Deuteron NMR relaxometry applied to confined liquid crystals

We discuss the capability of deuteron nuclear magnetic resonance (NMR) spectroscopy and relaxometry to reveal molecular ordering and dynamics in confined liquid crystals. The attention is focused on the high-temperature phase above the nematic-isotropic transition, which is — in the absence of the long-range orientational order — very suitable for the study of surface interactions. Deuteron NMR spectra and relaxation rates are presented for two representatives of confined liquidcrystal systems: 8CB in cylindrical cavities of Anopore membranes and 5CB with an embedded polymer network. A substantial increase in the transverse spin relaxation rate, stimulated by the surface-induced order in enclosures, has been observed. In cylindrical cavities, it exhibits a strong temperature dependence on approaching the phase transition, whereas in the polymer network dispersion it is temperature-independent. The increase of T ₂ ^?1 provides information on the effect of spatial constraints on molecular mobility and on the surface orientational order parameter. Using deuteron relaxometry, one can measure the degree of orientational order in the isotropic phase not only in cylindrical but also in spherical cavities and enclosures of irregular shape, where the standard approach based on quadrupolar splitting of the NMR spectrum fails.     

Insights on some chiral smectic phases

Combining layered positional order as smectic order and chirality can generate complex architectures since twist parallel to the layers is not allowed. This paper will review some new experimental results on different phases resulting from the competition between smectic positional order and twist orientational order. It concerns the TGB_A and the NL^*, that is the liquid line phase as well as the SmQ phase. Chiral effects in the isotropic phase will also be discussed.     

Free energy of semiflexible polymers and structure of interfaces

The free energy of semiflexible polymers is calculated as a functional of the compositional scalar order parameter and the orientational order parameter of second-rank tensor S_ij on the basis of a microscopic model of wormlike chains with variable segment lengths. We use a density functional theory and a gradient expansion to evaluate the entropic part of the free energy, which is given in a power series of .The interaction term of the free energy is derived with a random phase approximation. For the rigid rod limit, the nematic-isotropic transition point is given by , N and w being the degree of polymerization and the anisotropic interaction parameter, respectively, and the degree of ordering at the transition point is 0.33448. We also find that the contour length of polymer chains becomes larger in a nematic phase than in an isotropic phase. Interface profiles are obtained numerically for some typical cases. In the neighborhood of isotropic-isotropic interfaces, polymer chains tend to align parallel to the interface on the polymer-rich side and perpendicular on the poor side. When an isotropic region and a nematic region coexist, orientational order parallel to the interface is preferred in the nematic region. Received: 28 May 1998 / Revised: 12 August 1998 / Accepted: 8 September 1998     

On long-range order in low-dimensional lattice-gas models of nematic liquid crystals

The problem of the orientational ordering transition for lattice-gas models of liquid crystals is discussed in the low-dimensional case d = 1, 2. For isotropic short-range interactions, orientational long-range order at finite temperature is excluded for any packing of molecules on the lattice Z^d; on the other hand, for reflection-positive long-range isotropic interactions, we prove the existence of an orientational ordering transition for high packing (μ > μ₀) and low temperatures (β > β_c(μ)).     

Dynamic light scattering study of biaxial ordering in a thermotropic liquid crystal

Dynamic light scattering from orientational order fluctuations in a liquid crystalline tetrapode reveals successive, weakly first-order isotropic to uniaxial and uniaxial to biaxial nematic phase transitions. The order parameter relaxation rates exhibit temperature dependences consistent with Landau-de Gennes mean field theory. Combined with previous evidence of a second-order uniaxial-biaxial transition in a closely related tetrapode, the present study supports the existence of a nematic-nematic tricritical point in thermotropic liquid crystals.     

Fluctuations of orientational order in a uniaxial nematic liquid crystal with biaxial molecules and its response to an external field

Homogeneous thermal fluctuations of the orientational order parameters S and G of biaxial molecules in a uniaxial nematic liquid crystal are investigated in the framework of the molecular-statistical theory. It is demonstrated that the molecular biaxiality significantly affects the order parameters S and G, their temperature dependences in the nematic phase, the amplitude and the temperature dependence of the order parameter fluctuations in the nematic and isotropic phases, and the character of the transition from the nematic phase to the isotropic liquid phase. It is established that the fluctuations of the parameters S and G in the nematic phase are related to the temperature dependences of S and G and the susceptibilities χ_S and χ_G of the nematic liquid crystal to external fields, which leads to a change in the parameters S and G at a fixed director orientation. Explanations are offered for the known experimental data on the orientational ordering of biaxial molecules under the action of external fields in the isotropic phase of nematic liquid crystals.     

