共查询到20条相似文献,搜索用时 31 毫秒
1.
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
2.
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
3.
D. A. Petrov A. N. Zakhlevnykh A. V. Mantsurov 《Journal of Experimental and Theoretical Physics》2018,127(2):357-369
A statistical theory is proposed to describe a suspension of carbon nanotubes in a nematic liquid crystal. The mean-field approach is used, and dispersion attraction, the excluded volume effects, the diamagnetism of liquid crystal molecules, and the strong diamagnetism of nanotubes are taken into account. The influence of the volume fraction of impurity, temperature, and magnetic field on the orientational ordering of a liquid crystal matrix and carbon nanotubes is studied. The concentration and temperature phase transitions in the suspension are investigated for various magnetic fields. The concentration and field shifts of the point of the phase transition between nematic and isotropic or paranematic phases are studied. 相似文献
4.
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
5.
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
6.
Depletion interaction in a suspension of rodlike colloids with added non-adsorbing polymer coils is theoretically studied.
We calculate an overlap volume of depletion zone between two rodlike colloids, based on the second virial approximation. We
examine nematic-isotropic phase transition (NIT) and two-phase coexistence between an isotropic and a nematic phase at low
polymer concentrations. We find that the depletion interaction is dependent on the orientational order parameter of rodlike
colloids and leads to a decrease in the NIT concentration on the addition of polymer. The coexistence curves have a leaning-chimney
shape and are shifted to lower rod concentrations on increasing the polymer concentration.
Received 23 May 2001 and Received in final form 18 July 2001 相似文献
7.
We study theoretically the phase behavior of the continuum Random Anisotropy Nematic model. A domain-type pattern is assumed to appear in a distorted nematic liquid crystal (LC) phase. We map the model parameters to physical quantities characterizing LCs confined to Controlled-Pore Glasses and LC-aerosil dispersions. The domain size dependence on the disorder strength is obtained in accordance with the Imry-Ma prediction. The model estimates for temperature shifts of the paranematic-nematic phase transition and for the critical point, where this transition ceases to exist, are compared to the available experimental results.Received: 28 March 2004, Published online: 29 June 2004PACS:
61.30.-v Liquid crystals - 61.30.Dk Continuum models and theories of liquid crystal structure - 61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions 相似文献
8.
E. Vahedi H. Tajalli A. Ghanadzadeh Gilani J. Parka H. Nasirzadeh Milani 《Phase Transitions》2013,86(6):432-439
We have experimentally studied for the first time the electro-optical Kerr effect and the pre-transitional behavior in the isotropic phase of two antiferroelectric liquid crystal mixtures, W-232 and W-204D, composed of rod-like ester molecules exhibiting the direct smectic-A to isotropic (SmA-I) phase transition. The Kerr law has been confirmed for the two compounds and the variation of inverse Kerr constant with temperature above the smectic–isotropic transition temperatures were determined. Both the mixtures with very broad antiferroelectric phase around room temperature have similar sequence of the phases (i.e., Cr-SmC*A-SmC*-SmA-I). Although, the pre-transitional behavior is usually complex in the isotropic phase of the chiral smectic liquid crystal compounds, the investigated compounds showed a similar behavior compared to that of nematic–isotropic behavior. 相似文献
9.
We investigate theoretically some phase transitions in liquid crystals in the presence of a laser beam. We found, in non-absorbing nematics, a laser-induced one-way transition from a paranematic to a nematic phase. In absorbing nematics we found, in addition to this transition, a one-way transition from a nematic to a paranematic phase with increasing laser intensity. Further, we found a reentrant nematic or a reentrant paranematic via paranematic or nematic phase respectively. In the case of smectic A, laser absorption results in a coupling between the positional and orientational orders. As a result, the smectic A to nematic transition can change from second order to first order and the smectic C to smectic A transition can become first-order in the field of a laser. 相似文献
10.
We model a melt of monodisperse side-chain liquid-crystalline polymers as a melt of comb copolymers in which the side groups
are rod-coil diblock copolymers. We consider both excluded-volume and Maier-Saupe interactions. The first acts among any pair
of segments while the latter acts only between rods. Using a free-energy functional calculated from this microscopic model,
we study the spinodal stability of the isotropic phase against density and orientational fluctuations. The phase diagram obtained
in this way predicts nematic and smectic instabilities as well as the existence of microphases or phases with modulated wave
vector but without nematic ordering. Such microphases are the result of the competition between the incompatibility among
the blocks and the connectivity constraints imposed by the spacer and the backbone. Also the effects of the polymerization
degree and structural conformation of the monomeric units on the phase behavior of the side-chain liquid-crystalline polymers
are studied. 相似文献
11.
