共查询到20条相似文献,搜索用时 31 毫秒
1.
A.C. Ribeiro L. Oswald J.F. Nicoud D. Guillon Y. Galerne 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):503-512
The characterization by optical microscopy, X-ray diffraction and differential scanning calorimetry of two new liquid crystalline
compounds is presented. The compounds under consideration incorporate in their molecules two chiral centres (one asymmetric
carbon and one asymmetric sulphur of a sulphinate group). While the carbon chirality is fixed in the R configuration the sulphur
chirality may be either in the S or in the R configuration. A mesophase in a large temperature domain is evidenced for one of the two diastereomers, while both and mesophases are observed in a similar temperature domain for the other one. For both diastereomers and in both mesophases
it is possible to observe the coexistence of two different types of optical textures, namely planar cholesteric textures and
developable domains. The latter are coiled in a different way than proposed in an earlier publication. More precisely, they
form cylindrical or cone-like domains with double-twist properties as observed in the blue phases. The pitch of the helix
in the TGB mesophases is evaluated by means of optical reflection observations in samples exhibiting the planar cholesteric
textures. For both diastereomers it is also possible to evaluate structural parameters related to the organization within
the mesophases. Finally, the induced phase transition - is studied by the application of an AC electric field.
Received: 16 June 1997 / Revised: 17 October 1997 / Accepted: 20 November 1997 相似文献
2.
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
3.
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
4.
Shankar Rao DS Krishna Prasad S Raja VN Yelamaggad CV Anitha Nagamani S 《Physical review letters》2001,87(8):085504
We report the occurrence of a reentrant twist grain boundary phase, which we designate as Re- TGB(A). Microscopic observations on a nonsymmetric dimer showed the phase sequence Iso- N*- TGB(A)-Sm- A-Re- TGB(A)- TGB(C*). Here N* and Sm- A stand for the chiral nematic and smectic- A phases, TGB(A) is the twist grain boundary phase with smectic- A blocks, and TGB(C*) that with smectic- C* blocks and exhibiting features of both the smectic- C* and TGB phases. The reentrance of the TGB(A) phase is unambiguously demonstrated using x-ray diffraction, selective reflection, and optical rotation data. 相似文献
5.
Several new kinds of smectic-C twist grain boundary phases (TGBC) have been observed during the last few years. These pure
compounds or mixtures exhibit unusual textures with polygonal lattices (square or hexagonal grids) in the plane normal to
the TGB helix. The structure of these new phases seems to be complex and different from reported and predicted TGBC phases.
In this article, we review the main results obtained on these different new phases, and we propose new TGBC structures based
on the well-known splayed polarization --twisted director structures adopted by chiral smectic-C's in planar aligned (bookshelf)
cells. The observed square or hexagonal lattices are made of superimposed pairs of unwinding lines due to the suppression
of the helix within smectic-C blocks by the grain boundaries (unwinding walls). A lattice-free TGBC occurs if the helix within
smectic-C blocks is suppressed completely.
Received 15 March 2001 相似文献
6.
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
7.
F. Hardouin M.F. Achard J.-I. Jin Y.-K. Yun S.-J. Chung 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):47-56
Systematic physical chemistry studies are in progress concerning the occurrence of incommensurate low ordered smectic phases
in non-symmetric dimesogens varying molecular parameters from the standard compound KI-5. In the present study, the selected
molecules possess the same spacer length and the same cholesteryl unit. By means of X-ray diffraction on orientated samples,
commensurate phases, incommensurate fluid smectics and two-dimensional ones are clearly evidenced depending both on temperature
and molecular parameters. So these dimesogenic compounds respond to the frustration connected to the competition between two
incommensurate lengths with the formation either of an incommensurate phase or of a two-dimensional modulated phase. A new topology in a phase diagram results from this competition in a binary system
composed of two homologous dimesogens.
Received: 26 June 1997 / Revised: 7 October 1997 / Accepted: 29 October 1997 相似文献
8.
