Light scattering by some nematic liquid crystals due to phase transitions

New computer modelling of light scattering and its propagation through liquid crystal has been presented using T-matrix method in the structural phase transition regions. Numerical aspects of light scattering process, which are based on numerically solving Maxwell's equations, were calculated for some nematic liquid crystals. Firstly, we described in detail T-matrix method for computing light scattering from nematic liquid crystals and presented results of benchmark computations for the considered model. We reported results of extensive calculations for polydisperse, randomly oriented rod-like multilayered systems (nematic liquid crystals). Our results are associated with light scattering by ferroelectric and ferroelastic materials.     

Ordering and phase transitions in liquid crystals

Structures and properties of different types of liquid crystals are discussed from a uniform point of view. Apart from the traditional mesophases (nematics, cholesterics, smectics), some new examples, including polymeric, metalloorganic and ferromagnetic liquid crystals, are also investigated. Systems with several macroscopic scales on which the type of ordering may differ, are described and analysed. Phase transitions and certain types of critical behaviour in different liquid crystals are studied.     

The structure investigations of fog phase (BPIII) of liquid crystals

Summary The current understanding of the problems concerning the fog phase structure organization based upon experimental results of our group is presented. The correspondence between theoretical models and experimental results is discussed. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Reentrant phase transitions in liquid crystals

A comprehensive review of the recent developments regarding the phenomenon of reentrant phase transitions (RPT) in liquid crystals is presented. In addition to liquid crystals this phenomenon has been observed in amazingly diverse systems. A critical assessment of the experimental investigations concerning single and multiple reentrances is given. A brief account of the theoretical efforts is also given. The article ends with the identification of the factors which impede the proper understanding of the phenomenon.     

Rotational viscosity coefficients of nematic and smecticC liquid crystals: Statistical theory

Summary The rotational diffusion of a rodlike molecule in a nematic and smecticC liquid crystal is considered in the molecular-field approximation. The microscopic friction constant, which determines the molecular rotation drag, possesses an exponential temperature dependence with the activation energy determined by the isotropic part of the intermolecular interaction energy. The rotational viscous coefficients,γ ₁ andγ _⊥ are obtained by averaging of the corresponding microscopic stress tensor with the nonequilibrium distribution function. The additional activation energy, proportional to the corresponding order parameter, appears in the expressions for the rotational viscosity coefficients both in nematics andC smectics. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Optical nonlinearities in lyotropic liquid crystals

Summary The existence of an optical nonlinearity of the Kerr-like type in a lyotropic liquid crystal is described in c.w. operation. The nonlinearity is found using the liquid crystal as a cladding material in a waveguide, or studying the interface properties between the liquid crystal and an ordinary glass. The used material is potassium caprillate and the nonlinearity is ascribed to thermal effects. Work performed with partial support from MPI-40% (Italian Education Ministry) and GNEQP-CNR (Italian National Research Council-Quantum Electronics Group).     

Birefringence and phase transitions in liquid crystals

The birefringence of liquid-crystalline phases is the result of the parallel order of molecules exhibiting a polarizability anisotropy. The magnitude and sign of the birefringence are determined by the structure and order of the liquid-crystalline phase types as well as by the polarizability properties of the constituent molecules. The characteristic change of the birefringence at phase transitions between liquid-crystalline phases indicates more or less pronounced structural changes. The temperature dependence of the birefringence is due to the temperature change of the molecular order.

It is shown that the structural variety of the liquid crystalline state is reflected by a big variety of their optical anisotropy properties.     

Derivative-dependent surface-energy terms in nematic liquid crystals

Summary The boundary energy terms dependent on the first derivative of the director are analysed theoretically. Although these terms necessarily arise from nonlocal interactions, they are generally written as purely surface terms. We show that, in this case, the stable director configurations are described by functions with discontinuous derivatives at the boundaries. All the solutions given in literature, which are found by explicitly assuming the continuity of the derivatives, must, therefore, be revised. In a more correct formulation of the problem, the ranges of the interaction forces should be taken into account and continuous solutions are then obtained, which are generally well approximated by the discontinuous ones. The approximation, instead, is not good in the case of the energy term proposed by Dubois-Violette and Parodi, where the discontinuity has the form of a Dirac function. In this case a physically meaningful solution can be found only on the basis of a more suitable expression of the boundary energy. Work supported by CNR under contract No. 84.01186.11.     

Surface effects and defect structures in ferroelectric liquid crystals

Summary The influence of boundaries on the smectic structure dielectric properties and polarization reversal processes in thin films of ferroelectric liquid crystals are discussed. Work presented at the First USSR-Italy Bilateral Meeting on Liquid Crystals held in Portonovo, Ancona (Italy), September 30-October 2, 1987.     

One- and two-dimensional deformations in nematic liquid crystals: Influence of the flexoelectric polarization

Summary The elastic deformations in nematic liquid crystals are analysed in one and two dimensions. The influence of the flexoelectric polarization on the elastic properties of the medium is studied. It is shown that for one-dimensional deformations, the presence of the flexoelectric polarization introduces a renormalization of the elastic constants. On the contrary in the bidimensional case, the effect of the flexoelectricity is not a simple renormalization of the elastic constants. The profiles of the electric potential and of the tilt angle are evaluated for the cases in which the substrate is an insulating or conducting material. The validity of the calculations is discussed by considering the value of Debye's screening length of the nematic liquid crystal.     

