M. A. Osipov

Ferroelectric liquid crystals

Abstract The idea either to reveal or to construct a liquid ferroelectric was never abandoned by researchers since the discovery of ferroelectricity in solid crystals at the beginning of this century. In principle, nature does not forbid the existence of ferroelectric liquids. For instance, a centro-symmetric liquid can change its point symmetry from the K h , to C∝v , group with the formation of a uniform ferroelectric phase. A liquid without the inversion center can have a phase transition K → C∝, into the inhomogeneous, helical structure. In both these hypothetical cases the spontaneous polarization, P s , would play the role of the true order parameter. However, such liquid ferroelectric phases (proper ferroelectrics) have not been discovered as yet.

A new twist to molecular chirality: intrinsic chirality indices

We propose an intrinsic molecular chirality tensor based only on nuclear positions. The chirality tensor gives rise to two universal chirality indices, the first giving information about absolute chirality, and the second about the anisotropy chirality, i.e., the degree of chirality in different spatial directions. The formalism is derived using simple models obtained from the theory of optical activity. The indices are calculated analytically for a right angled tetrahedron, and numerically for a small selection of molecules.

Mean-field theory of a nematic liquid crystal doped with anisotropic nanoparticles

In the framework of molecular mean-field theory we study the effect of nanoparticles embedded in nematic liquid crystals on the orientational ordering and nematic–isotropic phase transition. We show that spherically isotropic nanoparticles effectively dilute the liquid crystal medium and decrease the nematic–isotropic transition temperature. At the same time, anisotropic nanoparticles become aligned by the nematic host and, reciprocally, improve the liquid crystal alignment. The theory clarifies the microscopic origin of the experimentally observed shift of the isotropic–nematic phase transition and an improvement of the nematic order in composite materials. A considerable softening of the first order nematic–isotropic transition caused by strongly anisotropic nanoparticles is also predicted.

A molecular model for tilting phase transitions between condensed phases of Langmuir monolayers

A model of interacting rigid rods is proposed to describe tilting phase transitions in monolayers of freely rotating long‐chain molecules with hexatic in‐plane order. The model takes into account steric repulsion and van der Waals attraction between neighboring rods as well as the orientational entropy of individual rods, all within a mean field approximation limited to the unit cell. Two variants of the model are proposed, with different constraints on the polar molecular headgroups. In the first, the headgroups are grafted to a hexagonal close‐packed (hcp) lattice, and in the second, the headgroup lattice deforms to accommodate to the tilt. For the monolayer on a solid substrate, tilt has two opposing actions on the internal energy. The decrease in the distance between rods acts to reduce the interaction energy, while the decrease in the overlapping length of the rods acts to increase it. As the area per molecule increases, the competition between these two effects drives the first‐order phase transitio...

The elastic constants of nematic and nematic discotic liquid crystals with perfect local orientational order

Explicit expressions for the Frank elasticity coefficients of nematic liquid crystals are obtained using the model of perfect local orientational order. The use of the density functional approach enables one to express the results in terms of the direct correlation function for a liquid of perfectly aligned particles. Assuming an ellipsoidal shape for the equipotential surfaces, the direct correlation function can be related to that of the sphere fluid by means of the affine transformation. Both rod-like and plate-like molecules are considered. The dependence of the elasticity coefficients on the axial ratio of the molecule is discussed, and the results for the ratio of the coefficients K 3/K 1 are found to be in reasonable agreement with experimental data for nematics composed of rigid molecules.

Continuously rotating chiral liquid crystal droplets in a linearly polarized laser trap

The transfer of optical angular momentum to birefringent particles via circularly polarized light is common. We report here on the unexpected, continuous rotation of chiral nematic liquid crystal droplets in a linearly polarized optical trap. The rotation is non-uniform, occurs over a timescale of seconds, and is observed only for very specific droplet sizes. Synchronized vertical motion of the droplet occurs during the rotation. The motion is the result of photo-induced molecular reorganization, providing a micron sized opto-mechanical transducer that twists and translates.

Tunability of wire-grid metamaterial immersed into nematic liquid crystal

We propose electrically tunable hybrid metamaterial consisting of special wire grid immersed into nematic liquid crystal. The plasmalike permittivity of the structure can be substantially varied due to switching of the liquid crystal alignment by external voltages applied to the wires. Depending on the scale of the structure, the effect is available for both microwave and optical frequency ranges.

Molecular model for de Vries type smectic-A-smectic-C phase transition in liquid crystals

We develop both phenomenological and molecular-statistical theory of smectic- A -smectic- C phase transition with anomalously weak smectic layer contraction. Using a general mean-field molecular model, we demonstrate that a relatively simple interaction potential suffices to describe the transition both in conventional and de Vries type smectics. The theoretical results are in excellent agreement with experimental data. The approach can be used to describe tilting transitions in other soft matter systems.

Density functional approach to the theory of interfacial properties of nematic liquid crystals

General expressions for the free energy of the nematic liquid crystal in the interfacial region are derived using the density functional approach. The anisotropic part of the surface tension and the coefficients of the Landau–de Gennes theory for the nematic free surface are expressed in terms of the direct correlation function and analytical results are obtained for a reference system composed of molecules with ellipsoidal equipotential surfaces. The assumption of perfect local orientational order is used to account for strong short‐range orientational correlations in the nematic liquid. It is shown that in such an idealized system the molecules can be aligned parallel or perpendicular to the free surface of the nematic, depending on the balance between attractive and repulsive forces. However, the condition of the stability of the nematic phase requires this alignment to be parallel. The possible role of specific intermolecular interactions, which can be very important in real liquid crystals, is also d...

Phase separation effects and the nematic–isotropic transition in polymer and low molecular weight liquid crystals doped with nanoparticles

Properties of the nematic–isotropic phase transition in polymer and low molecular weight liquid crystals doped with nanoparticles have been studied both experimentally and theoretically in terms of molecular mean-field theory. The variation of the transition temperature and the transition heat with the increasing volume fraction of CdSe quantum dot nanoparticles in copolymer and low molecular weight nematics has been investigated experimentally and the data are interpreted using the results of the molecular theory which accounts for a possibility of phase separation when the system undergoes the nematic–isotropic transition. The theory predicts that the nematic and isotropic phases with different concentrations of nanoparticles may coexist over a broad temperature range, but only if the nanoparticle volume fraction exceeds a certain threshold value which depends on the material parameters. Such unusual phase separation effects are determined by the strong interaction between nanoparticles and mesogenic groups and between nanoparticles themselves.