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1.
2.
A. C. Ribeiro Ph. Barois Y. Galerne L. Oswald D. Guillon 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(1):121-126
We report on optical and structural X-ray studies on a smectic C twist grain boundary phase (TGB) of a pure liquid crystalline material. It is shown that this TGB phase, which exists over a large range of temperature, is definitely different from previously reported and predicted TGB phases. The two main experimental features are: (i) the observation in planar geometry of an optical texture exhibiting a
square grid pattern, and (ii) the detection of a broad Bragg ring in reciprocal space instead of one (TGB) or two (TGB) sharp rings. We suggest a few possible tracks that can be explored to understand the complex structure of this new phase.
Received 25 January 1999 相似文献
3.
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
4.
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
5.
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
6.
M.B. Lacerda Santos M.A. Amato 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(3):393-400
Using a light-beating technique we have measured the damping time of thermal fluctuations of the nematic director for the
so called cylindrical or calamitic nematic (NC) phase of the lyotropic system K-laurate/decanol/. By varying the scattering angle in suitable geometries, we have been able to estimate the orientational diffusivities associated
to the three pure deformations of splay, twist and bend. A former measurement made in the disk-like ND phase of the same system yielded a large deviation between the splay and twist diffusivities. The effect was then attributed
to induced flows, or backflow, which could be responsible for the reduction of the splay viscosity. In fact, this is the analogous
effect, for disks, to the one recognized since long time ago arriving for rod-like molecules in a classical nematic, though
in this case it is associated with bend deformations. The analogy comes about thanks to the interchange of the role played
by disks and cylinders for, respectively, splay and bend fluctuations.The measurements reported here provide a new test on
the applicability of the backflow model to a nematic system composed of micelles, that is, aggregates made of amphiphilic
(surfactant) molecules, in its cylindrical-like variant, i.e. the NC phase. In addition, the comparative study made here with the previous results existing in the literature for the ND phase, allows us to conjecture on structural issues concerning lyotropic nematics.
Received: 29 April 1998 / Revised: 19 August 1998 / Accepted: 31 August 1998 相似文献
7.
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
8.
Experiments have shown that cholesteric droplets or cholesteric fingers may be put into motion by the action of an electric
field. The former rotate whereas the latter drift perpendicularly to their axes. In all cases, the texture moves without visible
material transport. The electric Lehmann effect was initially used to interpret these observations but, recently, alternative
explanations were found, based on electrohydrodynamics. Another experiment in this area was that of Padmini and Madhusudana
(Liq. Cryst. 14, 497 (1993)). Performed in 1993 with a compensated cholesteric liquid crystal under fixed planar boundary conditions, it
was also explained in terms of electric Lehmann effect. We conducted the same experiment and extended it to a π -twisted planar
geometry. Although our experimental results agree with those of Padmini and Madhusudana, we demonstrate that they are incompatible
with an electric Lehmann effect. By contrast, an explanation based on flexoelectricity allows us to interpret the whole data
set obtained in both geometries. The consequence is that there is at the moment no clear experimental evidence of the electric
Lehmann effect. 相似文献
9.
Fukuda J 《The European physical journal. E, Soft matter》2007,24(1):91-98
It was shown experimentally (P.V. Dolganov et al., Europhys. Lett. 76, 250 (2006)) and by numerical calculations (C. Bohley, R. Stannarius, Eur. Phys. J. E 23, 25 (2007)) that the c -director profile of a two-dimensional chiral smectic-C (SmC) film around a circular inclusion adopts dipolar rather than
quadrupolar configuration observed in achiral SmC films. We give an analytical argument on how spontaneous bend inherent in
chiral SmC liquid crystals influences the configuration of a SmC liquid crystal film around a circular inclusion imposing
tangential anchoring. We find how the angle α between two surface defects seen from the center of the inclusion depends on
the radius of the inclusion R and the strength of the spontaneous bend q . We show, however, that the contribution of the spontaneous bend to the free energy suffers from mathematical ambiguity;
it depends on the mathematical treatment of the outer boundary even when it is at infinity. This might indicate that the shape
as well as the treatment of the outer boundary of the film can significantly influence the equilibrium configuration of the
c -director and the position of the surface defects. 相似文献
10.
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
11.
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
12.
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
13.
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
14.
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
15.
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
16.
Marcerou JP Nguyen HT Bitri N Gharbi A Essid S Soltani T 《The European physical journal. E, Soft matter》2007,23(3):319-328
Usual ferroelectric compounds undergo a paraelectric-to-ferroelectric phase transition when the susceptibility of the electric polarization density changes its sign. The temperature is the only thermodynamic field that governs the phase transition. Chiral tilted smectics may also present an improper ferroelectricity when there is a tilt angle between the average long axis direction and the layer normal. The tilt angle is the order parameter of the phase transition which is governed by the temperature. Although the electric susceptibility remains positive, a polarization proportional to the tilt appears due to their linear coupling allowed by the chiral symmetry. Further complications come in when the chirality increases, as new phases are encountered with the same tilt inside the layers but a distribution of the azimuthal direction which is periodic with a unit cell of two (SmC(A)*, three (SmC(Fi1)*, four (SmC(Fi2)* or more (SmC(alpha)* layers. In most of these phases, the layer normal is a symmetry axis so there is no macroscopic polarization except for the SmC(Fi1)* in which the average long axis is tilted so the phase is ferrielectric. By studying a particular compound with only a SmC(Fi2)* and a SmC(alpha)* phase, we show that we recover the uniformly tilted ferroelectric SmC* when applying an electric field. We are thus led to build field-temperature phase diagrams for this class of compounds by combining different experimental techniques described here. 相似文献
17.
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
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 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
20.
B. Yahiaoui A. Gharbi M. Kléman J. Malthête 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):99-110
Electron microscopy observations of replicas of freeze-fractured samples of two columnar hexagonal phases of different nature (a lyotropic one, the inverse AOT in water; a thermotropic one, ) yield very different results: most defects at microscopic scales are screw dislocations in the lyotropic phase, longitudinal
edge dislocations in the thermotropic phase. A possible way to interpret these differences is as follows: in the lyotropic
the Lamé coefficients and μ and the bend modulus K3 would not display any anomaly compared to expected values; in the thermotropic the shear modulus μ would be ten times smaller
than the compressibility modulus , while K3 would still be comparable to (but larger than) the bend modulus of a small molecules liquid crystal. We present an elementary
theoretical model of the latter case which could explain the anomalous measurements of K3 and of the longitudinal compressibility (Ref. [#!ref10!#]) without contradicting more recent measurements of (Refs. [#!ref17!#,#!ref22!#]). Essentially, the hexagonal phase would be a phase with defects (longitudinal dislocations) akin to an hexatic phase but with some differences.
Re?u : 26 mai 1997 / Révisé : 20 Janvier 1998 /
Accepté : 27 avril 1998 相似文献