共查询到20条相似文献,搜索用时 31 毫秒
1.
2.
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
3.
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
4.
In this paper, we discuss the equilibrium phases and collapse transitions of a lyotropic nematic gel immersed in an isotropic
solvent. A nematic gel consists of a cross-linked polymer network with rod-like molecules embedded in it. Upon decreasing
the quality of the solvent, we find that a lyotropic nematic gel undergoes a discontinuous volume change accompanied by an
isotropic-nematic transition. We also present phase diagrams that these systems may exhibit. In particular, we show that coexistence
of two isotropic phases, of two nematic phases, or of an isotropic and a nematic phase can occur.
Received 15 February 2002 and Received in final form 14 June 2002 相似文献
5.
Linear stability analysis of capillary instabilities in a thin nematic liquid crystalline cylindrical fiber embedded in an
immiscible viscous matrix is performed by formulating and solving the governing nemato-capillary equations, that include the
effect of temperature on the nematic ordering as well as the effect of the nematic orientation. A representative axial nematic
orientation texture with the planar easy axis at the fiber surface is studied. The surface disturbance is expressed in normal
modes, which include the azimuthal wave number m to take into account non-axisymmetric modes. Capillary instabilities in nematic fibers reflect the anisotropic nature of
liquid crystals, such as the ordering and orientation contributions to the surface elasticity and surface normal and bending
stresses. Surface gradients of normal and bending stresses provide additional anisotropic contributions to the capillary pressure
that may renormalize the classical displacement and curvature forces that exist in any fluid fiber. The exact nature (stabilizing
and destabilizing) and magnitude of the renormalization of the displacement and curvature forces depend on the nematic ordering
and orientation, i.e. the anisotropic contribution to the surface energy, and accordingly capillary instabilities may be axisymmetric or non-axisymmetric.
In addition, when the interface curvature effects are accounted for as contributions of the work of interfacial bending and
torsion to the total energy of the system, the higher-order bending moment contribution to the surface stress tensor is critical
in stabilizing the fiber instabilities. For the planar easy axis, the nematic ordering contribution to the surface energy,
which renormalizes the effect of the fiber shape, plays a crucial role to determine the instability mechanisms. Moreover,
the unstable modes, which are most likely observed, can be driven by the dependence of surface energy on the surface area.
Low-ordering fibers display the classical axisymmetric mode, since the surface energy decreases by decreasing the surface
area. Decreasing temperature gives rise to the encounter with a local maximum or to monotonic increase of the characteristic
length of the axisymmetric mode. Meanwhile, in the presence of high surface ordering, non-axisymmetric finite wavelength instabilities
emerge, with higher modes growing faster since the surface energy decreases by increasing the surface area. As temperature
decreases, the pitches of the chiral microstructures become smaller. However, this non-axisymmetric instability mechanism
can be regulated by taking account of the surface bending moment, which contains higher order variations in the interface
curvatures. More and more non-axisymmetric modes emerge as temperature decreases, but, at constant temperature, only a finite
number of non-axisymmetric modes are unstable and a single fastest growing mode emerges with lower and higher unstable modes
growing slower. For nematic fibers, the classical fiber-to-droplet transformation is one of several possible instability pathways,
while others include chiral microstructures. The capillary instabilities' growth rate of a thin nematic fiber in a viscous
matrix is suppressed by increasing either the fiber or matrix viscosity, but the estimated droplet sizes after fiber breakup
in axisymmetric instabilities decrease with increasing the matrix viscosity.
Received 15 April 2002 and Received in final form 3 October 2002
RID="a"
ID="a"e-mail: alejandro.rey@mcgill.ca 相似文献
6.
We theoretically model the nucleation of nematic droplets during phase ordering in mixtures of a flexible polymer and a low-molecular-weight
liquid crystal. By appealing to classical nucleation theory (CNT), we calculate the energy barrier to nucleation and the size
of a critical nucleus. We study the influence of a metastable intermediate phase on the nucleation of the nematic. Below a
triple point in the phase diagram, there are two distinct mechanisms for the formation of a nematic nucleus: 1) direct nucleation
from the isotropic phase and 2) nucleation via a precursor metastable isotropic phase. We calculate the crossover concentration as a function of temperature, delineating
the regions of the phase diagram in which each mechanism prevails. In the latter case, the presence of a hidden metastable
isotropic-isotropic binodal may either promote or delay the nucleation of a nematic phase.
Received 9 August 2002
RID="a"
ID="a"e-mail: matuyama@chem.mie-u.ac.jp 相似文献
7.
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
8.
Spontaneous thermal expansion of nematic elastomers 总被引:1,自引:0,他引:1
We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic
groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional
segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium
thermal expansion on cooling from the isotropic phase. At higher concentration of main chains their behaviour dominates the
elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures,
one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber,
r(T) = / proportional to the effective nematic order parameter Q(T), is given by an average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental
data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators
are discussed in the end.