Aggregates of amphiphilic molecules in lyotropic liquid crystals

Summary Aamphiphilic molecules are made of two parts with different chemical affinities. They build interfaces which limit the contact between these two parts. We shall focus our attention on the case of soap molecules in the presence of water, in which the interfaces separate the paraffinic chains from the water and delimit aggregates of various shapes. Classically the aggregates can be infinite, with flat or cylindrical interfacial curvatures, and packed with long-range translational order in the well-known lamellar, cubic and hexagonal phases; or they can be finite, quasi-spherical and packed without any long-range order in the micellar phase. However, recent investigations show that a long-range ordering of finite aggregates is possible. There is one example of translational order, in which the micelles build a cubic structure, and much more examples now of orientational order. In the latter case the micelles, oblate or prolate spheroids, are dispersed in the solution with their axes nearly parallel to each other, a nematic phase is formed. We shall discuss the possible factors controlling the shapes of the aggregates and their interactions. Laboratoire associé au CNRS (LA No. 2).     

Theory for cholesteric ordering in lyotropic liquid crystals

Summary This paper presents an investigation of the chiral dispersion interaction between macromolecules in the solvent which is considered as a dielectric medium. In particular the results of the molecular statistical theory are reported. The expression for the helical pitch as a function of temperature on the solvent dielectric constant is used to interpret the experimental data. Moreover, the effect of the solvent chirality on the cholesteric ordering in polymer liquid crystals is considered. Finally the cholesteric ordering in the solution of chiral micelles is discussed. Work presented at the First USSR-Italy Bilateral Meeting on Liquid Crystals held in Portonovo, Ancona (Italy), September 30–October 2, 1987.     

Diffusion of testing particles in turbulent nematic liquid crystals

Summary We report on the diffusive motion of dielectric testing particles floating in a turbulent nematic fluid under the effect of an external oscillating electric field. The particle is viewed as a tracer of the underlying turbulence. We calculate the diffusion coefficients and the probability distribution of the particle displacements, thus showing that the motion of the particle is a classical Brownian diffusion. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Structure,elasticity and phase transitions in liquid crystals with deformations

ABSTRACT

A molecular-statistical theory describing the nematic liquid crystals (LCs) with spherical inclusions (or point defects) is proposed. At given size of inclusions and nematic order parameters at the surfaces of inclusions (zero in the case of point defects) and far from inclusions (where the nematic LC is almost uniform), the distribution of nematic order parameters in the bulk of LC with inclusions was found to be fully determined by the elastic constants of LC. We have found and explained the two-step heat-driven transformation from the nematic phase into the isotropic phase, with the intermediate phase in between. The nematic order parameters and the elastic constants are evaluated in the framework of a unified approach based on the features of pair interaction potentials of the individual LC molecules. It is shown that, in the case of K₃₃ < K₁₁, the point defects should destroy the conventional nematic phase.     

The effects of a laser field on phase transitions in liquid crystals

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.     

Structure investigation of liquid crystals composed of terminally polar molecules

Summary Recent experimental studies of X-ray scattering in mesogenic compounds with polar endgroups are discussed. The evidence of different types of layering in the nematic and smecticA phases was found. The lamellar packing in smeticA phases corresponds to either the monolayersA ₁ with an interlayer distance equal to the molecular lengthd≈L, or a bilayerA ₂ withd≈2L, or partially bilayerA _d phase whose period is incommensurate with the molecular lengthL<d<2L. The influence of the molecular structure of terminally polar molecules on the stability of layered structures of different types is discussed. The smectic ? phase with bilayer density wave modulated along the direction perpendicular to the directorn is described. The properties of liquid crystals consisting of polar molecules are discussed in terms of frustrated smectics with competing length scales. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

First-order polarization-activated optical transitions in smecticC and planar nematic liquid crystals

Summary It is shown that polarization-activated intrinsic optical bistability (OB) may be obtained in smecticC or planar nematic liquid crystals without external feed-back resonator. First-order transitions for twist are proved to occur, in principle, irrespective of the liquid-crystal material constants. The light intensity, needed to obtain OB, ranges from 10 kW/cm² to 1 MW/cm². To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

A model for the orientational order in liquid crystals

Summary A model for the orientational-order parameter in liquid crystals from the microscopic viewpoint and in conformity with its current understanding well established in the field of phase transitions is proposed here. This model has a close resemblance with the original ideas of Maier and Saupe. However, it is independent of the nature of molecules, rigid or flexible, and enables us to describe a phase with a single-order parameter tensor as expected. Moreover, an important advantage of this model in contrast to the prevailing ones is its, ability to separate the contributions of long-range order from local information inherent in any microscopic measurement such as by NMR and EPR techniques with some reasonable assumptions. All these are demonstrated here with the observed quadrupole splittings for different deuterium sites of long-chain molecules forming the uniaxial nematic phase. The role of molecular conformations on the observed splittings is also explicitly demonstrated by recovering quadrupole coupling constants and bond orientations at different molecular sites. These results are in reasonable agreement with available data. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Pseudo-molecular approach for the elastic constants of nematic liquid crystals interacting via anisotropic dispersion forces

The bulk and the surface-like elastic constants of a nematic liquid crystal are calculated for an ensemble of particles interacting via anisotropic dispersion forces using the pseudo-molecular method. The geometrical anisotropy of the molecules is also taken into account in the calculations by choosing a molecular volume of ellipsoidal shape. Analytical expressions for the elastic constants are obtained as a function of the eccentricity in the molecular volume shape. The method allows one to explore the dependence on the molecular orientation with respect to the intermolecular vector by analyzing the magnitude and the behaviour of macroscopic elastic parameters defining the nematic phase.