L. Yu. Iskakova

Structurization of ferrofluids in the absence of an external magnetic field

Structural transformations in a model ferrofluid in the absence of an external magnetic field have been theoretically studied. The results agree with well-known laboratory experiments and computer simulations in showing that, if the concentration of particles and their magnetic interaction energy are below certain critical values, most particles form separate linear chains. If these parameters exceed the critical values, most particles concentrate so as to form branched network structures. The passage from chains to network has a continuous character rather than represents a discontinuous first-order phase transition.

To the theory of kinetic properties of polar nematics

The non-equilibrium properties of nematic systems consisting of solid rod-like particles with constant magnetic or electric moment are investigated. The analysis is performed on the basis of the Fokker-Plank equation for one-particle orientational distribution function. By using approximate decisions of this equation, we derive the relaxation equation of the mean dipole moment of the system as well as its quadrupole order parameters. The relaxation times appearing in these equations are calculated. With the help of Doi-Edwards theory we evaluate Leslie-Ericksen coefficients of the system as well as the components of viscoelastic tension tensor for non-stationary flows.

STATISTICAL THERMODYNAMICS OF FERRONEMATIC

In this paper we investigate theoretically the equilibrium phase transitions in ferronematics taking into account the steric and magnetic interactions of colloid particles. The systems of two kinds are considered: in the first one the particles magnetic moment can be parallel or antiparallel to the director of nematics and in the second one it can only be parallel. Phase diagrams of separation of a system on the regions with high and low concentration of the particles in the external magnetic field are constructed. The external magnentic field is shown to stimulate this separation. It has been found that magnetic interaction of particles decreases the critical field of Fredericks effect and increases the deformation amplitude of structure of the system in this phenomenon. The existence of domain structures in thin gaps filled by ferronematics of the second kind is predicted.

Dynamics of the Frederiks effect in ferronematic

Abstract The kinetics of the Frederiks transition in ferronematics (suspensions of stretched ferromagnetic colloidal particles in a nematic solvent) have been studied. The particles are assumed to align rigidly along the nematic director, both the initial nematic director n 0 and the external magnetic field H being parallel to the plane of a gap containing a sample. On the basis of a microscopic analysis and the Fokker–Planck equation for a ferromagnetic particle the macroscopic equation describing the kinetics of the suspension remagnetization has been derived. This equation includes an empirical relaxation time τ . With the help of this equation two cases were considered, the first one concerns a suspension with zero initial magnetization and the second one concerns a suspension with nonzero magnetization frozen in a solvent. In the first case periodical structures can arise at the onset stage of a Frederiks transition. Their periods increase and their amplitudes decrease with the increasing τ so that for large τ such structures cannot occur. All structures vanish during the final stage of the transition. In the second case the Frederiks effect is initiated by an infinitesimal magnetic field. The structures arising during the phase transition are homogeneous in the directions of both n 0 and H .

Structure transformations in ferrosmectics

In this paper we have investigated theoretically deformational phase transitions in ferrosmectics. These transitions can be induced by external field normal to smectic layers as well as by attraction of particles belonging to different smectic layers. Ferrosmectic structure deformation has been found to occur in either the soft or rigid regime. Soft deformations have been studied extensively.

Shear elastic modulus of magnetic gels with random distribution of magnetizable particles

Magnetic gels present new type of composite materials with rich set of uniquie physical properties, which find active applications in many industrial and bio-medical technologies. We present results of mathematically strict theoretical study of elastic modulus of these systems with randomly distributed magnetizable particles in an elastic medium. The results show that an external magnetic field can pronouncedly increase the shear modulus of these composites.

Phase transitions in suspensions of needle-like particles in liquid crystals

The thermodynamical properties of suspensions of rigid stretched particles in liquid crystal solvent are studied with the help of modification of Mc Millan and Maier-Saupe theories. We have shown that the presence of particles increases the temperatures of phase transitions isotropic medium-nematic and nematic-smectic A in a solvent as well. The indirect interaction between particles through nematic solvent effectively acts as an attraction. This attraction may induce their condensation and phase transition of type “gas-liquid” in the system of particles.