Ferronematic Liquid Crystal Research Articles

The influence of the flexoelectric effect on the orientational transitions in ferronematic liquid crystals

The influence of flexoelectric polarization induced by external electric and magnetic fields in a ferronematic liquid crystal on the orientational transitions between uniform and nonuniform phases is analyzed. The Freedericksz transition field and the saturation field as functions of ferronematic material parameters are determined. Bifurcation diagrams of orientational phases are analyzed. It is shown that the orientational transitions can be of the first or second order depending on the value of the segregation parameter of the dispersed phase.

Orientational dynamics of a ferronematic liquid crystal in a rotating magnetic field

The behavior of the orientational structure of a ferronematic in a rotating uniform magnetic field is investigated using the continual theory. The time-dependent system of equations describing the dynamics of the ferronematic is derived. The dependences of the angles of rotation of the director and of the magnetization of the ferronematic on the velocity of field rotation are determined for various values of the material parameters. Two regimes (synchronous and asynchronous) of rotation of the ferronematic structure are detected. In the synchronous regime, the director rotates with the frequency of the magnetic field and a constant phase delay. The asynchronous regime is characterized by a time-dependent phase delay. The dependence of the critical angular velocity of magnetic field rotation, which determines the boundary between the synchronous and asynchronous regimes, on the magnetic field strength is derived.

Orientational bistability in ferronematic liquid crystals with negative diamagnetic anisotropy

In the framework of continuum theory we study magnetic field induced orientational transitions in a ferronematic, i.e. suspension of single-domain magnetic particles in a nematic liquid crystal. We consider the case of negative diamagnetic susceptibility anisotropy of a liquid crystal and soft planar coupling of impurity particles with a liquid crystal matrix. We found tricritical behavior of the threshold transition in a magnetic field from perturbed state into uniform planar state. This transition can be the first or second order, depending on the parameter of the magnetic phase segregation. We analytically derive the expression for the tricritical segregation parameter that determines the character of a transition. We show that ferronematic has a large magneto-optical non-linearity which is the result of the director reorientation under external field. Comparison of results of numerical calculations with experimental data has been carried out.

Structural Changes in Ferronematic Liquid Crystals Studied by Surface Acoustic Waves

The response in attenuation of surface acoustic wave (SAW) of frequency 30MHz propagating along ferronematic liquid crystals (6CHBT) to both electric and low magnetic field (up to 0.25 T) has been studied experimentally. Two low volume concentrations (Φ1=1×10-5 and Φ2=1×10-4) of spherical magnetic nanoparticles were added to liquid crystal during its isotropic phase. In contrast to undoped 6CHTB the distinctive SAW attenuation responses induced by both electric and magnetic fields in ferronematic liquid crystals below Fréedericksz transition have been observed suggesting structural changes and the orientational coupling between the magnetic moment of magnetic particles and the director of the liquid crystal. The geometrical re-ranking of magnetic nanoparticles was registered for some orientations of magnetic field, too. Observed results confirmed the significant influence of the presence of magnetic nanoparticles on the structural properties of 6CHTB.

Effect of electric and magnetic fields on the orientation structure of a ferronematic liquid crystal

We analyze uniform orientation phases in soft ferronematics (suspensions of magnetic nanoparticles in nematic liquid crystals) induced by electric and magnetic fields. It is shown that the competition between the electric and magnetic fields can lead to various sequences of orientation transitions in a ferronematic depending on the energy of coupling between the director and magnetization. We obtain and analyze phase diagrams of these transitions. A sequence of re-entrant transitions in the orientation structure (angular phase-homeotropic phase-angular phase-planar phase) is predicted for a certain range of the coupling energies and electric field strengths.

Weak coupling effects and re-entrant transitions in ferronematic liquid crystals

In the framework of continuum theory we study the orientational and magnetooptical properties of ferronematics (i.e., colloidal suspensions of magnetic nanoparticles in nematic liquid crystals) with soft planar coupling of a disperse phase with a liquid crystal matrix in a magnetic field. It is shown that there is a threshold energy of coupling between the dispersed phase and the liquid crystal matrix, below which (weak coupling) ferronematic has a complex magnetooptical response to an applied magnetic field. We show that at weak coupling initially a uniform ferronematic structure under the field action undergoes the sequence of re-entrant transitions “uniform phase–non-uniform phase–uniform phase–non-uniform phase”, which can be of the first or second order depending on a segregation parameter value. The tricritical value of the segregation parameter is found analytically as a function of the material parameters of a suspension. We study magnetic field action on the optical phase lag and the capacity of a ferronematic cell. Comparison of results of numerical calculations with experimental data is carried out.

