On the scattering of sound by a magnetic field in a MHD fluid

We investigate the effect of a localized magnetic field on the propagation of sound in an infinite fluid described by the magnetohydrodynamic equations (MHD). An externally imposed magnetic field will scatter an acoustic wave, and the scattered wave is related to the spatial structure of the magnetic field. Measuring it is thus a non-intrusive probe for the magnetic field. Simple examples likely to be encountered in practice are worked out, and estimates are given that suggest the practical feasability of this diagnostic tool in current MHD experiments.Received: 1 May 2003, Published online: 15 October 2003PACS: 42.25.Fx Diffraction and scattering - 47.65.+a Magnetohydrodynamics and electrohydrodynamics - 52.35.Dm Sound waves     

Frequency dependences of the velocity and absorption coefficient of sound waves in a magnetic fluid

On the basis of molecular-kinetic theory, frequency-dependent expressions are derived for the velocity and absorption coefficient of sound in a magnetic fluid with allowance for the contribution of structural relaxation in the presence of an external inhomogeneous magnetic field. The asymptotic behavior of the expressions is investigated for both low-and high-frequency limits. A numerical study is carried out by taking as an example a magnetic fluid consisting of kerosene with magnetic particles of Fe₃O₄ suspended in it. The results of the numerical study show that the calculated frequency dependences of the velocity and absorption coefficient of sound in a magnetic fluid are in good agreement with the static experimental data.     

Perturbation of magnetization of a magnetic fluid by ultralow thermal fluctuations accompanying a sound wave

The paper presents the results of theoretical and experimental research of the the effect of perturbation of the magnetization of a magnetic fluid caused by thermal fluctuations in an adiabatic sound wave in the initial area of the magnetization curve. Measurements are conducted on samples of a magnetic colloid with various viscosity of the dispersion medium in the frequency range of 20–60 kHz. In this frequency range, the studied samples are characterized by the absence of thermal relaxation of magnetization. Comparison of the conclusions of the thermal magnetization relaxation model with the results of experiment make it possible to obtain information on the rheological features of the neareast molecular environment of a particle, i.e., nanorheology.     

磁流体光栅的制作原理和磁性流体

介绍了磁流体光栅的制作原理、方法和磁性流体原理。     

On the molecular sound absorption in fluid mixtures

Measurements of sound absorption in binary mixtures of gases in which two relaxation processes can occur due to the specific heats of the molecular vibrations, have hitherto been evaluated by using a theory based mainly on gaskinetic considerations. The theory presented here, uses the thermodynamics of irreversible processes as developed byMeixner for sound absorption in fluid systems due to interior reactions of arbitrary kind and number. This treatment permits a concise derivation of the final formulas which are also valid for mixtures of real gases and fluids. According to which set of independent thermodynamical variables for the derivation is used, the final formulas seem to differ considerably. It is pointed out that these differences arise from introducing two types of relaxation times and may be eliminated by means of the relationship between these relaxation times.     

Transient dipolar sound wave in a viscous compressible fluid

The dipolar sound wave generated by a sudden impulse in an unbounded viscous compressible fluid is studied on the basis of the linearized Navier-Stokes equations. Due to viscosity the spherical wavefront is diffuse with a width which grows with the square root of time. The wavefront is followed by a spherical shell of static potential flow. The shell itself surrounds an expanding spherical region of viscous flow. At long times the energy in each of the three regions is of comparable magnitude, and decays with a t^−3/2 power law.     

Vorticity dynamics and sound generation in two-dimensional fluid flow

An approximate solution to the two-dimensional incompressible fluid equations is constructed by expanding the vorticity field in a series of derivatives of a Gaussian vortex. The expansion is used to analyze the motion of a corotating Gaussian vortex pair, and the spatial rotation frequency of the vortex pair is derived directly from the fluid vorticity equation. The resulting rotation frequency includes the effects of finite vortex core size and viscosity and reduces, in the appropriate limit, to the rotation frequency of the Kirchhoff point vortex theory. The expansion is then used in the low Mach number Lighthill equation to derive the far-field acoustic pressure generated by the Gaussian vortex pair. This pressure amplitude is compared with that of a previous fully numerical simulation in which the Reynolds number is large and the vortex core size is significant compared to the vortex separation. The present analytic result for the far-field acoustic pressure is shown to be substantially more accurate than previous theoretical predictions. The given example suggests that the vorticity expansion is a useful tool for the prediction of sound generated by a general distributed vorticity field.     

