Capillary oscillations of a charged viscous spheroidal droplet

Dispersion relations are derived for the capillary oscillations of a charged viscous spheroidal droplet by scalarization within perturbation theory using an expansion in two small parameters, viz., the magnitude of the perturbation of the spheroidal surface as a result of thermal fluctuations and the magnitude of the deviation of the equilibrium spheroidal droplet shape from a spherical shape. It is shown analytically that the motion spectrum of the liquid consists of two components that interact in the linear theory: toroidal vortex motion and poloidal potential motions. A numerical analysis reveals that the instability growth rates of the higher modes of a highly charged droplet increase with enhancement of the degree of spheroidal strain and decrease rapidly as the viscosity of the liquid increases. Zh. Tekh. Fiz. 68, 20–27 (April 1998)     

Rotation of a droplet in a viscous fluid

Unique capabilities for modeling the bulk motion of one liquid in another arise from the use of droplets made of a magnetic liquid. In this paper the low-frequency rotational motion of a magnetic droplet suspended in a viscous liquid is investigated. In this frequency range, the shape of the droplet does not depend on the field frequency and is determined only by its amplitude. An analytic solution has been found in the Stokes approximation to the problem, which generalizes the classic problem of Jeffrey to the case of a liquid ellipsoidal particle. This solution makes it possible to determine the velocity field inside and outside the liquid particle, the moments of the viscous forces acting on the droplet, its coefficient of rotational mobility. Zh. éksp. Teor. Fiz. 112, 1340–1350 (October 1997)     

The energy of a prolate spheroidal shell in a uniform magnetic field

The problem of the energy of a spheroidal magnetic shell, solved by methods of classical electrodynamics, arises, in particular, upon the study of thin-wall biocompatible microcapsules in connection with a pressing issue of targeted drug delivery. The drug inside a microcapsule should be released from the shell at a required instant of time by destroying the capsule’s shell. The placement inside a shell of magnetic nanoparticles sensitive to an external magnetic field theoretically makes it possible to solve both problems: to transport a capsule to the required place and to destroy its shell. In particular, the shell can be destroyed under the action of internal stress when the shape of a capsule is changed. In this paper, the analysis of the model of a magnetic microcapsule in the form of a prolate spheroidal shell is performed and formulas for the magnetostatic and magnetic free energy when the magnetic field is directed along the major axis of the spheroid are derived.     

Dynamics of a dielectric droplet suspended in a magnetic fluid in electric and magnetic fields

The behavior of a microdrop of dielectric liquid suspended in a magnetic fluid and exposed to the action of electric and magnetic fields is studied experimentally. With increasing electric field, the deformation of droplets into oblate ellipsoid, toroid and curved toroid was observed. At the further increase in the electric field, the bursting of droplets was also revealed. The electrorotation of deformed droplets was observed and investigated. The influence of an additional magnetic field on the droplet dynamics was studied. The main features of the droplet dynamics were interpreted and theoretically examined.     

Phase transitions in a few-electron quantum dot in a magnetic field: Wigner phases and broken-symmetry spin-singlet states

We develop a variational many-body approach within a second quantized formulation for a few-electron system in a parabolic two-dimensional quantum dot (QD). By way of application, the nature of the ground state of a two-electron system in a parabolic QD in a broad range of magnetic fields is theoretically investigated. Various phase transitions on the basis of the resulting analytical expressions for energy of the system have been investigated: First, the well-known transition from a maximum density droplet to a Wigner phase in a magnetic field is obtained, provided that the QD is in conditions of weak confinement. Furthermore, in the case of relatively strong QD confinement and weak magnetic fields, a rotationally symmetric spin-singlet state is the ground state of the system. However, in a strong magnetic field and for the same QD confinement, a broken-symmetry spin-singlet state appears to be energetically favored over the symmetric spin-singlet state. A first investigation of such a broken-symmetry spin-singlet phase in a QD in a magnetic field is shown to be an important application of the proposed technique. The text was submitted by the authors in English.     

