共查询到20条相似文献,搜索用时 531 毫秒
1.
G. Basappa N.V. Madhusudana 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(2):179-187
We present experimental studies on the effect of strong electric fields on the nematogen p-cyanophenyl p-n-heptyl benzoate
which has the strongly polar cyano end group and a large positive dielectric anisotropy. We use a local temperature measurement
to take into account heating effects and an electrical impedance analysis to determine both the dielectric constant () and the resistance (R) of the sample. We also measure the higher harmonic responses of the medium. The new results obtained in this study are: (i)
a detailed temperature dependence of the terms which describe (a) the quenching of macroscopic thermal fluctuations of the
nematic director and (b) the enhancement of the orientational order parameter due to Kerr effect, (ii) clear evidence for
the critical divergence of susceptibility as reflected in the third harmonic signal, (iii) an unusual enhancement of the conductivity
which shows a large peak just below the critical point , (iv) a significant peak in the second harmonic signal at and (v) evidence for a field induced nematic-nematic transition well inside the nematic range. We argue that the results
(iii)-(v) indicate the presence of polar short range order in the medium and hence support a molecular model in which such an order has been proposed.
Received: 15 July 1997 / Received in final form: 24 September 1997 / Accepted: 29 October 1997 相似文献
2.
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
3.
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
4.
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
5.
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
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.
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
8.
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
9.
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
10.
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
11.
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
12.
13.
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
14.
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
15.
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
16.
S. Faetti M. Nobili I. Raggi 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):445-453
In this paper we report an experimental investigation on the dynamics of the azimuthal director reorientation at a nematic-solid
interface. Three qualitatively different kinds of substrates have been investigated: I) intrinsically anisotropic SiO-substrates
(-evaporation), II) isotropic SiO-substrates (-evaporation) and III) rubbed PVA-substrates. In the case II), an in-plane anisotropy was induced cooling slowly the thermotropic
nematic liquid crystal (NLC) from the isotropic phase in the presence of a 0.75 T magnetic field. The reorientation dynamic
of the surface azimuthal director angle at the switching-on and off of a magnetic (or electric) field has been investigated.
All the substrates show comparable azimuthal anchoring energies and two dynamic regimes: a fast dynamic response, driven by
the bulk director reorientation and an extremely slow reorientation. The slow dynamics is explained in terms of anisotropic
adsorption of NLC molecules on the solid substrate and is well represented by a stretched exponential.
Received 7 December 1998 相似文献
17.
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
18.
H. Schiessel 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):373-380
We investigate the bending of flexible charged membranes due to the presence of rigid rodlike macroions in the framework of
the Debye-Hückel approximation. When the macroions are fixed in space at some distance from the bilayer the membrane bends
towards them; we calculate the exact deformation profile. On the other hand a macroion which is adsorbed on the membrane causes
a deflection of the bilayer. Finally, we consider swollen lamellar polyanion/charged-lipid complexes where the macroions are
intercalated between charged lipid bilayers. We predict the occurrence of a double adsorption (pinching effect) of the macroion
for sufficiently flexible membranes.
Received: 9 February 1998 / Revised: 9 June 1998 / Accepted: 2 July 1998 相似文献
19.
L. Ramos P. Fabre L. Fruchter 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):67-72
A new anisotropic magnetic fluid is obtained using a colloidal suspension of magnetic particles of nanometric size (ferrofluid)
as a component of a swollen lyotropic hexagonal phase. This doped hexagonal system exhibits specific behaviors when submitted
to a magnetic field of weak intensity. The field-induced instabilities are described and interpreted; they result from a high
anisotropy of the magnetic susceptibility of the medium, which is measured. It is finally shown that the magnetic properties
of the doped hexagonal phase allows one a determination of the compression modulus of the system.
Received 10 February 1998 相似文献
20.
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