Magnetically induced bistable behavior of ferronematic liquid crystals

In the framework of the modified model [S. V. Burylov and A. N. Zakhlevnykh, Phys. Rev. E 88, 012511 (2013)] of soft ferronematic liquid crystals, i.e., suspensions of needlelike ferroparticles in nematic solvents, we consider the ferronematic states with different mutual orientations of the director and magnetization. We study the transitions between states in an external magnetic field and show that these transitions are characterized by either continuous or discontinuous changes in the order parameter; i.e., they can be both the second-order and the first-order, respectively. In the latter case the magnetic field induced orientational hysteresis arises, which can be observed in experiments on the birefringence of ferronematic liquid crystals.

Polarization gratings using ferronematics—An elastic continuum theory

In our paper we apply the elastic continuum theory in order to describe the working of a polarization grating device based on the ferronematic liquid crystal and driven by magnetical field. Solving the Euler–Lagrange equations we obtain analytical expressions for the critical thickness and threshold magnetic field in strong and weak anchoring conditions. We discuss the influence of the ferroparticle presence and properties on the behavior of a device based on the ferronematic liquid crystal polarization gratings. We study also the dynamic behavior of this kind of device and we find the expression for switching on and off times. Our study may be useful in designing magneto-optical devices which include nanoparticles or microprocessed surfaces.

Capacitance changes in ferronematic liquid crystals induced by low magnetic fields

The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNT-COOH), and SWCNT functionalized with Fe(3)O(4) nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was φ(1)=10(-4) and φ(2)=10(-3). Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Fréederiksz transition threshold) has been observed, which is not present in the undoped nematic.

Influence of the segregation effect on the magnetic and optical properties of a compensated ferronematic liquid crystal

The Freedericksz transition in splay and bend geometry in a ferronematic (a suspension of single-domain magnetic particles in a nematic liquid crystal) is considered in the framework of the continuum theory. In zero magnetic field, the ferronematic was assumed to be compensated (i.e., having equal fractions of an impurity with magnetic moments directed parallel and antiparallel to the local director). Spatial distortions of the director and the concentration redistribution of the magnetic impurity in the ferronematic layer are studied as functions of the applied magnetic field and the segregation parameter. It is shown that the magnetic-field induced Freedericksz transition from the homogeneous to the inhomogeneous state has a threshold nature. The transition field as a function of the material parameters of the ferronematic is determined analytically. Magnetization of the ferronematic and its optical properties for a strong and weak segregation as functions of the magnetic field are studied.

Magnetite nanorod thermotropic liquid crystal colloids: Synthesis, optics and theory

We have developed a facile method for preparing magnetic nanoparticles which couple strongly with a liquid crystal (LC) matrix, with the aim of preparing ferronematic liquid crystal colloids for use in magneto-optical devices. Magnetite nanoparticles were prepared by oxidising colloidal Fe(OH)2 with air in aqueous media, and were then subject to alkaline hydrothermal treatment with 10moldm−3 NaOH at 100°C, transforming them into a polydisperse set of domain magnetite nanorods with maximal length ∼500nm and typical diameter ∼20nm. The nanorods were coated with 4-n-octyloxybiphenyl-4-carboxylic acid (OBPh) and suspended in nematic liquid crystal E7. As compared to the conventional oleic acid coating, this coating stabilizes LC-magnetic nanorod suspensions. The suspension acts as a ferronematic system, using the colloidal particles as intermediaries to amplify magnetic field–LC director interactions. The effective Frederiks magnetic threshold field of the magnetite nanorod–liquid crystal composite is reduced by 20% as compared to the undoped liquid crystal. In contrast with some previous work in this field, the magneto-optical effects are reproducible on time scales of months. Prospects for magnetically switched liquid crystal devices using these materials are good, but a method is required to synthesize single magnetic domain nanorods.

Optical Transmission Factor of a Ferronematic Liquid Crystal Under Magnetic Field Induced Orientational Transitions

In the framework of continuum theory we study the light transmission factor of a ferronematic liquid crystal as a function of external magnetic field, the anchoring energy with the boundary of a layer, and material parameters of a ferronematic. We show that magnetic field induced Fréedericksz transition in ferronematics in twist geometry can be either a first- or second-order phase transition depending on the anchoring energy between the director and magnetic particles, i.e., it exhibits the tricritical behavior. The light transmission factor for first- or second-order transition is also studied.

Reentrant Phase Transitions in Ferronematic Liquid Crystals

Within the continuum theory we study the influence of codirectional electric and magnetic fields on the orientational structure of a ferronematic layer taking into account the segregation effects. Recently in [Phys. Rev. E 81, 051710 (2010)] the tricritical behavior of the magnetic Freedericksz transition in ferronematics has been theoretically predicted. This paper shows that an external electric field can change the character of this transition. We find reentrant Freedericksz transitions in ferronematic liquid crystals subjected to the electric and magnetic fields. These transitions can be either first- or second-order ones. We derive the analytical expression for the width of the area of the reentrant orientational transitions.