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.     

Monte Carlo simulations of magnetic particle suspensions with a simple assessment method for the particle overlap between magnetic spheroids

We have developed a simple assessment method for the overlap between spheroidal particles, which neither requires the complex manipulation of vectors and matrices that is indispensable in the ordinary methods, nor is based on a model potential. Moreover, we have developed an evaluation method for the interaction energy arising from the overlap of the steric layer coating spheroidal particles. This is based on a sphere-connected particle model, but some modifications are introduced in order to express an appropriate repulsive interaction energy at the deepest overlapping position. We have investigated the phase change in a magnetic spheroidal particle suspension for a two-dimensional system by means of Monte Carlo simulations. In the case of no external magnetic field, if the magnetic particle-particle interaction is sufficiently strong to favour cluster formation, long raft-like clusters tend to be formed in a dilute situation. With decreasing values of area fraction, a chain-like structure in a dense situation transforms into a raft-like structure within a narrow range of the particle area fraction. Similarly, the raft-like clusters are preferred in a weak applied magnetic field, but an increase in the field strength induces a phase change from a raft-like into a chain-like structure.

Highlights of the present paper:

1. A simple assessment method has been proposed for the overlap between two spheroidal particles.

2. The particle overlap assessment is free from a complex mathematical manipulation regarding vectors and matrices.

3. A modified sphere-connected model has been proposed in order to more accurately evaluate a repulsive interaction due to the overlap of the steric layers coating spheroidal particles.

4. 2D Monte Carlo simulations have been performed to elucidate the phenomenon of a phase change by magnetic spheroidal particles on a material plane surface.

5. A phase change between a raft-like and a chain-like aggregate structure is able to be controlled by the area fraction of particles and an external magnetic field.

     

Field dependent shape variation of magnetic fluid droplets on magnetic dots

The morphology of magnetic fluid droplets on magnetic thin film dots is studied experimentally, including the aspect ratio and the contact angle variation of the droplets. Under a uniform external magnetic field, the droplet's aspect ratio increases with the external field and with the diameter of the magnetic dot due to the concentrated magnetic flux inside the magnetic fluid droplet. Similar to the electrical wetting phenomenon, the induced magnetic dipoles in the magnetic film and in the magnetic fluid near the solid–liquid interface change the solid–liquid interfacial tension, and in consequence reduce the apparent contact angle of the magnetic fluid droplet.     

Stabilization of electron-hole liquid in uniaxially strained germanium in a strong magnetic field

Luminescence spectra of uniaxially and uniformly strained high-purity germanium crystals at liquid-helium temperatures in a magnetic field of up to 14 T have been investigated. In strongly strained Ge crystals, a new line has been detected on the low-energy side of the excitonline in magnetic fields higher than 4 T. Studies of this line’s characteristics as functions of pressure, temperature, and magnetic field have led us to conclude that its presence is due to recombination of electron-hole pairs in an electron-hole liquid. The experimental data suggest that the metallic electron-hole liquid is stabilized in a strong magnetic field. By approximating the shape of the newly detected line using the model of metallic electron-hole liquid, we have obtained the electron-hole liquid density n _EHL(B) and Fermi energies E _Fe,h of electrons and holes. The liquid binding energy ø as a function of magnetic field has been estimated.     

Effect of a strong magnetic field on the quadrupole and magnetic orders and crossover in the Jahn-Teller magnet DyVO<Subscript>4</Subscript>

The effect of a strong magnetic field H||[001] on magnetic properties of the Jahn-Teller compound DyVO₄ is studied. New phase transitions discovered and investigated experimentally and theoretically include the breaking of quadrupole order (enhancement of the crystal symmetry) and breaking of antiferromagnetic order as well as the effect of convergence of energy levels of the Dy³⁺ ion (crossover). The differential magnetic susceptibility of a DyVO₄ crystal is measured in fields up to 36 T in the temperature interval 1.4?15 K. The observed magnetic susceptibility anomalies and phase transitions are described using a unified theoretical approach.     

