Nematic Phase Transition Research Articles

Cell size dependence of orientational order of uniaxial liquid crystals in flat slit

In order to investigate the ordered structure of nematic liquid crystal molecules confined in a nanoslit, we carried out a classical molecular dynamics simulation of uniaxial prolate Gay–Berne particles in a flat, structureless slit at several temperatures. When the slit gap is so small that the system is not assumed as the bulk, particles in the slit possess orientationally ordered structures different from ones in the bulk. The weak spacial orientational correlation existed when the temperature corresponded to the isotropic phase in the bulk system. The first order isotropic–nematic phase transition was not clearly observed and the transitional phenomenon of the creation and annihilation of the uniaxial domains were observed. These results revealed that the ordered structure depends on the number of particles, in other words, cell size, and that the system with 100,000 or more particles gives reasonable results of an infinitely wide slit. The number of particles is converted into up to 220 particles of the length of the base.

Calorimetric study of activated kinetics of the nematic and smectic phase transitions in an aligned nano-colloidal liquid crystal+aerosil gel

This study explores the calorimetric analysis of an aligned nano-colloidal aerosil dispersed octyl-cyanobiphenyl gel. This system was prepared by solvent dispersion method (SDM). Heating scans were performed at different heating rates from 20 to 1 K min−1 using DSC. Aligned samples follow Arrhenius behavior and showed a temperature shift in SmA-N and N-I transitions towards lower temperature. These samples show a decreased activated kinetics and an interesting relationship with their enthalpy. This behavior can be explained in terms of surface and molecular interaction between aerosil nano-particles and 8CB molecules and produced strain in the system.

Theory of the nodal nematic quantum phase transition in superconductors

We study the character of an Ising nematic quantum phase transition deep inside a $d$-wave superconducting state with nodal quasiparticles in a two-dimensional tetragonal crystal. We find that, within a $1∕N$ expansion, the transition is continuous. To leading order in $1∕N$, quantum fluctuations enhance the dispersion anisotropy of the nodal excitations and cause strong scattering, which critically broadens the quasiparticle (qp) peaks in the spectral function, except in a narrow wedge in momentum space near the Fermi surface where the qps remain sharp. We also consider the possible existence of a nematic glass phase in the presence of weak disorder. Some possible implications for cuprate physics are also discussed.

High-resolution x-ray study of nematic–smectic-Aand smectic-A–reentrant-nematic transitions in liquid-crystal–aerosil gels

We have studied the effects of quenched random disorder created by dispersed aerosil nanoparticle gels on the nematic to smectic- A (N- SmA ) and smectic- A to reentrant nematic ( SmA -RN) phase transitions of thermotropic liquid-crystal mixtures of hexyloxycyanobiphenyl (6OCB) and octyloxycyanobiphenyl (8OCB). These effects are probed using high-resolution synchrotron x-ray diffraction techniques. We find that the reentrant characteristics of the system are largely unchanged by the presence of the aerosil gel network. By comparing measurements of the smectic static structure amplitude for this 8OCB- 6OCB+aerosil system with those for butyloxybenzilidene-octylaniline (4O.8)+aerosil gels, we find that the short-range smectic order in the smectic- A phase is significantly weaker in the reentrant system. This result is consistent with the behavior seen in pure 8OCB-6OCB mixtures. The strength of the smectic ordering decreases progressively as the 6OCB concentration is increased. Detailed line shape analysis shows that the high- and low-temperature nematic phases (N and RN) are similar to each other.

Entropy-driven phase transition in a system of long rods on a square lattice

The isotropic–nematic (I–N) phase transition in a system of long straight rigid rods of lengthk on square lattices is studied by combining Monte Carlo simulations and theoretical analysis.The process is analyzed by comparing the configurational entropy of the system with thatcorresponding to a fully aligned system, whose calculation reduces to the one-dimensional(1D) case. The results obtained (1) allow us to estimate the minimum value ofk which leads to the formation of a nematic phase and provide an interesting interpretationof this critical value; (2) provide numerical evidence for the existence of a second phasetransition (from a nematic to a non-nematic state) occurring at a density close to1 and (3) allow us to test the predictions of the main theoretical models developed to treatthe polymer adsorption problem.

