共查询到20条相似文献,搜索用时 9 毫秒
1.
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
2.
P. Cluzeau P. Barois H.T. Nguyen C. Destrade 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(1):73-82
We report X-ray diffraction experiments performed on an antiferroelectric compound exhibiting a very rich polymorphism (). The structural study of the unknown phases only allows us to exclude some phenomenological models. The use of oriented
planar samples prepared between solid glass plates generate by cooling from the phase a chevron structure of tilted layers already well characterized for the phase. The extensive analysis of the evolution of the chevron structure through the numerous smectic-smectic phase transitions
provides some original information in three distinct areas: fundamental data on the important physical parameters in the chevron
structure formation, detection of the smectic-smectic phases transition by small change of the chevron structure, and information
on the local molecular order induced by the alignment layer (interaction with a rubbed polymer).
Received: 13 November 1996 / Received in final form: 19 January 1997 / Accepted: 30 January 1998 相似文献
3.
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
4.
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
5.
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
6.
7.
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
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.
We have used a new experimental method to produce and deform three-dimensional monodisperse foams. Uniaxial deformation causes
transitions in the foam in which the number of close-packed bubble layers perpendicular to the direction of elongation changes
by one. We predict the critical strains at which such transitions occur by calculating the foam energy as a function of strain.
These calculations are approximate with simplifying assumptions regarding the geometry of the bubbles. The foam deforms by
nucleation and subsequent glide of dislocations which consist, in one configuration, of pairs of 12- and 16-faced cells along
a close-packed direction of the foam. We describe these line defects and identify the topological transformations that occur
in glide. These are neighbour switchings associated with a 4-sided face that rotates changing the adjacencies of the cells.
These T4 operations occur in an avalanche and cause movement of the dislocation while preserving its identity.
Received 16 July 2001 相似文献
10.
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
11.
Ribeiro AC Heinrich B Cruz C Nguyen HT Diele S Schröder MW Guillon D 《The European physical journal. E, Soft matter》2003,10(2):143-151
We present the structural study of a biforked compound exhibiting a Colr-Colh phase transition. This study was performed by means of DSC, optical microscopy, X-ray diffraction, dilatometry and molecular
modelling. The molecular packing within the two columnar mesophases of this compound is discussed. For the first time, it
is shown that the clusters of adjacent molecules which form the elementary section of a column are tilted with respect to
the columnar axis in the rectangular mesophase. From the whole set of experimental data, the number of molecules in a columnar
section is found to be about 5 in the rectangular mesophase, and 4 in the hexagonal one. The transition between the two columnar
mesophases is discussed with respect to its slow kinetics.
Received 1st October 2002 Published online: 16 April 2003
RID="a"
ID="a"e-mail: ribeiro@cii.fc.ul.pt
RID="b"
ID="b"UMR 7504 相似文献
12.
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
13.
P.K. Mukherjee H. Pleiner H.R. Brand 《The European physical journal. E, Soft matter》2001,4(3):293-297
A simple Landau-type free energy function is presented to describe the smectic-A-isotropic phase transition. Varying the coupling
between orientational and positional order parameters, a smectic-A-isotropic or a nematic-isotropic phase transition occurs.
Within this model the smectic-A-isotropic phase transition is found to be always more strongly first order than the nematic-isotropic
phase transition. The theoretical results are found to be in good agreement with all published experimental results.
Received 27 June 2000 相似文献
14.
We study stable “bookshelf” smectic-A structures within a very thin plane-parallel cell of thickness L in which the mismatch between surface preferred (d
s) and intrinsic (d0) smectic layer thicknesses occurs. The Landau-Ginzburg approach based on a complex smectic order parameter is used. For a
weak enough smectic positional anchoring strength W smectic layers adopt the modified bookshelf profile. In a thick enough cell with increasing W a lattice of edge dislocations is continuously formed at the confining surfaces and then depinned from them. The structure
with dislocations is formed when the condition d
0/(
d
0/d
s - 1) ∼ 2 is fulfilled, where is the positional surface anchoring extrapolation length. If the cell is thin enough the dislocations formed at opposite
cell plates annihilate and consequently the smectic layers adopt a locked bookshelf structure. This transition is discontinuous
and takes place when d
0/(L
d
0/d
s - 1) ∼ 5 is realized. To observe these transitions in a cell of thickness L∼ 1μm the conditions W∼ 10-6
J/m
2 and
d
0/d
s - 1∼ 5 . 10-4 have to be fulfilled. All the three qualitatively different structures coexist at the triple point.
Received 21 February 2002 相似文献
15.
P. Pieranski L. Sittler P. Sotta M. Imperor-Clerc 《The European physical journal. E, Soft matter》2001,5(3):317-328
Monocrystals of the cubic lyotropic liquid crystal phase V1 are studied in droplets of the mixture C
12
EO
6/water surrounded by water vapor of controlled pressure p. Shapes of monocrystals are found to depend on the conditions of growth from the lamellar phase and on the nature of the
substrate. After the growth, when the lamellar phase is exhausted and crystals are in equilibrium with water vapors, their
shapes are shown to depend on the pressure p. Thermodynamic aspects of these phenomena are discussed.
Received 22 November 2000 相似文献
16.
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
17.
A disclination line populated with point defects that break the translational symmetry forms near a free nematic (N) interface
in a confined geometry. The disclination line is, however, absent in the smectic-A phase (SmA). We use this fact to control
the formation of point defect distributions on a disclination line by directional melting of the SmA phase in a temperature
gradient. A threshold velocity ( v
th) exists below which a defect-free disclination line is formed. The frequency of nucleation of point defects increases steadily
for v > v
th and exhibits a remarkable regularity. We derive an empirical scaling for v
th in terms of the experimental tuning parameters. We propose a simple model that allows to understand the formation of the
point defects.
Received 1 October 2002 / Published online: 15 April 2003
RID="a"
ID="a"Permanent address: Departament de Quımica Fısica, Universitat de Barcelona, Martı i Franquès 1, Barcelona 08028, Spain; e-mail: jignes@qf.ub.es 相似文献
18.
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
19.
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
20.
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