Exchange interactions and spin states in a V15 magnetic molecular nanocluster

Magnetization reversal in a V₁₅ magnetic molecular cluster is studied theoretically and the results of calculations are compared with the available experimental data. Analytical and numerical methods (e.g., a modified Lanczos method) are used for calculating the energy spectrum of the cluster and for determining a set of exchange constants, which ensures good quantitative agreement between the results of theoretical calculations and experimental data on the behavior of magnetic susceptibility in the range of weak as well as strong fields. The fine structure of transitions from a low-to a high-spin state is predicted (in the range of ultrahigh fields, each of three transitions is a combination of two close transitions).     

横向静磁场对电磁悬浮液滴稳定性的影响

在利用电磁悬浮技术实现液滴悬浮的过程中,液滴内部往往存在剧烈对流、外部伴随快速旋转和质心的水平位移等不稳定因素;因此,实现液滴的稳定悬浮是完善电磁悬浮技术的关键.本文采用实验观测的方法,通过U形静磁场组件对液滴所在空间施加横向静磁场,利用高速相机记录了不同磁场强度下纯铜熔融液滴的振荡变形过程;分析了横向静磁场对悬浮铜液滴振荡频率、振幅以及旋转的影响.实验发现:对于熔融前的固态铜颗粒,若静磁场强度超过0.3 T,铜颗粒几乎以静止状态悬浮.熔融后,当施加0.15 T的静磁场,与未加静磁场时相比,液滴俯视图轮廓线拟合出的椭圆分别与x轴和y轴的交点坐标之差R-、椭圆面积A和椭圆长轴长度Dmax的振幅分别减小了25%,76%和60%;随着磁场强度的继续增加,振幅和频率继续减小,但在静磁场强度为0.3 T时,相比静磁场强度为0.2 T,频率增加了1 Hz.横向静磁场还抑制了悬浮铜液滴的旋转,当磁场强度增加到0.53 T时,悬浮液滴只在10?的角度范围内摆动.这些结果表明,施加横向静磁场能够有效提高悬浮液滴的稳定性.     

Instability of the flat surface of a magnetic fluid in a cylindrical cavity in the presence of a vertical magnetic field

The problem of a magnetic liquid which completely fills a vertical cylindrical cavity in an undeformable horizontal layer of a magnet having the same magnetic properties as the liquid is considered. The entire system is immersed in a uniform vertical magnetic field. in a linear formulation of the problem an approximate solution in the form of series is obtained for the evolution of an initial small deviation of the free surface of the liquid from its flat equilibrium shape. An experiment is performed which shows that the initially flat free surface takes on a stable domed shape as the field strength is increased (from zero) and that a further increase in the field in a certain restricted range leads to the formation of an annular corrugation. The structures observed, which are the result of the nonlinear stage in the development of the initial perturbation, are qualitatively similar to the first two modes of the solution obtained. Zh. Tekh. Fiz. 68, 23–30 (January 1998)     

Hydrogen atom in strong magnetic field: a high accurate calculation in spheroidal coordinates

A B-spline-type basis set method for the calculation of hydrogen atom in strong magnetic fields in the frame of spheroidal coordinates has been introduced. High accurate energy levels of hydrogen in the magnetic field, with strength ranging from 0 to 1000 a.u., have been obtained. For the ground state, 1s₀, energies with at least 11 significant digits have been obtained. For the low-lying excited state, 2p₋₁, energies with at least 9 significant digits are obtained. The method has also been applied to the calculation of hydrogen Rydberg states in laboratory magnetic fields. Energy spectra with at least 10 significant digits are presented. A comparison with other results in the literatures has been performed. Our results are comparable to the most accurate one up to date. A possible extension to the cases of parallel and crossed electric and magnetic fields have been discussed.     