Synthesis and characterisation of main‐chain hydrogen‐bonded supramolecular liquid crystalline complexes formed by azo‐containing compounds

A series of azopyridine‐containing hydrogen bonding acceptors (4a–c) with flexible spacers of oligo(methylene) were synthesised. Hydrogen‐bonded polymeric complexes 4/5 and trimeric complexes 4/62 , where 5 and 6 are aromatic dicarboxylic acids and monocarboxylic acids, respectively, were prepared and their liquid crystallinity was examined using differential scanning calorimetry and polarising optical microscopy. The study showed that most of the complexes displayed reversible thermotropic nematic phase. The isotropic to nematic phase transition temperatures of polymeric complexes 4/5 and trimeric complexes 4/62 in general decreased with the increase in length of spacers and terminal groups in the corresponding proton acceptors 4 and the proton donors 5 and 6, respectively. Hydrogen bonding interactions in complexes 4/5 and 4/62 were studied by X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.

Characteristics of selective reflection of chiral nematic liquid crystalline gels with a nonuniform pitch distribution

The particles of photopolymerizable monomer/chiral dopant composites with a crystalline (Cr)-chiral nematic (N*) phase transition were prepared. By mixing the particles with different pitches of the N* phase together with the Cr phase and making the liquid crystal (LC) monomer molecules cross-linked by photopolymerization in the planarly oriented N* phase, a chiral nematic liquid crystal (N*-LC) gel film with a nonuniform pitch distribution was obtained. The bandwidth of the reflection spectrum and the location of reflection band of the gel film can be controlled accurately by controlling the pitch lengths of the N* phase of the particles.

Presmectic wetting and supercritical-like phase behavior of octylcyanobiphenyl liquid crystal confined to controlled-pore glass matrices

The influence of controlled-pore glass (CPG) confinement on the phase behavior of octylcyanobiphenyl liquid crystal (LC) is studied by means of x-ray scattering and high precision calorimetry. For CPG samples with pore diameter 2R>24 nm, the smectic order parameter temperature dependence eta(T) reveals apparent presmectic ordering far above the bulk smectic A-nematic (SmA-N) phase transition for both nontreated and silane-treated CPG matrices. The behavior of eta(T) is qualitatively similar in all samples, well obeying the mean field approach (MFA) in which the surface wetting tendency plays the dominant role. In contrast, the critical fluctuations remain important in the specific heat data, which cannot be described within the MFA. We show experimentally that randomness and surface wetting become dominant over finite-size effects for 2R approximately<10 nm, in agreement with theoretical analysis. In nontreated samples, the noncritical character of the static disorder and the interfacial LC-CPG coupling almost completely suppress the quasi-SmA-N and nematic-isotropic phase transitions at 2R approximately 15.1 and approximately 7.5 nm, respectively.

Glassiness of Thermotropic Liquid Crystals across the Isotropic−Nematic Transition

