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1.
Dolganov PV Nguyen HT Joly G Dolganov VK Cluzeau P 《The European physical journal. E, Soft matter》2008,25(1):31-37
New mechanisms of droplet nucleation and self-organization in ferroelectric membranes are described. The droplets may be accompanied
by different number of topological defects (zero, one, two) whose location may be on the droplet boundary or in the membrane.
Nucleation and self-organization of droplets with total topological charge S = 0 , S = + 1 and S = - 1 were investigated. We found that an S = - 1 topological defect may be the center of both droplet nucleation and chain formation. This mechanism of chaining drastically
differs from the droplet self-organization described earlier which is realized by attraction of droplet-defect pairs. Our
observations demonstrate new possibilities for manipulating the inclusions and their self-organization in smectic membranes. 相似文献
2.
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
3.
The formation of regular colloid patterns in free-standing smectic films at the transition from the smectic-C to the isotropic
or nematic phase is well known experimentally. The self-organization of isotropic or nematic droplets is caused by their mutual
interaction, mediated by elastic distortions of the local director in the surrounding liquid crystal. These distortions are
related to the anchoring conditions of the director at the droplet border. We describe analytically the energetics of the
liquid crystal environment of a single droplet in one-constant approximation. A method of complex analysis, Conformal Mapping,
is employed. Following a suggestion of Dolganov et al. (Phys. Rev. E. 73, 041706 (2006)), energetics of chain and grid patterns built from the colloids are investigated numerically in order to explain
experimentally observed formations and their director fields. 相似文献
4.
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
5.
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
6.
Self-organization of cholesteric and nematic droplets in smectic free-standing films is investigated. Strong tangential anchoring
at the droplet boundary leads to nucleation of a topological defect and formation of a topological dipole from the droplet
and the defect. The interaction between droplets results in assembly of the droplets in dimers and line chains. Topological
dipoles of the droplet-defect pairs can be oriented in dimers with polar (ferroelectric) and nonpolar (antiferroelectric)
ordering. In this paper we found novel dimer structures formed by droplets with different handedness of the c-director field near the droplets. Depending on the relative magnitude of the droplet dipoles the resulting structure may
be dipolar or quadrupolar. Formation of antiferroelectric dimers is discussed on the basis of electromagnetic analogy. 相似文献
7.
J.C. Loudet O. Mondain-Monval P. Poulin 《The European physical journal. E, Soft matter》2002,7(3):205-208
We study experimentally the dynamics of a topological defect located around a colloidal particle suspended in a thermotropic
nematic liquid crystal. The considered defect consists of a disclination loop encircling the particle at the equator. Under
specific conditions, it is shown that this disclination continuously shrinks to a hedgehog defect located in the immediate
vicinity of the particle. This phenomenon corresponds to a transition between an elastic quadrupolar configuration and an
elastic dipolar configuration. We performed a basic numerical calculation to get an estimate of the dissipated energy during
the transition; we compare the results with theoretical predictions that describe the elastic energy of particles surrounded
by defects.
Received 21 December 2001 相似文献
8.
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
9.
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
10.
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
11.
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
12.
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
13.
The interaction between particle-like sources of the
nematic director distortions (e.g., colloids, point
defects, macromolecules in nematic emulsions) allows for a useful
analogy with the electrostatic multipole interaction between
charged bodies. In this paper we develop this analogy to the level
corresponding to the charge density and consider the general
status of the pairwise approach to the nematic emulsions with
finite-size colloids. It is shown that the elastic analog of the
surface electric charge density is represented by the two
transverse director components on the surface imposing the
director distortions. The elastic multipoles of a particle are
expressed as integrals over the charge density distribution on
this surface. Because of the difference between the scalar
electrostatics and vector nematostatics, the number of elastic
multipoles of each order is doubled compared to that in the
electrostatics: there are two elastic charges, two vectors of
dipole moments, two quadrupolar tensors, and so on. The
two-component elastic charge is expressed via the vector of
external mechanical torque applied on the particle. As a result,
the elastic Coulomb-like coupling between two particles is found
to be proportional to the scalar product of the two external
torques and does not directly depend on the particles' form and
anchoring. The real-space Green function method is used to develop
the pairwise approach to nematic emulsions and determine its form
and restrictions. The pairwise potentials are obtained in the
familiar form, but, in contrast to the electrostatics, they
describe the interaction between pairs (dyads) of the elastic
multipole moments. The multipole moments are shown to be uniquely
determined by the single-particle director field, unperturbed by
other particles. The pairwise approximation is applicable only in
the leading order in the small ratio particle
size-to-interparticle distance as the next order contains
irreducible three-body terms. 相似文献
14.
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
15.
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
16.
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
17.
Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions
in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure
dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T*AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the
quadrupolar nematic ordering Qij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity
of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with
pressure. 相似文献
18.
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
19.
The relaxation of director fields in freely suspended smectic films is studied experimentally by means of polarizing microscopy,
and analyzed by solving the torque balance equation under appropriate initial and boundary conditions. We consider in particular
the role of anchoring conditions of the c-director at particles and defects in the film. The structure of regular relaxation
patterns allows to determine the elastic anisotropy of smectic materials. The splay elastic constant can exceed the bend constant
by a factor of two and more. A remarkable consequence of this anisotropy is the stick-slip-like relaxation around a central
defect of topological strength s = + 1. 相似文献
20.
The effect of silica aerosils on the kinetics of the first-order nematic-isotropic (NI) phase transition is phenomenologically described in the framework of the time-dependent Landau-Ginzburg equation. A steady-state solution to the equation is presented such that the NI interface may propagate with a solitary-like wave profile under constant quenching. The results provide a plausible basis for the interpretation of the dynamical effects of quenched disorder in the liquid-crystal systems, caused by randomly interconnected porous media, such as aerosils. In the low silica aerosil ρs ( ≤0.1 g/cm^3) regime, the calculated values of the interface velocity v(T,ρs), the interface thickness κ(T,ρs), and the critical radius of a spherical nucleus of new nematic phase in a bulk isotropic environment, composed of polar molecules, such as 4-n-octyl- 4′- cyanobiphenyl and 4-n-heptyl- 4′- cyanobiphenyl shows that the effect of silica aerosils on the kinetics is reflected in a shifting of the set of temperature-dependent curves to lower temperature values.-1 相似文献