共查询到20条相似文献,搜索用时 15 毫秒
1.
A.A. Boulbitch P. Tolédano 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):355-362
The behaviour of the antiferroelectric SmCA liquid crystal phase under applied electric field is discussed theoretically. The phase diagram involving the SmA, SmCA and SmC
A
* phases is worked out and shown to exhibit a Lifshitz critical point. The deformation of the bilayer structures induced by
the field transforms the SmCA phases into a ferrielectric phase whose specific configuration is described.
Received: 23 October 1997 / Revised: 8 April 1998 / Accepted: 14 July 1998 相似文献
2.
We studied the formation of χ disclination lines in planar cholesteric samples placed in a temperature gradient near the cholesteric
to smectic A phase transition. We observed that the first simple line which forms close to the smectic-cholesteric front zigzags
when it is perpendicular to the direction of planar anchoring and is straight for other orientations. This instability is
similar to Herring instability for crystalline surfaces. We show numerically that it originates from a strong increase of
the elastic anisotropy close to the transition. In addition, we propose a new method to measure the pitch divergence at the
smectic to cholesteric phase transition. 相似文献
3.
Yu.A. Nastishin M. Kléman J. Malthête H.T. Nguyen 《The European physical journal. E, Soft matter》2001,5(3):353-357
We have shown that cholesteryl nonanoate, a thermotropic compound which is well known to exhibit pretransitional effects at
the smectic A (SmA) cholesteric (N*) transition (W.L. McMillan, Phys. Rev. A 4, 1238 (1971); 6, 936 (1972)), has in fact a TGBA phase in between. Our arguments rely on the observation of new TGBA defects, either in Robinson
spherulites cooled from the N* phase or in free-standing films. The same new defects can be obtained in a well-documented
TGBA phase of a tolane compound. We analyze qualitatively the TGBA defects in both geometries, in particular their relation
to the disclination radius of the N* Robinson spherulites.
Received 12 February 2001 相似文献
4.
Following our experimental observations of disclination lines in freely suspended droplets and free-standing films (Yu.A.
Nastishin et al., Eur. Phys. J. E 5 353 (2001)), topological defects of the twist grain boundary (TGBA) phase are considered according to two aspects: topological
and energetical. There are two classes of line defects, disclinations (as in the cholesteric (N*) phase and the liquid vortices phase (NL*), relating to the directors tripod symmetries) and dispirations (relating to the translation-rotation symmetries); there are no topological point defects. Differences between N*, NL* and
TGBA disclinations are physical, not topological. The absence of focal conic domains in the TGBA phase is an immediate consequence of the materialization of the helical axis
(along the χ-director); the same feature, coupled to the trend to parallelism of the smectic layers, accounts for the predominance
of λ-lines. Finally, the presence of defects akin to developable domains is explained in the frame of the leastcurvaturemodel, that requires the introduction of a third type of defects: the densitiesofedgedislocations of the smectic layers.
Received 20 February 2002 相似文献
5.
We describe a chiral axial next nearest neighbour xy-model to account for the various subphases exhibited by antiferroelectric liquid crystals made of chiral rod-like molecules.
The assumed form of the interlayer interactions is based on physical processes which are discussed. Using a discrete model,
the predicted sequence of transitions is SmA-SmC-SmC-FIH-FII-FIL-SmC
*
A, where FI stands for a ferriphase, as seen in many compounds. The ferri and SmC phases are characterized by relatively large angles between the c-vectors of successive layers and occur only when the compounds have high optical purity. The calculated field induced structures
exhibit a plateau of the apparent tilt angle at , where is the tilt angle of the molecules in the ferriphase. The conoscopic figures in the presence of a field and ellipsometric
parameters in the absence of a field have also been generated, which agree extremely well with the experimental results. Recent
anomalous X-ray scattering studies prove the xy-character of the configurations, though the commensurate structures that are found in the ferriphase require an extension
of the model to include lock-in terms.
Received 23 August 1999 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.
Yves Galerne 《The European physical journal. E, Soft matter》2000,3(4):355-368
We propose complements to the Renn model of the liquid crystal TGBC* phase. We argue that the rotation angle per helislab is spontaneously in the radian range, not too small to limit the energy
cost of the twist grain boundaries between the helislabs, not too large to preserve the double twisting efficiency. Taking
the elastic interactions between the helislabs into account, we show that the structure may undergo two different lock-ins,
provided that the uncontrolled interactions at the sample surfaces are small enough. First, for appropriate values of the
elastic constants, an angular lock-in may fix the rotation angle per helislab at values exactly commensurate to π. Three characteristic
lengths of the TGBC* phase, the TGB period and the thicknesses of both the smectic blocks and of the helislabs are then commensurate to one another,
with moreover, integer ratios at the most efficient commensurabilities. The TGBC* phase could thus exist in several versions, incommensurate and commensurate, according to the steps of a restricted Devil's
staircase. A second elastic lock-in should then arise in the commensurate TGBC* phases, to set the arrays of disclination lines between the helislabs in simple rectangular lattices, arranged in a helical
manner. Being placed right behind one another, the disclination lines then coincide when observed along the TGB axis. This
could explain the typical textures with square or hexagonal grids, observed in oriented samples. The commensurability of the
TGBC* phases could be analyzed with X-ray scattering experiments in the same manner as already reported for the TGBC phase.
Received 30 November 1999 and Received in final form 5 May 2000 相似文献
8.
9.