Braga WS Kimura NM Luders DD Sampaio AR Santoro PA Palangana AJ 《The European physical journal. E, Soft matter》2007,24(3):247-250
The phase diagram is an interesting field of research, particularly in lyotropic liquid crystals (LLC). In this way, one of the most important phase diagrams of this LLC system was reported by Yu and Saupe. Two uniaxial (calamitic--N(C) and discotic--N(D)) and one biaxial nematic (N(B)) phases were determined by these authors. Furthermore, in this phase diagram the classical isotropic phase (I was observed at high temperature as well as a reentrant isotropic phase (I(RE)) which takes place at lower temperature. Later, this phase diagram was also studied by several authors and in all cases the I(RE)-N(C)-I phase transitions were not observed. In this work, we present a study of this phase diagram through digital image processing and refractometry optical techniques. The occurrence of these phase transitions is investigated and characterized. In addition, the order parameter is obtained based on the Vuks hypothesis from a particular point, in the range of the N(C) phase, where the absolute value of the optical birefringence (Deltan is maximum. 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.
Sebastião PJ Simeão Carvalho P Chaves MR Nguyen HT Ribeiro AC 《The European physical journal. E, Soft matter》2006,20(1):55-61
We present a polarising optical microscopy study of the low-temperature anticlinic-like tilted mesophase of the liquid-crystal
compound octylphenyl-2-chloro-4-(p-cyano-benzoyloxy) (DB8Cl). This mesophase has been described as a bilayer smectic structure in which the molecules within each layer are organised
in an anticlinic way. The optical textures observed in samples with planar orientation show a double stripe pattern, with
the lines aligned parallel to the rubbing direction, characteristic of a double periodic modulation of the refractive index
of the material. The long-period modulation is temperature dependent and disappears for thin sample cells (< 5μm). The short-period
modulation is nearly independent of the thickness of the cells. The experimental results are analysed in terms of a model
which considers that there is a special distribution of the principal optical axis which may be in or out of the polariser-analyser
plane. The observed periodic variation of the principal optical axis could not be interpreted in terms of the original structure
proposed for this phase. DB8Cl presents a structure formed by dimers that can be viewed as flexible bent-core-like molecules, showing similarities with
phases found in banana-like systems, but exhibiting a much more complex structure. 相似文献
11.
We obtained monocrystalline droplets in a thermotropic cubic phase, of approximate size 100μm, deposited on a flat surface.
The facets of these soft crystals are explored using both an optical microscope and an AFM. The height of individual steps
on the principal facets and the lateral distance between steps in vicinal facets are measured using AFM in imaging (tapping)
mode. Moreover, the elastic modulus is measured locally, using the AFM tip (in contact mode) as a local rheological probe. 相似文献
12.
R. Kemkemer D. Kling D. Kaufmann H. Gruler 《The European physical journal. E, Soft matter》2000,1(2-3):215-225
In culture migrating and interacting amoeboid cells can form nematoid arrangements in analogy to a nematic liquid crystal
phase. A nematoid arrangement is formed if the interaction has an apolar symmetry. Different cell types like human melanocytes
(= pigment cells of the skin), human fibroblasts (= connective tissue cells), human osteoblasts (= bone cells), human adipocytes
(= fat cells) etc., form a nematoid structure. Our hypothesis is that elastic properties of these nematoid structures can
be described in analogy to that of classical nematic liquid crystals. The orientational elastic energy is derived and the
orientational defects (disclination) of nematoid arrangements are investigated. The existence of half-numbered disclinations
shows that the nematoid structure has an apolar symmetry. The density- and order parameter dependence of the orientational
elastic constants and their absolute values are estimated. From the defect structure, one finds that the splay elastic constant
is smaller than the bend elastic constant (melanocytes). The core of a disclination is either a cell free space or occupied
by non oriented cells (isotropic state), by a cell with a different symmetry, or by another cell type.
Received 3 May 1999 and Received in final form 29 September 1999 相似文献
13.
14.
