Investigation of electroconvective flow regimes in liquid dielectrics possessing unipolar injection conductivity

Isothermal electroconvection in a dielectric liquid arising in a plane-parallel electrode system due to unipolar injection of charges from the cathode is considered. Stationary flow regimes formed after the crisis of the equilibrium layer stability loss are numerically modeled. Variations of initial conditions yield two flow regimes providing comparatively higher volume charge densities in the first case near the cathode and in the second case near the anode. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 3–7, June, 2008.     

Dynamics of electroconvective nematic liquid crystal structures in a nonharmonic electric field

The emergence of electroconvection in a nematic liquid crystal under the action of a nonharmonic electric field is investigated. Analysis is carried out using a 2D model. We propose new forms of the varying electric field acting on the system, for which subharmonic oscillations exist: (a) electric field of a trapezoidal form and (b) external field varying in accordance with the law of “joined cosines.” The behavior of synchronous excitations in the insulating and conducting regimes, as well as subharmonic oscillations, is analyzed. The parametric instability domains are found, and the critical frequencies of transition between different response regimes are determined. The stability maps of the nematic liquid crystal are constructed on the frequency-voltage amplitude plane.     

Dynamics of electroconvective wave flows in a modulated electric field

Wave regimes of finite-amplitude electroconvection in a poorly conducting fluid heated from above in a modulated field of a horizontal capacitor are analyzed. Low conductivity of the fluid is ensured by the charges injected from the anode and intrinsic (dissociated) ions. A bifurcation diagram of solutions is constructed. Apart from the regimes of modulated standing and traveling waves, stable waves changing their direction of motion are detected, as well as waves with a chaotically varying phase velocity.     

Electroconvection under injection from cathode and heating from above

We study the electroconvection that appears in a nonuniformly heated, poorly conducting liquid in a parallel-plate horizontal capacitor due to the action of an external static electric field on the charge injected from the cathode. It is shown that the heating of the layer from above prevents steady-state convection and that, unlike the isothermal situation, electroconvection can appear in the oscillatory manner as a result of direct Hopf bifurcation. The effect of the heating intensity, the intensity of charge injection from the cathode, and the charge mobility on the thresholds of oscillatory and monotonic electroconvection is analyzed and the characteristic scales and frequencies of critical perturbations are determined. The nonlinear wave and steady-state regimes of the 2D convective structures formed in the poorly conducting liquid under the action of thermogravitational and injection mechanisms of convection are analyzed. The domains of existence of standing, traveling, and modulated waves are determined.     

Electroconvection in homeotropic nematic liquid crystals

Electroconvection is a classical example of pattern-forming phenomena in liquid crystals, typically observed in nematics with negative dielectric and positive conductivity anisotropies. This article focuses on how electroconvection in the homeotropic geometry differs from that in planar alignment. The influence of an additional magnetic field on the pattern characteristics and on secondary instabilities (the normal roll–abnormal roll transition) is discussed. The homeotropic alignment offers unique possibilities also for studying defect motion. Basic characteristics of some patterns of large wavelength are presented and compared with those of the classical Carr–Helfrich structures. Finally, electroconvection in substances with negative conductivity anisotropy is addressed.     

Electroconvection of a poorly conducting fluid in a steady electric field

The emergence of electroconvection and nonlinear flows of a nonisothermal poorly conducting fluid in a steady electric field of a horizontal capacitor is investigated. The main mechanism of charge formation in the fluids under investigation is electric conduction due to the temperature dependence of the charge mobility. The Galerkin method is used for obtaining a system of nonlinear differential equations for the amplitudes of spatial modes; on the basis of this system, steady-state, periodic, and random nonlinear regimes of the fluid flow are investigated. The scenarios of transition to chaotic flows (via intermittence and a subharmonic cascade) are discovered. The interrelation between the electroconductive and thermogravitational mechanisms of convection is analyzed, and hysteretic transitions between monotonic and oscillatory regimes are studied. Qualitative comparison with the experiment is carried out.     

Onset of parametric electroconvection in a nematic liquid crystal

The behavior of a nematic liquid crystal (NLC) in an external ac electric field is studied in terms of the weak electrolyte model. Since electroconvection of certain types of NLCs can occur in a constant field in an oscillatory mode, a quasi-periodic response of a nematic to the external action is possible in an ac field, as well as a sharp reduction of the electroconvective stability thresholds at resonance frequencies. Stability maps for NLCs are plotted on the plane of amplitude-frequency parameters of the external field. Phase portraits are constructed for subharmonic and synchronous perturbations. The optical response of a NLC in the ac electric field is studied; the time dependences of the relative intensity of light, which passed through the nematic liquid crystal cell, are obtained.     