O. Dhez S. König D. Roux F. Nallet O. Diat 《The European physical journal. E, Soft matter》2000,3(4):377-388
A lyotropic system, consisting of a lecithin (DMPC) and a non-ionic surfactant (C12E5) in water was studied. The system exhibits a lamellar-to-nematic phase transition. The nematic phase appears as the temperature
is decreased and only exists in a very limited temperature and concentration range, for specific lipid-to-surfactant ratios.
While a lamellar phase is found at higher temperatures in both mixed and pure C12E5 systems, the transition to the nematic phase at lower temperatures coincides with a micellar phase in the pure C12E5 system. The transition appears to be driven by the strong temperature dependence of the surfactant film spontaneous curvature.
The structural properties of the lamellar phase close to the lamellar-to-nematic boundary have been studied by polarised light
microscopy and small-angle neutron and X-ray scattering experiments. The signature of a helical defect with Burgers vector
of magnitude 2 is apparent in our data, close to the lamellar-to-nematic phase transition. The proliferation of screw dislocations
in the lamellar phase might be a plausible mechanism for driving this transition.
Received 6 July 1999 and Received in final form 17 April 2000 相似文献
10.
D. Pociecha E. Gorecka J. Przedmojski 《The European physical journal. E, Soft matter》2000,1(2-3):137-140
Tilt-driven phase transitions between hexatic smectic phases: SmF-HexB and reversed HexB-SmF have been studied in compounds
belonging to two enaminoketone derivative homologue series. The tilt angle order parameter has been measured and its temperature
dependence near the phase transition point has been described by applying mean-field model. For both phase sequences the tricritical
points have been observed on phase transition lines in binary mixtures of respective materials having first and second order
phase transitions between hexatic phases.
Received 21 June 1999 相似文献
11.
H.R. Brand P.E. Cladis H. Pleiner 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):347-353
We discuss the macroscopic behavior of smectic CG liquid crystals. Smectic CG is the most general tilted smectic phase that is fluid in the layers. It is characterized by global C1 symmetry. Consequently, it is ferroelectric, pyroelectric and piezoelectric, opening up a number of possible applications
for such a phase. As smectic CG-phase has a macroscopic hand due to its structure, it is a natural candidate to explain the recent experimental observations
of left and right-handed helices in a system composed of achiral molecules. We also discuss critically to what extent smectic
CG could be important for liquid crystalline phases formed by banana-shaped molecules. Phase transitions involving a smectic
CG phase and defects of its in-plane director are briefly discussed.
Received: 25 March 1998 / Revised: 15 June 1998 / Accepted: 15 July 1998 相似文献
12.
B.I. Ostrovskii S.N. Sulyanov N.I. Boiko V.P. Shibaev W.H. de Jeu 《The European physical journal. E, Soft matter》2001,6(4):277-285
The phase behaviour and structure are reported of a new type of frustrated side-chain liquid crystalline (LC) polymer, a polyacrylate
with phenylbenzoate mesogenic side groups and a narrow polydispersity. At a high degree of polymerisation the LC polymers
show a nematic, a smectic-Ad, a re-entrant nematic and a C phase, for shorter chains only a nematic and a C phase. This constitutes a new example of nematic
re-entrance for which the driving field is the length of the polymer chain. The smectic-Ad layers consist of partially overlapped side groups while in the C phase the side chains are rearranged into chevron-like
blocks of bilayers. We propose an explanation of the frustrated phase behaviour in terms of these two different competing
length scales and their coupling to the backbone conformations.
Received 28 February 2001 and Received in final form 6 August 2001 相似文献
13.
B.I. Ostrovskii D. Sentenac I.I. Samoilenko W.H. de Jeu 《The European physical journal. E, Soft matter》2001,6(4):287-294
We report on the thin-film behaviour of a polyacrylate with phenyl benzoate mesogenic side groups and a narrow polydispersity.
Depending on the degree of polymerisation, these polymers show a nematic, a smectic-A d, a re-entrant nematic and a C phase with a two-dimensional monoclinic lattice. X-ray reflectivity and atomic-force microscopy
have been used to characterize the structure and surface morphology. The system exhibits two stable side-chain packing configurations
with incommensurate spacings that can be both stabilized at a free surface. Thin films in the nematic phase show a structural
dewetting induced by the growth of surface domains of the C phase. Additionally, surface-induced ripples with a nanoscale
lateral period form at the air-film interface. We attribute these patterns to a coupling between the local liquid crystalline
ordering of the mesogenic side groups and the surface curvature energy.
Received 28 February 2001 and Received in final form 6 August 2001 相似文献
14.
We model a melt of monodisperse side-chain liquid-crystalline polymers as a melt of comb copolymers in which the side groups
are rod-coil diblock copolymers. We consider both excluded-volume and Maier-Saupe interactions. The first acts among any pair
of segments while the latter acts only between rods. Using a free-energy functional calculated from this microscopic model,
we study the spinodal stability of the isotropic phase against density and orientational fluctuations. The phase diagram obtained
in this way predicts nematic and smectic instabilities as well as the existence of microphases or phases with modulated wave
vector but without nematic ordering. Such microphases are the result of the competition between the incompatibility among
the blocks and the connectivity constraints imposed by the spacer and the backbone. Also the effects of the polymerization
degree and structural conformation of the monomeric units on the phase behavior of the side-chain liquid-crystalline polymers
are studied. 相似文献
15.
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
16.
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
17.
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
18.
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
19.
20.
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