受限导致硬椭球液体的取向有序

Monte Carlo模拟研究了两平行硬墙受限下各向异相的硬椭球液体形成的有序结构. 墙壁对椭球粒子的过量吸附和椭球的长短径之比有关;对于高密度的椭球液体,它在一个临界的长短径之比 a/b=2.9处达到最大值,表明在墙表面附近发生了取向有序. 墙表面附近的密度分布和取向序参量进一步证实这结论. 相同密度条件下体相中的取向序参量计算表明体相结构均为各向同性,这是由于体系的密度依然低于各向同性相向相列向转变的临界密度. 该受限椭球液体中墙表面的取向有序是由墙面的几何受限所导致的.     

Spectral and orientational properties of new group of naphthalene derivatives of bicarboxylic acid in liquid crystal 7CB

The spectral and orientational properties of some dichroic fluorescent dyes (naphthalene derivatives of bicarboxylic acid) in some isotropic solvents and in an anisotropic matrix have been studied. The order parameter of the dyes in the nematogenic liquid crystal 7CB has been determined from measurements of polarized electronic absorption and fluorescence. The influence of the dye addition on the nematic-isotropic phase transition temperature of 7CB has been investigated. It was found that some of the dyes studied could be successfully utilized in guest-host liquid crystal display devices.     

Anisotropic defect-mediated melting of two-dimensional colloidal crystals

The melting transition of anisotropic two-dimensional (2D) crystals is studied in a model system of superparamagnetic colloids. The anisotropy of the induced dipole-dipole interaction is varied by tilting the external magnetic field off the normal to the particle plane. By analyzing the time-dependent Lindemann parameter as well as translational and orientational order we observe a 2D smecticlike phase. The Kosterlitz-Thouless-Halperin-Nelson-Young scenario of isotropic melting is modified: dislocation pairs and dislocations appear with different probabilities depending on their orientation with respect to the in-plane field.     

Local Disorder in Ln2Ti2O7 (Ln = Gd,Tb, Dy) Pyrochlores

JETP Letters - The orientational dynamics in the isotropic phase of a comb-like nematic polymer with mesogenic and functional side groups is studied using the Kerr effect and dielectric...     

Optical Kerr effect in nematic liquid crystals: a molecular dynamics simulation study

Molecular dynamics simulations of the Gay-Berne model have been undertaken and optical response properties calculated for the isotropic and nematic phases. The components of the optical response were calculated for the coordinate system with respect to the director. Furthermore, the response for the components was separated into the orientational and collision induced contributions in order to analyse the mechanism of the optical response. It was observed, in particular, that one of the depolarized components of the response function does not vanish after long times for the nematic phase, unlike in the isotropic phase. This means that the orientation of the director can be permanently changed by instant irradiation with polarized light. The results give a microscopic picture of the optical Kerr effect in nematic liquid crystals.     

Surface freezing and a two-step pathway of the isotropic-smectic phase transition in colloidal rods

We study the kinetics of the isotropic-smectic phase transition in a colloidal rod/polymer mixture by visualizing individual smectic layers. First, we show that the bulk isotropic-smectic phase transition is preceded by a surface freezing transition in which a quasi-two-dimensional smectic phase wets the isotropic-nematic interface. Next, we identify a two-step kinetic pathway for the formation of a bulk smectic phase. In the first step a metastable isotropic-nematic interface is formed. This interface is wetted by the surface-induced smectic phase. In the subsequent step, smectic layers nucleate at this surface phase and grow into the isotropic bulk phase.     

Photo-orientation at liquid crystal—polymer interfaces

It is suggested that liquid crystal—polymer interfaces are coupled systems, in which the components mutually influence the orientational state of each other. The photo-orientation process at liquid crystal-polymer interfaces provides a striking example of such a coupling. Experiments show that the anisotropic structure generated by polarised light at a polymer surface is strongly affected by the phase of the liquid crystal covering the polymer. Photo-orientation is significantly more efficient when the liquid crystal is in the isotropic phase than when it exhibits orientational order. The observations are interpreted by assuming that in the smectic and nematic phases the liquid crystal stabilises to a large extent polymer chain-segments aligned parallel to the director, while it blocks the photo-induced formation of chain-segments in the perpendicular direction. Other situations, in which the coupling between the liquid crystal and the polymer can be important, are also discussed briefly.     