Study of the transition from conduction to injection in an electrohydrodynamic flow in blade-plane geometry

A dielectric fluid can be set into motion with the help of electric forces, mainly Coulomb force. This phenomenon, called electroconvection, can be induced by electrohydrodynamic conduction, injection, and induction. Conduction is based on the dissociation/recombination phenomenon, generates heterocharge layers, and occurs for low electric field values. Injection produces homocharge layers in the electrode vicinity and requires stronger electric fields to be initiated. This study is an experimental observation of the transition from conduction to injection of a dielectric liquid in blade-plane geometry using Particle Image Velocimetry. In addition, the electric current is measured to completely understand the flow behavior.     

Structure and the magnetic and magneto-optical properties of Co-Sm-O nanogranular films

The magnetic properties and magneto-optical effects in nanocomposites based on Co-Sm-O films prepared through pulsed plasma sputtering of a SmCo₅ target are investigated. It is shown that, depending on the technological conditions and regimes of subsequent annealing, the films can have different structures from cobalt nanoparticles distributed in the dielectric samarium oxide matrix with a magnetic phase volume of more than 60% to a continuous polycrystalline cobalt film with embedded samarium oxide nanoparticles. The evolution of the spectra of the magneto-optical Kerr effect and the field dependences of the magnetization is studied as a function of the film structure.     

Numerical simulations of electro-thermo-convection and heat transfer in 2D cavity

In this article the electro-thermo-convective phenomena in a dielectric liquid enclosed in a 2D cavity and subjected to the simultaneous action of an electric field and a thermal gradient is studied. We solved directly the full set of coupled equations of Electro-Hydro-Dynamic (EHD) and energy equation using a finite volume method. In order to characterize the influence of the electric field on heat transfer the liquid is first heated (from a lateral wall) till the thermal steady state is obtained and then the electric potential and injection of electric charge is applied. Two cases of injection are considered: from the lower electrode and from a lateral wall (left or right). The flow pattern and Nusselt number strongly depend on the non-dimensional characteristic parameters: electrical parameter, Rayleigh number, Prandtl number and mobility parameter M. The convective motion passing from a purely thermal convection to a purely electrical convection and the number of electro-thermo-convective rolls patterns are investigated.As a consequence of the analysis of the combined effect of electric and thermal fields on the flow structure and on Nusselt number, we have also evaluated the heat transfer enhancement due to electroconvection. It is shown that the injection of electric charge increases the heat transfer and Nusselt number is independent of Rayleigh number for high enough values of T.     

Direct transition to electroconvection in a homeotropic nematic liquid crystal

We present an experimental and theoretical investigation of a variant of electroconvection using an unusual nematic liquid crystal in an isotropic configuration (homeotropic alignment). The significance of the system is a direct transition to the convecting state due to the negative conductivity anisotropy and positive dielectric anisotropy. We observe at onset rolls or squares depending on the frequency and amplitude of the applied ac voltage with a strong signature of the zigzag instability. Good agreement with calculations based on the underlying hydrodynamic theory is found. We also construct an extended Swift-Hohenberg model which allows us to capture complex patterns like squares with a quasiperiodic modulation.     

有耗异向介质层覆盖普通介质圆柱电磁特性的研究

研究了线电流源平行放置在有耗异向介质层覆盖的无限长介质圆柱附近模型的电磁特性.首先,提出电磁模型并给出此电磁模型的精确解.其次,利用精确解进行数值计算,得到不同电磁参数情况下该电磁模型的近场分布图形,并通过方向性系数和归一化辐射阻抗研究其远场特性.由于有耗异向介质的负折射特性和损耗的同时作用,相比于无耗异向介质层以及有耗普通介质层,得到了有耗异向介质层覆盖普通介质圆柱模型在近场和远场特性上极为不同的电磁特性. 关键词： 线电流源 异向介质 方向性系数 辐射阻抗     

Electroconvection in nematics above the splay Fréedericksz transition

We present a basic model for an instability leading to a novel type of electroconvection patterns observed above the splay Fréedericksz transition in nematics. Such patterns, with wave vector perpendicular to the director easy axis, are found in planar sandwich cells under crossed polarizers, they do not produce shadowgraph images at onset. An adaptation of the classical Carr Helfrich mechanism is introduced. The ground state is a tilted director field uniform in the cell plane. The proposed mechanism destabilizes this director field and leads to a structure with modulated out-of-plane (twist) deformations. Experimental confirmation is provided by polarizing microscopy. All experimental observations are qualitatively explained with the proposed model.     