The orientational dynamics of thermotropic liquid crystals across the isotropic-nematic phase transition have traditionally been investigated at long times or low frequencies using frequency domain measurements. The situation has now changed significantly with the recent report of a series of interesting transient optical Kerr effect (OKE) experiments that probed orientational relaxation of a number of calamitic liquid crystals (which consist of rod-like molecules) directly in the time domain, over a wide time window ranging from subpicoseconds to tens of microseconds. The most intriguing revelation is that the decay of the OKE signal at short to intermediate times (from a few tens of picoseconds to several hundred nanoseconds) follows multiple temporal power laws. Another remarkable feature that has emerged from these OKE measurements is the similarity in the orientational relaxation behavior between the isotropic phase of calamitic liquid crystals near the isotropic-nematic transition and supercooled molecular liquids, notwithstanding their largely different macroscopic states. In this article, we present an overview of the understanding that has emerged from recent computational and theoretical studies of calamitic liquid crystals across the isotropic-nematic transition. Topics discussed include (a) single-particle as well as collective orientational dynamics at a short-to-intermediate time window, (b) heterogeneous dynamics in orientational degrees of freedom diagnosed by a non-Gaussian parameter, (c) fragility, and (d) temperature-dependent exploration of underlying energy landscapes as calamitic liquid crystals settle into increasingly ordered mesophases upon cooling from the high-temperature isotropic phase. A comparison of our results with those of supercooled molecular liquids reveals an array of analogous features in these two important classes of soft matter systems. We further find that the onset of growth of the orientational order in the parent nematic phase induces translational order, resulting in smectic-like layers in the potential energy minima of calamitic systems if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. We discuss implications of this startling observation. We also discuss recent results on the orientational dynamics of discotic liquid crystals that are found to be rather similar to those of calamitic liquid crystals.

NMR Study of Orientational Order in the 7OCB Nematic Liquid Crystal

Liquid crystals are anisotropic materials whose properties vary with the degree of alignment along a common direction known as a director. The nematic phase with the lowest ordering of all the liquid crystal phases is characterized by the state in which the molecules have no positional order, but tend to align along the director. Thus, the orientational order-parameter is an important quantity for determining the capability of a nematic liquid crystal because it is a measure of the tendency of molecules to point along the director. The nematic liquid crystal 4-heptyloxyl-4’cyanobipheny (7OCB) undergoes a first-order phase transition from an isotropic liquid (iso) phase to a nematic (N) phase at 347 K [1, 2]. The orientational ordering and the nematic phase transition of 7OCB were studied by several experiments [1–6]. In a measurement of the dielectric permittivity, Rzoska et al. determined the critical exponent β for the orientational order-parameter to be 0.25 from the temperature dependence of the dielectric strength, indicating a tricritical behavior of the nematic phase transition [1]. In addition, the other permittivity quantities were found to show changed characteristics for this behavior around the nematic phase transition. The aim of the present work is to examine such behavior of the nematic phase transition in 7OCB. Therefore, we analyzed the chemical shift of C NMR spectra in the N phase and determined the temperature dependence of the orientational order-parameter to examine the nematic phase transition. In this paper, obtained results were reported along with the procedure for the spectral analysis.

On a leveraging effect of weak azimuthal inhomogeneities explaining the nonuniqueness of critical Maier-Saupe order parameters

The Maier--Saupe theory of liquid crystals predicts a unique critical order parameter for the transition point of isotropic--nematic phase transitions. Experimental studies have shown that critical order parameters are shifted away from that unique value. We show that micro-defects and weak azimuthal inhomogeneities can result due to a leveraging effect in the experimentally observed macro-shifts of the critical order parameter. We demonstrate the leveraging effect by computing the corresponding micro-shifts of the order parameter in the isotropic phase with the help of an effective order parameter. Our approach accounts for critical order parameters observed in experimental studies which is explicitly demonstrated for methyl-deuterated para-azoxyanisole (PAA-d6), 4-n-pentyl-4′-cyanobiphenyl (5CB), N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Finally, in line with the effective order parameter approach and the dynamical mean field approaches by Hess, Doi, and Edwards, predictions are derived that provide novel testbeds for experimental studies.

Growth of tactoidal droplets during the first-order isotropic to nematic phase transition of F-actin

We report on the observed characteristics of the first-order phase transition of F-actin from the isotropic state to the nematic liquid-crystalline state. Solutions of short average filament length F-actin at appropriate concentrations phase separate to form tactoidal droplets. These tactoids are the result of the minimization of their free energy and show a bipolar director field connecting two opposite poles. The tactoids are shown to form through two distinct mechanisms: nucleation and growth and spinodal decomposition. Both mechanisms produce tactoids with final domain sizes that are of the same order of magnitude. Additionally, analysis of the system shows several features of metastability. The solution can exist in a variety of steady states near equilibrium and can be easily perturbed, settling in one prescribed by the path followed in phase space.