Received 25 June 2001 and Received in final form 29 September 2001 相似文献
9.
The elastic anisotropy of nematic elastomers 总被引:2,自引:0,他引:2
We examine the robustness of order in nematic elastomers under mechanical strains imposed along and perpendicularly to the
director when director rotation is prohibited. In contrast to electric and magnetic fields applied to conventional nematics,
mechanical fields are shown theoretically and experimentally to greatly affect the degree of nematic order and related quantities.
Unlike in liquid nematics, one can impose fields perpendicular to the director, thereby inducing biaxial order which should
be susceptible to experimental detection. Nematic elastomers with unchanging director and degree of order should theoretically
have the same elastic moduli for longitudinal and transverse extensions. This is violated when nematic order is permitted
to relax in response to strains. Near the transition we predict the longitudinal modulus to be smaller than the transverse
modulus; at lower temperatures the converse is true, with a crossover a few degrees below the transition. The differences
are ascribed to the different temperature dependence of the stiffness of uniaxial and biaxial order. We synthesised side chain
single-crystal nematic polymer networks, performed DSC, X-ray, birefringence, and thermo-mechanical characterisations, and
then obtained linear moduli from stress-strain measurements.
Received 29 September 2000 相似文献
10.
We develop a continuum theory of linear viscoelastic response in oriented monodomain nematic elastomers. The expression for
the dissipation function is analogous to the Leslie-Ericksen version of anisotropic nematic viscosity; we propose the relations
between the anisotropic rubber moduli and new viscous coefficients. A new dimensionless number is introduced, which describes
the relative magnitude of viscous and rubber-elastic torques. In an elastic medium with an independently mobile internal degree
of freedom, the nematic director with its own relaxation dynamics, the model shows a dramatic decrease in the dynamic modulus
in certain deformation geometries. The degree to which the storage modulus does not altogether drop to zero is shown to be
both dependent on frequency and to be proportional to the semi-softness, the non-ideality of a nematic network. We consider
the most interesting geometry for the implementation of the theory, calculating the dynamic response to an imposed simple
shear and making predictions for effective moduli and (exceptionally high) loss factors.
Received 16 October 2000 and Received in final form 10 December 2000 相似文献
11.
O. Chanal B. Chabaud B. Castaing B. Hébral 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(2):309-317
We analyse experimental velocity measurements on the axis of a low temperature gaseous helium jet. From independent increments
arguments, we reproduce the behaviour of structure functions. We show where this approach fails and how the intermittency
phenomenon is a small correction. The physical arguments under the multiplicative cascade models for this intermittency imply
an acceleration of this cascade close to the dissipative range, which we are able to evidence. This acceleration could be
responsible of the apparent Extended Self Similarity between structure functions of various orders.
Received 13 October 1999 and Received in final form 19 May 2000 相似文献
12.
The anchoring of nematic liquid crystals on surfaces with grafted liquid-crystalline chain molecules is studied by computer
simulations and within a mean-field approach. The computer simulations show that a swollen layer of collectively tilted chains
may induce untilted homeotropic (perpendicular) alignment in the nematic fluid. The results can be understood within a simple
theoretical model. The anchoring on a layer of mutually attractive chains is determined by the structure of the interface
between the layer of chain molecules and the solvent. It is controlled by an interplay between the attractive chain interactions,
the translational entropy of the solvent and its elasticity. A second-order anchoring transition driven by the grafting density
from tilted-to-homeotropic alignment is predicted.
Received 5 July 2001 and Received in final form 16 November 2001 相似文献
13.
V.J. Anderson E.M. Terentjev S.P. Meeker J. Crain W.C.K. Poon 《The European physical journal. E, Soft matter》2001,4(1):11-20
We study the phase ordering colloids suspended in a thermotropic nematic liquid crystal below the clearing point and the resulting aggregated structure. Small () PMMA particles are dispersed in a classical liquid crystal matrix, 5CB or MBBA. With the help of confocal microscopy we
show that small colloid particles densely aggregate on thin interfaces surrounding large volumes of clean nematic liquid,
thus forming an open cellular structure, with the characteristic size of inversely proportional to the colloid concentration. A simple theoretical model, based on the Landau mean-field treatment,
is developed to describe the continuous phase separation and the mechanism of cellular structure formation.
Received 13 March 2000 and Received in final form 6 June 2000 相似文献
14.