Bellini T Caggioni M Clark NA Mantegazza F Maritan A Pelizzola A 《Physical review letters》2003,91(8):085704
Water-in-oil microemulsions of nanodroplets in the isotropic phase of a thermotropic liquid crystal exhibit, with decreasing temperature and in anticipation of a demixing transition, enhanced correlation in fluctuations of both molecular orientation and droplet concentration. Mean field modeling of this pretransition behavior, on the basis of a lattice in which the nanodroplets are introduced as holes, shows that the observed interdroplet attractive interaction is produced by the disordering effect of the droplets on the liquid crystal and mediated solely by paranematic fluctuations. 相似文献
12.
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
13.
Using density-functional theory, we have analyzed the phase behavior of a model liquid crystal confined between two parallel, planar surfaces (i.e., the so-called slit pore). As a result of confinement, a rich phase behavior arises. The complete liquid-crystal phase diagram of the confined fluid is mapped out as a function of wall separation and chemical potential. Strong commensuration effects in the film with respect to wall separation lead to enhanced smectic ordering, which gives capillary smectization (i.e., formation of a smectic phase in the pore), or frustrated smectic ordering, which suppresses capillary smectization. These effects also produce layering transitions. Our nonlocal density-functional-based analysis provides a unified picture of all the above phenomena. 相似文献
14.
The optical birefringence of rodlike nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 microm length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels. 相似文献
15.
Lapeña AM Glotzer SC Langer SA Liu AJ 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,60(1):R29-R32
Partially phase-separated liquid-crystal/polymer dispersions display highly fibrillar domain morphologies that are dramatically different from the typical structures found in isotropic mixtures. To explain this, we numerically explore the coupling between phase ordering and phase-separation kinetics in model two-dimensional fluid mixtures phase separating into a nematic phase, rich in liquid crystal, coexisting with an isotropic phase, rich in polymer. We find that phase ordering can lead to fibrillar networks of the minority polymer-rich phase. 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.
V. A. Ryzhov A. V. Lazuta V. P. Khavronin I. I. Larionov I. O. Troaynchuk D. D. Khalyavin 《Solid State Communications》2004,130(12):803-808
The data on the resistance and magnetoresistance (MR) as well as measurements of the linear and nonlinear susceptibilities are presented for a Nd0.75Ba0.25MnO3 single crystal with the Curie temperature TC≈129 K. Although this compound remains insulating in the ferromagnetic state, its resistance has an anomaly near TC and it reveals the colossal magnetoresistance. The data on the magnetic response are well described by the dynamic scaling theory for 3D isotropic ferromagnets in the paramagnetic critical region at τ>τ*≈0.11, τ=(T−TC)/TC. Below τ* an anomalous critical behavior is found that suggests the coexistence of two magnetic phases. This behavior is discussed in terms of a phase separation which can occur in the moderately doped manganites exhibiting an orbital ordering. 相似文献
18.
19.
E. R. Soulé A. D. Rey 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,83(3):357-367
We present theory and simulation of simultaneous chemical demixing and phase ordering in
a polymer-liquid crystal mixture in conditions where isotropic-isotropic phase separation
is metastable with respect to isotropic-nematic phase transition. In the case the
mechanism is nucleation and growth, it is found that mesophase growth proceeds by a
transient metastable phase that surround the ordered phase, and whose lifetime is a
function of the ratio of diffusional to orientational mobilities. In the case of spinodal
decomposition, different dynamic regimes are observed depending on the mobility ratio:
metastable phase separation preceding phase ordering, phase ordering preceding phase
separation, or simultaneous phase ordering and phase separation. Not only the overall
dynamics but also the final structure of the material can be different for each kinetic
regime. 相似文献
20.
We use a phenomenological model to find theoretically the non-linear dielectric effect (NDE) in the isotropic phase above the isotropic to chiral smectic-C (I-SmC*) phase transition. We find an analytical expression for the NDE in the isotropic phase above the I-SmC* phase transition. The temperature dependence of the NDE is presented in the isotropic phase of the I-SmC* phase transition. 相似文献