Orientation-dependent depletion interaction in rodlike colloid-polymer mixtures

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     

Determination of the Maier-Saupe strength parameter from dielectric relaxation experiments: a molecular dynamics simulation study

Molecular dynamics simulations have been performed to investigate the rotational motion in the nematic and isotropic phases of a model mesogenic system in which the interactions between the molecules are represented by the Gay-Berne potential. First-rank end-over-end rotational relaxation times, analogous to those measured using dielectric relaxation spectroscopy for real mesogens with a longitudinal electric dipole, have been determined as a function of temperature and density. The relaxation times at temperatures throughout the nematic region are found to be larger than the values extrapolated from the isotropic phase to the same temperature. The simulation results are compared with the extended Debye theory for dielectric relaxation in the nematic phase. This relates the reduction in the relaxation rate to the retardation factor which depends on the Maier-Saupe strength parameter, and in turn is defined uniquely by the second-rank orientational order parameter. The simulations indicate that the retardation factor at constant strength parameter is density dependent, a feature neglected in the relaxation theory. We compare the simulation results where possible with experiment.     

Spectral features of a nematic liquid crystal consisting of biaxial molecules with internal rotation

Expressions for the components of the optical permittivity tensor of a nematic liquid crystal consisting of biaxial π-conjugated molecules with a conformational degree of freedom (internal rotation) are obtained. A relationship between the intensity and dichroism of absorption bands in isotropic and nematic phases and the parameters of conformational, orientational, and mixed conformational and orientational orders of the molecules is established. A distribution function for the molecules is obtained that takes into account the mutual correlation of their conformational and orientational degrees of freedom. The effect of this correlation on the above-noted order parameters, the dichroism of the absorption bands, and the dependence of the measured oscillator strengths of molecular transitions on the character and degree of orientational order of biaxial molecules is studied. On the basis of comparison with experimental data, the relative role of different terms in the distribution function that are responsible for the correlation between the conformational and orientational degrees of freedom of the molecules is ascertained.     

Hard dumbells: Monte Carlo pressures and virial coefficients

The equation of state was determined by Monte Carlo simulation for 108 dumb-bells subject to periodic boundary conditions. The isotropic fluid-solid phase transition was observed. An orientational phase transition was not observed. Virial coefficients through B₅ were calculated.     

Morphology and structure of thin liquid-crystalline films at nematic-isotropic transition

We review the main features of very thin nematic liquid-crystalline films on solid substrates, focusing on 5CB on oxidized silicon wafers. By discussing the theoretical aspects of the observed structures, we show that the phenomena at work include isotropic capillary condensation and that the coexistence of isotropic and nematic terraces in thin films is a result of the interplay of several mechanisms. Further theoretical as well as experimental efforts are needed to completely understand the wetting behavior of these systems.Received: 1 August 2003PACS: 68.08.Bc Wetting - 68.15. + e Liquid thin films - 61.30.Hn Surface phenomena in liquid crystals including anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions     

A dissipative particle dynamics study of the realignment of a nanodroplet of a nematic in a weak external magnetic field

We present a dissipative particle dynamics (DPD) approach for simulating the realignment of a nematic nanodroplet suspended in an isotropic fluid following a switch in the direction of an applied external magnetic field. The interaction of the mesogens with the external field is weak relative to the inter-molecular interactions. The simulations were used to investigate the way orientational equilibrium is re-established. The results reveal that the realignment process of the nanodroplet is consistent with its fluid structure. The reorientation of the nanodroplet as a whole is found to be caused by an internal structural rearrangement rather than a coherent rotation of the centres of mass of the mesogens about the centre of the nanodroplet. The switch in the field direction furthermore is found to induce a transient spatial variation in the orientational order of the long axes of the mesogens: the orientational order parameters decreases on moving from the core of the nanodroplet to the surface in contact with the isotropic environment. The results highlight differences between the time evolution of the orientation of the long molecular axes in the field and the rotations of the centres of mass of the mesogens about the centre of the nanodroplet.