Molecular Dynamic Simulation of Side‐Chain Liquid Crystalline Elastomer Under Load

AbstractSummary: We present a molecular dynamic simulation of a side chain liquid crystalline elastomer (LCE) under load. The LCE is composed of a flexible tetrafunctional diamond like network with rod‐like mesogens attached to the network. As a precursor of the LC elastomer a flexible polymer network in a low molecular liquid‐crystal (LC) solvent was used. The phase behavior of the LCE under uniaxial stretching up to the deformations of λ = 1.5 and 2.0 at different densities was studied. As in the non‐stretched case upon density increase an isotropic to nematic phase transition occurs. However, in contrast to thermotropic side chain LC elastomers the stress induced shift transition is not observed. The stretching slightly increases the anisotropy of translational diffusion of mesogens in the nematic state. The stress‐strain dependence for LCE both in the isotropic and the nematic states is obtained. Elastic modulus increases at high values of order parameter.

Interactions and dynamics of mesogenic molecules in the vicinity of the isotropic to nematic phase transition

A role of the molecular structure and polarity in the dielectric behavior of nematogenic compounds in the vicinity of the isotropic to nematic phase transition is discussed. Temperature dependences of the static permittivity and its derivative, in relation to the basic thermodynamic quantities (internal energy, entropy and Helmholtz free energy), as well as an anomaly in the rotational diffusion of the mesogenic molecules and corresponding activation energy, are analyzed.

Nuclear magnetic resonance parameters of atomic xenon dissolved in Gay-Berne model liquid crystal

We present constant-pressure Monte Carlo simulations of nuclear magnetic resonance (NMR) spectral parameters, nuclear magnetic shielding relative to the free atom as well as nuclear quadrupole coupling, for atomic xenon dissolved in a model thermotropic liquid crystal. The solvent is described by Gay-Berne (GB) molecules with parametrization kappa=4.4, kappa{'}=20.0 , and mu=nu=1 . The reduced pressure of P{*}=2.0 is used. Previous simulations of a pure GB system with this parametrization have shown that upon lowering the temperature, the model exhibits isotropic, nematic, smectic- A , and smectic- B /molecular crystal phases. We introduce spherical xenon solutes and adjust the energy and length scales of the GB-Xe interaction to those of the GB-GB interaction. This is done through first principles quantum chemical calculations carried out for a dimer of model mesogens as well as the mesogen-xenon complex. We preparametrize quantum chemically the Xe nuclear shielding and quadrupole coupling tensors when interacting with the model mesogen, and use the parametrization in a pairwise additive fashion in the analysis of the simulation. We present the temperature evolution of {129/131}Xe shielding and 131Xe quadrupole coupling in the different phases of the GB model. From the simulations, separate isotropic and anisotropic contributions to the experimentally available total shielding can be obtained. At the experimentally relevant concentration, the presence of the xenon atoms does not significantly affect the phase behavior as compared to the pure GB model. The simulations reproduce many of the characteristic experimental features of Xe NMR in real thermotropic LCs: Discontinuity in the value or trends of the shielding and quadrupole coupling at the nematic-isotropic and smectic-A-nematic phase transitions, nonlinear shift evolution in the nematic phase reflecting the behavior of the orientational order parameter, and decreasing shift in the smectic-A phase. The last observation is due to the preference of the xenon solutes to occupy the interlayer space where the density of the medium is reduced as compared to the layers. There are systematic deviations, however, in the magnitude of the shielding and its discontinuities, as well as the distribution of the solutes in the translationally ordered smectic-A phase, between the simulation and experiment. These deficiencies are believed to result from the lack of flexibility of the GB model.

