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1.
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
2.
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
3.
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
4.
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
5.
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
6.
Cluzeau P Bonnand V Joly G Dolganov V Nguyen HT 《The European physical journal. E, Soft matter》2003,10(3):231-240
The behaviour of freely suspended smectic-C* ( SmC*) films at the bulk SmC*-cholesteric ( N*) phase transition has been investigated using polarized-reflected-light microscopy. Our experimental observations show that
above the bulk SmC*- N* phase transition the N* order appears in different ways according to the film thickness. In thin films, the conventional layer-by-layer thinning
occurs. In films of intermediate thickness N* inclusions nucleate inside the SmC* film. The distortions of the in-plane orientational order of the SmC* host phase induce elastic interactions between the inclusions and lead to their self-organization in chain-like structures.
Both the dynamic of the chaining and the parameters driving the equilibrium distance between the inclusions in the chain are
investigated. In thick films, N* fingers grow inside the film. The influence of the experimental conditions on the various processes is analysed.
Received 1 July 2002 / Published online: 15 April 2003
RID="a"
ID="a"e-mail: Philippe.cluzeau@univ-lille1.fr 相似文献
7.
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
8.
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
9.
The molecular dynamics (MD) simulation, based on a realistic atom-atom interaction potential, was performed on 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic phase. The rotational viscosity coefficients (RVCs) γ
i, (i = 1, 2) and the ratio of the RVCs λ = - γ
2/γ
1 were investigated. Furthermore, static and frequency-dependent dielectric constants and ε were calculated using parameters obtained from the MD simulation. Time correlation functions were computed and used to determine
the rotational diffusion coefficient, D
⊥. The RVCs and λ were evaluated using the existing statistical-mechanical approach (SMA), based on a rotational diffusion
model. The SMA rests on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian
motion in a certain potential of mean torque. According to the SMA, γ
i are dependent on the orientational order and rotational diffusion coefficients. The former was characterized using: i) orientational
distribution function (ODF), and ii) a set of order parameters, both derived from analyses of the MD trajectory. A reasonable
agreement between the calculated and experimental values of γ
i and λ was obtained.
Received 22 March 2000 and Received in final form 8 October 2000 相似文献
10.
William P. Acker Ali Serpengüzel Richard K. Chang Steven C. Hill 《Applied physics. B, Lasers and optics》1990,51(1):9-16
The strong stimulated Raman scattering (SRS) from diesel fuel droplets has the potential of providing the relative concentration of multicomponent fuel and the absolute size of individual droplets. The morphology-dependent resonances (MDRs) of a sphere cause the droplet to act as an optical resonator which greatly lowers the SRS threshold. The number density, quality factor, and frequency shift of several MDRs are calculated as a function of the ratio of the index of refraction of the liquid and the surrounding gas, which approaches unity at the thermodynamic critical condition for the fuel spray. The SRS spectra of monodispersed droplets of toluene, pentane, Exxon-Aromatic-150, and Mobil D-2 are presented. The exponential growth region of the SRS intensity I
1S as a function of the input laser intensity I
input is investigated for the toluene carbon ring breathing mode v
2 and the pentane C-H stretching region. The I
1S ratio of toluene and pentane is measured as a function of the ratio of the toluene and pentane concentration for monodispersed droplets. The reduced fluctuation in I
1S when I
input is changed from multimode to single-mode is displayed as a histogram of the I
1S of the v
2 mode of toluene droplets. 相似文献
11.
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
12.
P. Patrício M. Tasinkevych M.M. Telo da Gama 《The European physical journal. E, Soft matter》2002,7(2):117-122
We use a two-dimensional (2D) elastic free energy to calculate the effective interaction between two circular disks immersed
in smectic-C films. For strong homeotropic anchoring, the distortion of the director field caused by the disks generates topological
defects that induce an effective interaction between the disks. We use finite elements, with adaptive meshing, to minimize
the 2D elastic free energy. The method is shown to be accurate and efficient for inhomogeneities on the length scales set
by the disks and the defects, that differ by up to 3 orders of magnitude. We compute the effective interaction between two
disk-defect pairs in a simple (linear) configuration. For large disk separations, D, the elastic free energy scales as ∼D
-2, confirming the dipolar character of the long-range effective interaction. For small D the energy exhibits a pronounced minimum. The lowest energy corresponds to a symmetrical configuration of the disk-defect
pairs, with the inner defect at the mid-point between the disks. The disks are separated by a distance that is twice the distance
of the outer defect from the nearest disk. The latter is identical to the equilibrium distance of a defect nucleated by an
isolated disk.
Received 26 October 2001 and Received in final form 14 December 2001 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.
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
15.
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
16.
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
17.
The annihilation of the nematic hedgehog and anti-hedgehog within an infinite cylinder of radius R is studied. The semi-microscopic lattice-type model and Brownian molecular dynamics are used. We distinguish among the i)
early pre-collision, ii) late pre-collision, iii) early post-collision, and iv) late post-collision stages. In the pre-collision stage our results agree qualitatively with the existing experimental observations and also continuum-type simulations. The
core of each defect exhibits a ring-like structure, where the ring axis is set perpendicular to the cylinder symmetry axis. For ξ(0)d/(2R) > 1 the interaction between defects is negligible, where ξ(0)d describes the initial separation of defects. Consequently, the defects annihilate within the simulation time window for ξ(0)d/(2R) < 1. For close enough defects their separation scales as ξd
(tc - t)0.4±0.1, where tc stands for the collision time. In elastically anisotropic medium the hedgehog is faster than the anti-hedgehog. In the early pre-collision stage the defects can be treated as point-like particles, possessing inherent core structure, that interact via the nematic director field. In the late pre-collision stage the cores reflect the interaction between defects. After the collision a charge-less ring structure is first formed. In the early post-collision stage the ring adopts an essentially untwisted circular structure of the radius ξr. In the late post-collision stage we observe two qualitatively different scenarios. For μ = ξr/R < μc ∼ 0.25 the ring collapses leading to the escaped radial equilibrium structure. For μ > μc the chargeless ring triggers the nucleation growth into the planar polar structure with line defects. 相似文献
18.
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
19.
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
20.
We study stable “bookshelf” smectic-A structures within a very thin plane-parallel cell of thickness L in which the mismatch between surface preferred (d
s) and intrinsic (d0) smectic layer thicknesses occurs. The Landau-Ginzburg approach based on a complex smectic order parameter is used. For a
weak enough smectic positional anchoring strength W smectic layers adopt the modified bookshelf profile. In a thick enough cell with increasing W a lattice of edge dislocations is continuously formed at the confining surfaces and then depinned from them. The structure
with dislocations is formed when the condition d
0/(
d
0/d
s - 1) ∼ 2 is fulfilled, where is the positional surface anchoring extrapolation length. If the cell is thin enough the dislocations formed at opposite
cell plates annihilate and consequently the smectic layers adopt a locked bookshelf structure. This transition is discontinuous
and takes place when d
0/(L
d
0/d
s - 1) ∼ 5 is realized. To observe these transitions in a cell of thickness L∼ 1μm the conditions W∼ 10-6
J/m
2 and
d
0/d
s - 1∼ 5 . 10-4 have to be fulfilled. All the three qualitatively different structures coexist at the triple point.
Received 21 February 2002 相似文献