Orientational transitions in a ferronematic layer with bistable anchoring at the boundary

A possibility of the first-order transition, as well as reentrant transitions, induced by an external magnetic field between the homeotropic phase and the hybrid homeotropically planar phase in a ferronematic liquid crystal (ferronematic) with bistable anchoring at the layer boundary is demonstrated in the framework of a continuum theory. The critical values of the material parameters of the ferronematic, the anchoring energy, the thickness of the layer, and the magnetic field strength, for which this transition is possible, are determined. The cases of positive and negative diamagnetic anisotropy of the ferronematic are considered.

First Order Orientational Transitions in Ferronematic Liquid Crystals

The influence of external magnetic field on the orientational structure of a ferronematic liquid crystal is analyzed. We use modified Rapini potential for the anisotropic part of coupling energy between the ferronematic and the limiting surfaces. We show that under magnetic field action one of three ferronematics phases is possible: the uniform state, the disturbed state and the saturation state. We find that the transition between these ferronematic phases can be either the first- or the second order transition depending on the surface anchoring anisotropy and segregation parameter. We show the influence of the segregation effect on the orientational transitions in ferronematics.

The influence of magnetic field on refractive index profile of ferronematic cell

This paper presents the study of the influence of magnetic field on the refractive index profile of homeotropically oriented 6CHBT-based ferronematic liquid crystal cell. The direction of magnetic field was chosen in a way which ensures the one-dimensional deformation of directors field. The FN cell was placed between polarizers. On a basis of Burylov-Raikher model and measurements of light transmission throughout the ferronematic layer the mean volume concentration of magnetic particles dispersed in 6CHBT and refractive index profiles induced by magnetic filed were determined. Moreover the range of magnetic field intensity was determined in which the distribution of tilt angle can be described in a scope of the Burylov-Raikher theory and the small tilt angle approximation.

On an effective model for ferronematic liquid crystals

AbstractThe tools of homogenization theory are used to develop an effective model for ferronematics–colloidal suspensions of small ferromagnetic particles in a nematic liquid crystalline medium. The results are presented for dilute suspensions of identical prolate spheroidal particles. For needle‐like particles of large eccentricity, the model reduces to a known expression [5]. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic field-induced orientational phases of ferronematics in shear flow

We examine the effect of shear flow on the orientational phase transitions induced by a magnetic field in ferronematic liquid crystals. Continuum approach based on the generalized Leslie–Ericksen theory is used to describe the dynamics of ferronematic liquid crystals. We consider three orientations of the magnetic field in a plane of shear flow. Stationary solutions for the director and the magnetization are obtained as functions of the magnetic field strength for different values of material parameters. Our results show that shear flow can lead to the shift of the field thresholds or to a “smoothing” of the magnetic field-induced transitions in ferronematics. In the limiting case of pure nematic liquid crystals, we revealed threshold effects, which are unstipulated by the orientational elasticity of a liquid crystal, in contrast to the conventional Fréedericksz transition.

Thermotropic behaviour of the complex liquid crystal system containing 8CB [4-cyano-4'-(n-octylbiphenyl)] and organic ferrofluid

A ferronematic liquid crystal (FNLC) system containing 8CB (4-cyano-4'-[n-octylbiphenyl]) and magnetite particles was prepared by using the two steps method. The magnetite particles were dispersed in the organic (liquid crystalline) media by using toluene. However, the toluene affected the thermotropic and structural behaviour of the pure basic (8CB) substance, drastically. The product was stable as no any sedimentation and coagulation has been observed during several weeks after the preparation. However, the distribution of magnetite particles was found not to be homogeneous in the nm size range.

LATTICE BOLTZMANN KINETIC SCHEME FOR ELECTRO-OPTICAL AND MAGNETO-OPTICAL EFFECTS IN FERRONEMATIC LIQUID CRYSTALS

A new lattice Boltzmann (LB) scheme for ferronematic liquid crystals (FNLCs) is presented. Ferronematics are the colloidal suspension of ferromagnetic nanoparticles in nematic liquid crystals, which have interesting potential applications due to their susceptibility to both the electric field and the magnetic one. We extend the LB model for liquid crystals using our kinetic scheme for magnetic fluids. This model makes it possible to study the coupling effects of electric field and magnetic field in the FNLC efficiently. The LB method is suitable for the numerical investigation of complex fluids such as FNLCs because of its simple algorithm based on the microscopic physics.