The onset of rotational motion of dusty plasma structures in strata of a glow discharge in a magnetic field

An interpretation is given to the previously observed action of a magnetic field on the state of a dusty plasma structure in strata of a glow discharge. The conditions of previous experiments are analyzed, in which a nonuniform rotation and a change in the degree of order of a dusty plasma structure (the translational order), as well as a correlation between them, were revealed. Based on this analysis and on data in the literature on dusty plasmas in a magnetic field, a hypothesis is made that the reason for the rotation of the structure is the ion drag force. Additional experiments on the observation of the onset of rotational motion of a structure in “weak” and “strong” magnetic fields are conducted. It is shown that rotation reversal (and rearrangement of the order of the structure) is caused by changes in the direction of ionic flows—from internal regions of the structure to its periphery and vice versa—in the weak and strong magnetic fields. The results obtained agree qualitatively with the hypothesis adopted, as well as with the data of the two-dimensional theory of strata.     

Linear dynamics of a magnetic subsystem in a quasi-Ising magnet

Microwave absorption at frequencies from 37 to 85 GHz was studied for a Dy_0.3Y_2.7Fe₅O₁₂ single crystal in pulsed magnetic fields of up to 30 T at T=4.2 K. The magnetic field was aligned with the [100] direction. For the fields above 4 T, several soft magnetic-resonance modes were observed, most of them being caused by the static phase transitions induced by a strong external magnetic field. The field-independent absorption lines away from the points of phase transition may be due to the dynamic Jahn-Teller magnetic effect.     

Orientational Ordering of a Liquid-Crystal Suspension of Carbon Nanotubes in a Magnetic Field

A statistical theory is proposed to describe a suspension of carbon nanotubes in a nematic liquid crystal. The mean-field approach is used, and dispersion attraction, the excluded volume effects, the diamagnetism of liquid crystal molecules, and the strong diamagnetism of nanotubes are taken into account. The influence of the volume fraction of impurity, temperature, and magnetic field on the orientational ordering of a liquid crystal matrix and carbon nanotubes is studied. The concentration and temperature phase transitions in the suspension are investigated for various magnetic fields. The concentration and field shifts of the point of the phase transition between nematic and isotropic or paranematic phases are studied.     

Photon-drag effect in a two-dimensional electron gas in high magnetic fields

We present the first measurements concerning the photon drag effect in a two-dimensional electron gas based on intersubband transitions in high magnetic fields. It is shown that the excitation mechanism of the drag voltage in a magnetic field differs obviously from the case of zero magnetic field. The longitudinal as well as the Hall drag voltage show strong oscillations around zero when the magnetic field is swept. Both consist of a B-symmetrical and an antisymmetrical part with the same periodicity in B as the magnetoresistanceR_xx. The drag voltage oscillations are strongly correlated to the relative position of Fermi energy and Landau levels and are independent of the photon energy in the range of usable laser lines.     

Charged mobile complexes in magnetic fields: A novel selection rule for magnetooptical transitions

The implications of magnetic translations for internal optical transitions of charged mobile electron-hole (e-h) complexes and ions in a uniform magnetic field B are discussed. It is shown that transitions of such complexes are governed by a novel exact selection rule. Internal intraband transitions of two-dimensional (2D) charged excitons X ⁻ in strong magnetic fields are considered as an illustrative example. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 504–509 (25 October 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Optical band gap width in GaAs in megagauss magnetic fields

The band gap width in GaAs in magnetic fields of up to 10 MG is calculated using a five-band kp model. The selection rules for interband electron transitions in strong magnetic fields are found, and the dependences of the interband transition probabilities on a magnetic field are calculated. The electronic spectra calculated in the five-band model are compared with those calculated in the Kane model and in the tight-coupling approximation. The calculations are shown to agree with experimental data if the contribution from the density-of-states tails and excitonic effects to light absorption is taken into account.