Ferrielectricity in smectic-C^{*} dechiralization-line lattices.

Recent experiments probing a new ferroelectric liquid crystal (CLF08) confined in cells with planar alignment have shown dielectric and optic anomalies suggesting the onset of ferrielectric ordering within the surface lattice of dechiralization lines. We present a phenomenological theory describing the corresponding phase transition sequence SmA→SmC^{*}→Ferri. Phase diagrams and thermodynamic, dielectric, and optic properties are worked out and compared with experiments.

Coupling of replicate order-parameters in incommensurate multiferroics.

The specific properties of incommensurate multiferroic phases resulting from the coupling of order-parameter replicates are worked out using the illustrative example of iron vanadate. The phase difference between the order-parameter copies induces an additional broken symmetry phase corresponding to the lowest symmetry of the system and varies critically at the transition to the multiferroic phase. It reflects the temperature dependence of the angle between paired spins in the antiferromagnetic spiral structure.

Directional magnetoelectric effects in MnWO4: magnetic sources of the electric polarization.

The ferroelectric order and magnetic field induced effects observed in the spiral phase of MnWO4 are described theoretically. It is demonstrated explicitly that the Dzyaloshinskii-Moriya antisymmetric interactions contribute to the correlation between spins and electric dipoles in the incommensurate and commensurate ferroelectric phases of magnetic multiferroics. However, other single-site symmetric interactions are shown to be involved in the magnetoelectric process, suggesting the possible existence of an electric polarization originating from purely symmetric effects.

Theory of two- and three-dimensional bent-core mesophases.

An extension of the vector-wave theory of antiferroelectric bent-core liquid crystals to columnar and soft-crystalline mesophases is presented. We enumerate and describe the phases resulting from the condensation of one, two, and three vector waves in the isotropic liquid of bent-core molecules. Beside the one-dimensional B2 and smectic- CP phases, 24 columnar and soft-crystalline orthorhombic, monoclinic, and triclinic phases can be stabilized when two or three waves have nonparallel wave vectors.

Macroscopic and molecular symmetries of unconventional nematic phases.

The effects of the molecular symmetry on macroscopic properties of conventional and unconventional nematic phases are investigated theoretically. These effects concern: (1) the form of the molecular orientation fluctuations, (2) the thermodynamic behavior and the list of stable ordered phases, (3) the effective symmetry of the molecules in the ordered phases. The order-parameter consists of tensors forming the main harmonics of the fluctuation distribution.

Theory of the cooperative evaporation of volatile droplets.

A theoretical approach to the cooperative process of evaporation of volatile liquid droplet arrays is proposed which combines the equation of the liquid-gas interface with a Landau-type description. The spatial and temporal behaviors of the process are shown to be governed by the spontaneous symmetry breaking mechanism of a parent droplet configuration. The applicability of the approach is illustrated by the theoretical description of recent experiments in closed and open systems.

Theory of uniaxial and biaxial nematic phases in bent-core systems.

We present a phenomenological theory of the uniaxial and biaxial nematic phases recently observed in bent-core mesogenic systems. To take into account the molecular anisotropy we introduce two types of symmetry transformation: "external" rotations which turn the molecules with respect to the laboratory axes and "internal" rotations which turn them with respect to the molecular axes. We show then that the description of bent-core nematic phases involves two isomorphic copies of the conventional tensor order parameter instead of one for conventional rod like and disk like molecules.

Theory of chiral periodic mesophases formed from an achiral liquid of bent-core molecules.

The theory of unconventional one-dimensional periodic mesophases forming in a liquid of achiral bent-core molecules is presented. The order parameter is a polarization wave. Three distinct phases which can be stabilized directly from the isotropic liquid phase are associated with linear, circular, and elliptic polarizations of the wave.

Theory of the incommensurate smectic-A phases in dimesogenic liquid crystals.

A mechanism assuming a mismatch between the basic smectic periodicity and the amplitude and phase modulations periods of the mass density wave order parameter is proposed, for describing the incommensurate smectic-A structure found in dimesogenic liquid crystals. The different sequences of phases found in this family of compounds are described theoretically.

Theory of the anticlinic pretilted surface phase in tilted chiral smectic films.

In freely suspended chiral smectic films, the molecules tilt at the surfaces above the bulk ferroelectric transition temperature. The tilt angle at the two surfaces can be either equal (synclinic surface phase) or opposite (anticlinic surface phase) on each surface of the film. We propose a realistic theoretical model accounting for the stability and properties of these two structures, based on competing surface interactions.

Theory of polar biaxial nematic phases.

A theoretical model is proposed for describing the polar biaxial and uniaxial nematic phases observed in thermotropic liquid crystals formed from rodlike polyester molecules. The polarity and biaxiality are shown to result from the same molecular mechanism, i.e.