Prenematic Behavior of the Electric-Field-Induced Increment of the Basic Thermodynamic Quantities of Isotropic Mesogenic Liquids of Different Polarity

Temperature dependences of the static dielectric permittivity and its derivative, obtained for isotropic mesogenic liquids composed of the molecules of different polarity in relation to the basic thermodynamic quantities (internal energy, entropy, and Helmholtz free energy), are analyzed. A role of the molecular polarity in the dielectric behavior of the liquids in the vicinity of the isotropic to nematic phase transition is discussed.

Retraction Note: Nematic Ordering Pattern Formation in the Process of Self-Organization of Microtubules in a Gravitational Field.

[This retracts the article DOI: 10.1007/s10867-006-9032-x.].

Local quantum criticality at the nematic quantum phase transition

We discuss the finite temperature properties of the fermion correlation function near the fixed point theory of the nematic quantum critical point (QCP) of a metallic Fermi system. We show that though the fixed point theory is above its upper critical dimension, the equal time fermion correlation function takes on a universal scaling form in the vicinity of the QCP. We find that in the quantum critical regime, this equal-time correlation function has an ultra local behavior in space, while the low-frequency behavior of the equal-position auto correlation function is that of a Fermi liquid up to subdominant terms. This behavior should also apply to other quantum phase transitions of metallic Fermi systems.

Surface ordering and capillary phenomena of confined hard cut-sphere particles.

Isothermal-isobaric and Gibbs ensemble Monte Carlo (GEMC) computer simulations of = 1500 and = 3000 hard cut spheres of aspect ratio / = 0.1, respectively, are carried out in order to investigate the effects of confinement on the isotropic (I)-nematic (N) phase transition. We first consider the free system, and confirm the stabilisation of isotropic (I), nematic (N) and columnar (Col) states. We examine in detail the I-N transition and find coexistence densities of =0.355 and =0.368. A slab geometry is then considered for two types of walls: a hard wall, which excludes the particles entirely, and an 'adsorbent' wall which excludes the centre of mass of the particles. The adsorbent wall is found to favour planar (edge-on) alignment, which results in the formation of a first layer of adsorbed molecules, which then acts as a rough hard wall for subsequent particles, and promotes disordered states. Using Gibbs ensemble simulations we determine the capillary phase diagram of the system, and the adsorption as a function of pore width. The capillary phase diagram obtained from Gibbs ensemble simulations corresponds to one with a first-order capillary isotropisation transition, with an associated capillary critical point for a wall separation of ∼3. The hard walls are seen to promote homeotropic (face-on) alignment of the cut spheres, and promote the stabilisation of the nematic phase. In this case the capillary phase diagram obtained from the GEMC simulations exhibits a first-order capillary nematisation transition, and a capillary critical point for a wall separation of ∼4.

Effect of suspended clay particles on isotropic–nematic phase transition of liquid crystal

Sterically stabilised nano-platelets were prepared by treating montmorillonite clay with both a surfactant and a polymeric stabiliser. These nano-platelets formed stable suspensions in a thermotropic liquid crystal, 5CB. This is in marked contrast with previous work on preparing liquid crystal suspensions of either spheres, which formed gels on cooling the solvent into the nematic phase, or clay platelets stabilised only with low molecular weight surfactant, which tended to aggregate. In the isotropic state of the liquid crystal, static light scattering showed that the clay nano-platelets were freely suspended, and no aggregation was detected even after repeated temperature cycling into and out of the nematic phase. Small-angle X-ray scattering showed that the clay was delaminated nearly completely in the liquid crystal, with some stacks of a few clay nano-platelets having formed. Differential scanning calorimetry of the liquid crystal/clay suspensions showed a small but non-monotonic shift of the transition temperature compared to that of the pure liquid crystal. This behaviour is similar to that of liquid crystal confined in porous media, with an initial increase of the transition temperature on adding clay being ascribed to the effect of surface anchoring facilitating the formation of the nematic phase, whereas a decrease at higher clay concentrations (or equivalently, for smaller pores) is ascribed to confinement effects frustrating the formation of the nematic phase. This interpretation is supported by polarising light microscopy which showed the nematic domain size becoming smaller on increasing the clay concentration.