Igor P. Pinkevich

Weak anchoring effects in ferronematic systems

We study the magnetically induced behaviour of a ferronematic cell with finite anchoring energy at the cell surfaces. We calculate the dependence of orientational and concentration profiles on the magnetic field, the director anchoring energy, and the cell thickness. We find a new intermediate thickness high field state, in which there is no ferroparticle segregation and a highly ordered director field. This contrasts with previous work in which only a restricted set of system parameters and strong anchoring at the cell surfaces were considered.

Magnetic Field Induced Director Reorientation in the Nematic Cell with Time-Dependent Anchoring Due to Adsorption/Desorption of LC Molecules

ABSTRACT The time evolution of director orientation in the cell subject to the external magnetic field in the case of time-dependent boundary conditions is investigated. The case when one substrate has strong anchoring and at other substrate director anchoring is determined by process of adsorption/desorption of LC molecules on it is considered. The problem is solved in one constant approximation and Rapini-type potential for interaction between LC molecules in the cell and adsorbed LC molecules is proposed. During the time evolution the orientational distribution function of molecules adsorbed on the cell substrate possesses two maximums. Time evolution of the director orientation, anchoring energy and easy axis at the substrate is investigated numerically.

Influence of Small Spherical Particles on the Spatial Director Distribution and Light Scattering in a Nematic Cell

Abstract Director spatial distribution near a small spherical particle is calculated for the case of weak director anchoring with the particle surface. The differential cross-section of the light scattering by the director inhomogenuity near the particles is calculated in dipole approximation. We use the Percus - Yevick approximation for hard spheres to account for interference effects. It is studied the influence of the external ac electric field on the director profile and magnitude of light scattering cross-section.

Optical Study of the Phase Transitions in Liquid Crystals

Abstract The temperature dependencies of vibrational bands parameters corresponding to intramolecular vibrations have been investigated for supercooled MBBA (N-p-methoxybenzylidene p-n-butylaniline) (from 163 K to 315 K) by Raman spectroscopy. The formation mechanisms of vibrational bands for various phases of MBBA have been established. Besides the nature of heterogeneous broadening for amorphous and nematic phases have been studied1–6.

Influence of light induced molecular conformational transformations and anchoring energy on cholesteric liquid crystal pitch and dielectric properties

Abstract The cholesteric liquid crystal (CLC) pitch and director orientation are obtained as the functions of the anchoring energy W of a director and its pretilt angles Δϑ1, Δϑ2 on the cell orienting surfaces. If the anchoring energy is strong enough, the monotonic change of the CLC twisting ability due, for example, to the light-induced molecular conformational transformatins causes jumps of the CLC pitch and mean values of dielectric susceptibility tensor ⟨ϵ1J⟩. Values of Jumps of ⟨ϵ1J⟩ components are determined by anchoring energy W and pretilt angles Δϑ1, Δϑ2 that may be used for experimental estimations of W, Δϑ1, Δϑ2.

Director Reorientation in a Cell with Time-Dependent Anchoring Due To Adsorption/Desorption of LC Molecules

The time evolution of the director orientation in a nematic cell due to the adsorption/desorption processes of LC molecules on the cell substrate is considered. It is shown that boundary conditions on the cell surface evolve in accordance with the orientation distribution function of LC molecules in the cell bulk. During the time evolution the orientational distribution function of molecules adsorbed on the cell substrate possesses two maximums. Switching off the magnetic field at the suitable moment one can obtain a bistable nematic system.

Rayleigh-Gans Theory of Light Scattering in Filled Nematics

Abstract The differential cross-sections of the light scattering by the director inhomogeneity near small spherical and cylindrical particles in nematic liquid crystals are calculated in the Rayleigh-Gans approximation. We use the Percus-Yevick approximation to account for interference effects. The influence of the boundary conditions at the particle surface as well as of external electric field on the angular dependence and magnitude of the light scattering cross-section is studied.

Magnetic field induced orientational bistability in a ferronematic cell

The equilibrium states of a suspension of single-domain ferroparticles in a nematic liquid-crystalline homeotropic cell subject to an external magnetic field are studied. We predict the existence of magnetic field induced orientational bistability in such a system in a magnetic field ∼10 2 Oe. The existence of the bistability phenomenon is governed by conditions on the cell thickness and on the director anchoring energy. The effect can be controlled using a small bias magnetic field normal to the unperturbed director. The director reorientation in a magnetic field causes an effective change in refraction index, which enables the orientational bistability to be exploited in optical devices.

Light-Scattering by Small Solid Spherical Particles Dispersed in a Nematic Cell

Abstract The light scattering differential cross-section by small identical spherical particles in nematic liquid crystal is numerically calculated at the infinitely rigid director anchoring with particles surface. It is taking into account the influence of the correlations between the positions of the particles.

Spectroscopy of phase transitions in liquid crystals

Abstract The results of study of structural genesis of the nonoriented nematic MBBA at different temperature pre-history are reported. The temperature dependent transformations of the vibrational Raman spectra and their connection with the change of MBBA structure were investigated in the spectral region 1100–1600 cm−1 and in the temperature interval 163–333 K. The method to be used allows to prognosticate the structural evolution of MBBA and to identify its phase states. In particular it is found that the several coexisting fractions of different conformers may be obtained in the temperature region lower the glass transition temperature in dependence on the sample cooling rate.