V.S.U. Fazio L. Komitov C. Radüge S.T. Lagerwall H. Motschmann 《The European physical journal. E, Soft matter》2001,5(3):309-315
The influence of capillary flow on the alignment of the nematic liquid crystal 5CB on fatty acid Langmuir-Blodgett monolayers
was studied by optical second-harmonic generation (SHG). The surface dipole sensitivity of the technique allows probing the
orientation of the first liquid crystal monolayer in the presence of the liquid crystal bulk. It was found that capillary
flow causes the first monolayer of liquid crystal molecules in contact with the fatty acid monolayer to be oriented in the
flow direction with a large pretilt (78 degrees), resulting in a quasi-planar alignment with splay-bend deformation of the
nematic director in the bulk. The large pretilt angle also suggests that the Langmuir-Blodgett film itself is affected by
the flow. The quasi-planar flow-induced alignment was found to be metastable. Once the flow ceases, circular domains of homeotropic
orientation nucleate in the sample and expand until the whole sample becomes homeotropic. This relaxation process from flow-induced
quasi-planar to surface-induced homeotropic alignment was also monitored by SHG. It was found that in the homeotropic state
the first nematic layer presents a pretilt of 38 degrees almost isotropically distributed in the plane of the cell, with a
slight preference for the direction of the previous flow.
Received 8 November 2000 and Received in final form 12 March 2001 相似文献
15.
C. Chevallard M. Clerc P. Coullet J.-M. Gilli 《The European physical journal. E, Soft matter》2000,1(2-3):179-188
We have experimentally observed the pattern instabilities of an Ising wall formed in a nematic or cholesteric liquid crystal
layer. We have deduced an envelope equation, relevant close to the Fréedericksz transition, from which we derived an equation
for the dynamics of the interface in the vicinity of its bifurcation. In the case of the zig-zag instability, this model is
characterized by a conservative and variational order parameter whose gradient satisfies a Cahn-Hilliard equation. We have
also investigated the influence of slightly broken symmetries on the dynamical behaviour of the system. The disappearance
of the interface translational invariance or of the reflection symmetry along the wall axis may induce new interfacial patterns
which have been both experimentally and theoretically pointed out.
Received 5 August 1999 and Received in final form 13 September 1999 相似文献
16.
We calculate the ratio of moduli for extension parallel and perpendicular to the director of nematic elastomers. Elastomers
in practice are not ideal nematics with a jump from a finite to zero-order parameter with increasing temperature. Some classes
behave as if under effective external fields. These are really internal in origin and leave the system subcritical (still
with jumps) or supercritical. We give expressions for general non-ideality and memory of formation conditions, along with
its translation into values of the “fields". For the case of supercritical fields, we find that the modulus ratio deviates
more from unity, in accord with experiment, than in the subcritical case.
Received 29 September 2000 相似文献
17.
Tasinkevych M Silvestre NM Patrício P Telo da Gama MM 《The European physical journal. E, Soft matter》2002,9(4):341-347
The interaction between two disks immersed in a 2D nematic is investigated i) analytically using the tensor order parameter
formalism for the nematic configuration around isolated disks and ii) numerically using finite-element methods with adaptive
meshing to minimize the corresponding Landau-de Gennes free energy. For strong homeotropic anchoring, each disk generates
a pair of defects with one-half topological charge responsible for the 2D quadrupolar interaction between the disks at large
distances. At short distance, the position of the defects may change, leading to unexpected complex interactions with the
quadrupolar repulsive interactions becoming attractive. This short-range attraction in all directions is still anisotropic.
As the distance between the disks decreases, their preferred relative orientation with respect to the far-field nematic director
changes from oblique to perpendicular.
Received 1 October 2002 and Received in final form 12 November 2002
RID="a"
ID="a"e-mail: miko@cii.fc.ul.pt 相似文献
18.
F.N. Braun H. Yokoyama 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(3):377-379
Capillary waves and director fluctuations reduce the surface tension of a non-anchoring unbound nematic surface by comparable
amounts. These are relatively small effects in three dimensions, but in two dimensions they become more significant. We examine
the conditions in two dimensions under which they dominate explicitly within the framework of a model of the Maier-Saupe type.
We find that for reasonable physical parameters of the model the onset of the fluctuation dominated regime generally preempts
the nematic-isotropic transition. We conclude that processes which are sensitive to line tension, such as Ostwald ripening
during two-dimensional liquid-gas phase separation, are much more strongly coupled to anisotropic molecular interactions and
associated nematic ordering than in three dimensions.
Received 10 February 1999 and Received in final form 29 March 1999 相似文献
19.
H. Gruler U. Dewald M. Eberhardt 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(1):187-192
Interacting cells have the ability to form liquid crystal phases: (i) A cluster of a polar nematic liquid crystal is formed
by cells which emit molecules for attracting other cells and (ii) an apolar nematic liquid crystal is formed by elongated
cells which have an anisotropic steric repulsion. The angle distribution function is predicted by using the characteristics
of an automatic controller where the extracellular guiding field is approximated by two-dimensional mean-field. The nematic
liquid crystal state is quite well described by the model.
Received 7 September 1998 and Received in final form 4 March 1999 相似文献
20.
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