Parametric Instability of Surface Waves on the Second Harmonic of Electron Cyclotron Frequency in a Plasma Layer

The parametric instability of surface waves on the second harmonic of electron cyclotron frequency (SWCF) in a plasma filled dielectric wave guide is examined in a kinetic approximation. The studied surface waves are extraordinary polarized modes and propagate across the external steady magnetic field. The amplitude of the electrical pump wave is assumed to be small. Simple expressions for increments of the parametric instability of the SWCF are calculated. The otained results can be used in controlled fusion researches in order to avoid undesirable regimes of plasma periphery heating in that fusion devices which use the resonance electron cyclotron heating method.     

Static dielectric constant assessment from capacitance over a wide range of electrode separations

Static dielectric constant extraction from two-electrode capacitance measurement over a wide range of electrode separations and dielectric constants involves careful assessment of fringe fields. Finite-element method has been employed to compute capacitance and quantify fringe fields for parallel electrode capacitor of (finite thickness, radii r, electrode separation d), with a homogeneous dielectric medium extending up to the geometric limits of the electrodes. Two distinct regimes, in the fringe field contributions are seen. A procedure to extract the static dielectric constant has been proposed for the first regime and a validation has been provided for the same.     

Magnetic field effects on the electric modulus properties of nematic mixture E7

The molecular dynamics of the homogeneously aligned nematic liquid crystal mixture E7 subject to a magnetic field has been studied. The dielectric spectra study has revealed a low bias magnetic field effect on the evolution of dielectric relaxation spectra occurred at lower (∼kHz) (δ-relaxation) and higher (∼MHz) (α-relaxation) frequency regions. The complex electric modulus, which converted from experimental dielectric spectra, has been analyzed with theoretical model of Debye relaxation. The obtained fitting parameters of relaxation time and strength of dielectric components are shown to vary systematically with the strength of applied magnetic field. A microscopic molecular dynamic model has been proposed to describe the two-step variation of E7 molecular under the bias magnetic field. The results provide implication for magneto-modulation of liquid crystal molecular dynamics under the bias magnetic field.     

Electroconvection in the presence of autonomous unipolar injection and residual conductivity

The problem of the finite-amplitude electroconvection of a nonuniformly heated fluid filling a horizontal capacitor in the presence of the autonomous unipolar injection of a charge from an anode has been solved. In addition to the injected charge in a carrier fluid, the presence of proper ions (appearing owing to dissociation) ensuring the low residual conductivity of the fluid is assumed. The effect of the ratio of proper and injected ions, as well as the heating degree, on the thresholds of monotonic and oscillating electroconvection has been analyzed. The bifurcation diagrams of the solutions have been plotted for the cases of the heating of a layer from below and above. The electroconvection modes of the nonuniformly heated liquid that appear because of forward and backward bifurcations have been revealed.     

Wave chaotic behaviour generated by linear systems

It is shown that regimes with dynamical chaos are inherent not only to nonlinear system but they can be generated by initially linear systems and the requirements for chaotic dynamics and characteristics need further elaboration. Three simplest physical models are considered as examples. In the first, dynamic chaos in the interaction of three linear oscillators is investigated. Analogous process is shown in the second model of electromagnetic wave scattering in a double periodical inhomogeneous medium occupying half-space. The third model is a linear parametric problem for the electromagnetic field in homogeneous dielectric medium which permittivity is modulated in time.     

On the limit distribution of layer block spin variables in the mean spherical model

The limit distribution of the layer block spin variables of the mean spherical model under Neumann–Dirichlet boundary conditions is investigated in the presence of an inhomogeneous external field which changes sign at distance Lx (0x1) from the Neumann boundary. The behaviour of the equation of state is studied in different temperature and field regimes: high-temperature bulk limit, critical finite-size scaling regime, and low-temperature moderate-field regime. A new classes of critical behaviour for the characteristic function of the limit distributions are obtained and studied in the three different regimes.