the coupling between flow and other hydrodynamic variables that can occur in the optically isotropic tetrahedratic
phase (Td) characterized by a third rank tensor order parameter Tijk. We point out that an applied electric field or an applied temperature gradient will lead to flow. Reciprocally we predict
that, for example, a shear flow applied to a tetrahedratic phase leads to an induced electric field and a temperature gradient.
Similarities to recent experimental observations in the vicinity of the isotropic-B7 phase transitions in materials formed
by banana-shaped molecules are discussed.
Received 31 July 2001 相似文献
12.
The depolarisation ratio for the Raman-active phenyl stretching mode has been measured over the whole of the mesophase range,
and the orientational order parameters deduced, in the uniaxial nematic liquid crystal octylcyanobiphenyl (8CB). Linearly
polarised light was incident normally on a homogeneously aligned sample and a χ2 minimisation routine performed on the 360° depolarisation ratio profile. The order parameters 〈P
200〉 and 〈P
400〉 , together with the differential polarisability ratio, r , are used as fitting parameters and measured as a function of temperature. Interestingly, we show that the value for r , conventionally measured in the isotropic phase and assumed to remain constant, has a clear temperature dependence, ranging
from -0.032±0.008 in the isotropic phase through to -0.245±0.015 at the nematic-to-smectic A phase transition. The measured
order parameters 〈P
200〉 and 〈P
400〉 varied from 0.35- 0.55±0.02 and 0.180- 0.245±0.02 , respectively, across the 8 ° C wide nematic phase range. The values of both 〈P
200〉 and 〈P
400〉 are in excellent agreement with theory, but it is noteworthy that 〈P
400〉 shows a much better quantitative match than has been reported in previous work. Crucially the temperature dependence of
r is shown to be a contributing factor in the low 〈P
400〉 values that have been conventionally reported from Raman scattering measurements. The potential for fitting the entire angular
depolarisation ratio distribution in liquid crystalline systems that are described by more order parameters, specifically
biaxial materials, is discussed. 相似文献
13.
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
14.
J.C. Loudet O. Mondain-Monval P. Poulin 《The European physical journal. E, Soft matter》2002,7(3):205-208
We study experimentally the dynamics of a topological defect located around a colloidal particle suspended in a thermotropic
nematic liquid crystal. The considered defect consists of a disclination loop encircling the particle at the equator. Under
specific conditions, it is shown that this disclination continuously shrinks to a hedgehog defect located in the immediate
vicinity of the particle. This phenomenon corresponds to a transition between an elastic quadrupolar configuration and an
elastic dipolar configuration. We performed a basic numerical calculation to get an estimate of the dissipated energy during
the transition; we compare the results with theoretical predictions that describe the elastic energy of particles surrounded
by defects.
Received 21 December 2001 相似文献
15.
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
16.
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
17.
Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions
in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure
dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T*AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the
quadrupolar nematic ordering Qij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity
of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with
pressure. 相似文献
18.
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
19.
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
20.
Martínez-Ratón Y Cinacchi G Velasco E Mederos L 《The European physical journal. E, Soft matter》2006,21(2):175-188
It is known that when hard spheres are added to a pure system of hard rods the stability of the smectic phase may be greatly
enhanced, and that this effect can be rationalised in terms of depletion forces. In the present paper we first study the effect
of orientational order on depletion forces in this particular binary system, comparing our results with those obtained adopting
the usual approximation of considering the rods parallel and their orientations frozen. We consider mixtures with rods of
different aspect ratios and spheres of different diameters, and we treat them within Onsager theory. Our results indicate
that depletion effects, and consequently smectic stability, decrease significantly as a result of orientational disorder in
the smectic phase when compared with corresponding data based on the frozen-orientation approximation. These results are discussed
in terms of the τ parameter, which has been proposed as a convenient measure of depletion strength. We present closed expressions
for τ, and show that it is intimately connected with the depletion potential. We then analyse the effect of particle geometry
by comparing results pertaining to systems of parallel rods of different shapes (spherocylinders, cylinders and parallelepipeds).
We finally provide results based on the Zwanzig approximation of a fundamental-measure density-functional theory applied to
mixtures of parallelepipeds and cubes of different sizes. In this case, we show that the τ parameter exhibits a linear asymptotic
behaviour in the limit of large values of the hard-rod aspect ratio, in conformity with Onsager theory, as well as in the
limit of large values of the ratio of rod breadth to cube side length, d, in contrast to Onsager approximation, which predicts τ ∼ d
3. Based on both this result and the Percus-Yevick approximation for the direct correlation function for a hard-sphere binary
mixture in the same limit of infinite asymmetry, we speculate that, for spherocylinders and spheres, the τ parameter should
be of order unity as d tends to infinity. 相似文献