磁性液体在磁场中产生光的双折射效应机理

磁性液体是一种特殊的高分子稳定胶体,在磁场中会产生光的双折射效应,对磁性液体在胶体学科方面展开研究,发现磁性液体在磁场中的弱絮凝行为表现异常明显,显示出特有的方向性,且又不至胶体系统失稳,证明了磁性液体中的磁性微粒在磁场中聚集成方向性的链状而又不失稳的临界状态存在。从而揭示了方向性弱絮凝是磁性液体在磁场中产生光的双折射效应的机理。     

Plasmon-assisted sound amplification in semi-conductors under strong magnetic fields

We derive an expression for the contribution from plasma effects to the sound amplification in piezo-electric semiconductors subjected to a strong d.c. magnetic field applied parallel to the drift field. It is shown that this contribution largely differs from that in the absence of a magnetic field both in the strength of the effect and in angular distribution of the emitted phonons.     

Free oscillations of magnetic fluid in strong magnetic field

The paper presents the esults of measuring the elastic parameters of an oscillatory system (coefficient of pondermotive elasticity, damping factor, and oscillation frequency) whose viscous inertial element is represented by a magnetic fluid confined in a tube by magnetic levitation in a strong magnetic field. The role of elasticity is played by the pondermotive force acting on thin layers at the upper and lower ends of the fluid column. It is shown that, by measuring the elastic oscillation frequencies of the magnetic fluid column, it is possible to develop a fundamentally new absolute method for determining the saturation magnetization of a magnetic colloid.     

Fast sound in expanded fluid Hg accompanying the metal-nonmetal transition

The dynamic structure factor S(Q,omega) of expanded fluid Hg has been measured up to the metal-nonmetal transition region at 9.0 g cm(-3) (1723 K and 1940 bars) using high-resolution inelastic x-ray scattering, at momentum transfers, Q, from 0.2 to 4.8 A(-1). Analysis in the framework of generalized hydrodynamics reveals that the frequencies of the collective excitations increase faster with Q than estimated from the macroscopic speed of sound. The effective sound velocity at 9.0 g cm(-3) estimated from the dispersion relation is triple the ultrasonic sound velocity. The present result suggests the existence of fast sound in expanded fluid Hg accompanying the metal-nonmetal transition.     

Elasticity of a magnetic fluid in a strong magnetic field

Complex measurements of the following elastic-magnetic parameters of a magnetic fluid suspended by magnetic levitation within a horizontal tube in a strong magnetic field were performed: the oscillation frequency and decay coefficient; the static, ponderomotive, and dynamic elasticity coefficients; the fluid displacement under hydrostatic pressure; magnetization curve; and the magnetic field strength and gradient. Calculations based on a model of ponderomotive elasticity with correction for the resistance of a viscous fluid in motion and on the fluid column displacement for two magnetic fluid samples agree well with the experimental magnetization curve. The discussed technique holds promise for research into magnetophoresis and nanoparticle aggregation in magnetic colloids.     

Notes on the absorption of sound in high magnetic fields

We study the absorption of sound by a free electron-impurity system in a quantizing magnetic field and derive a formula for the absorbed power on the basis of linear response theory. In the approximation of constant relaxation time and in the limit of high temperatures (1/τ?kT) this formula is essentially equivalent to the formulas used by Gurevichet al. and by Svirskii. Without these restricting assumptions we obtain general conditions for the appearance of quantum oscillations. We discuss the influence of spin flip scattering on the shape of the 0⁺-absorption line. Furthermore, we show that the usual kinetic approach leads to the incorrect results given by Quinn.     

Longitudinal sound measurements on UPt3 in a magnetic field

A new feature (a peak) is found in the magnetic field dependence of the longitudinal ultrasound attenuation in UPt₃. Below the peak, the attenuation varies linearly with field. Above the peak Δα = α(H_c2)-α(H) ∼(H_c2-H)². The peak, at the change over from linear to parabolic behavior, may correspond to a phase transition in the vortex lattice.     

Electrokinetic phenomena in a kerosene-based magnetic fluid

We propose the methods for studying electrokinetic phenomena in magnetic colloidal systems (magnetic fluids), which make it possible to use the magnetic properties of particles of the disperse phase. Electrophoresis and the sedimentation potential in a kerosene-based magnetic fluid are studied. It is shown that only a small part (approximately one-thousandth) of all disperse particles in the magnetic fluid under investigation are charged, the sign of the particle charge being negative.