Depletion interaction in a suspension of rodlike colloids with added non-adsorbing polymer coils is theoretically studied.
We calculate an overlap volume of depletion zone between two rodlike colloids, based on the second virial approximation. We
examine nematic-isotropic phase transition (NIT) and two-phase coexistence between an isotropic and a nematic phase at low
polymer concentrations. We find that the depletion interaction is dependent on the orientational order parameter of rodlike
colloids and leads to a decrease in the NIT concentration on the addition of polymer. The coexistence curves have a leaning-chimney
shape and are shifted to lower rod concentrations on increasing the polymer concentration.
Received 23 May 2001 and Received in final form 18 July 2001 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.
H. Stark J. Stelzer R. Bernhard 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):515-523
Recently, it was observed that water droplets suspended in a nematic liquid crystal form linear chains [Poulin et al., Science 275, 1770 (1997)]. The chaining occurs, e.g., in a large nematic drop with homeotropic boundary conditions at all the surfaces. Between each pair of water droplets a
point defect in the liquid crystalline order was found in accordance with topological constraints. This point defect causes
a repulsion between the water droplets. In our numerical investigation we limit ourselves to a chain of two droplets. For
such a complex geometry we use the method of finite elements to minimize the Frank free energy. We confirm an experimental
observation that the distance d of the point defect from the surface of a water droplet scales with the radius r of the droplet like .When the water droplets are moved apart, we find that the point defect does not stay in the middle between the droplets,
but rather forms a dipole with one of them. This confirms a theoretical model for the chaining. Analogies to a second order
phase transition are drawn. We also find the dipole when one water droplet is suspended in a bipolar nematic drop with two
boojums, i.e., surface defects at the outer boundary. Finally, we present a configuration where two droplets repel each other without a
defect between them.
Received 11 December 1998 相似文献
17.
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
18.
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
19.
D. Spoliansky V. Ponsinet J. Ferré J.-P. Jamet 《The European physical journal. E, Soft matter》2000,1(2-3):227-235
Ferrosmectics and ferrohexagonals are magnetic liquid crystals which consist of a lamellar and a hexagonal phase in which
magnetic nanoparticles are incorporated. The magnetic field variation and the relaxation of the linear optical birefringence
of ferrosmectics and ferrohexagonals and its time relaxation are measured in different geometries and compared with that of
a ferrofluid sample. We interpret our data by the existence of a mean orientation of the magnetic particles in the lyotropic
structure, which appears to be non-random in zero field. We conclude that the magnetic moments of the particles are preferentially
aligned in the plane of the ferrosmectic layers and along the axis of the ferrohexagonal cylinders, respectively. To account
for this preferred alignment, we propose that some of the particles in the ferrosmectic are adsorbed on the surfactant layers
with their moment aligned in the lamellar plane, while the orientation of the moments in the ferrohexagonal is restricted
to a cone around the cylinder axis.
Received 31 December 1998 and Received in final form 22 September 1999 相似文献
20.
O. Pelletier C. Bourgaux O. Diat P. Davidson J. Livage 《The European physical journal. E, Soft matter》2000,2(2):191-198
We report small-angle X-ray scattering experiments performed in both the isotropic and nematic phases of aqueous V2O5 suspensions. We show that the scattering in the isotropic phase can be well described in the whole accessible q-range by only considering the form factor of non-interacting ribbons. We investigate the influence of concentration and pH on the dimensions of V2O5 ribbons and show that these parameters do not have any significant effect, as long as the system stays well within the chemical
stability domain of the ribbons. We then show that nematic single domains display an anisotropic small-angle scattering pattern,
even at scattering vectors small compared to that at which a characteristic correlation peak is observed. This feature is
expected for a nematic phase, but was rarely observed. We finally try to describe this scattering within the framework of
theories developed for the structure factor of a nematic polymer, and we reach the conclusion that chain ends are certainly
important to understand this pattern.
Received 21 July 1999 and Received in final form 17 December 1999 相似文献