Mark I. Shliomis

NEGATIVE VISCOSITY OF FERROFLUID UNDER ALTERNATING MAGNETIC FIELD

A stationary magnetic field induces an increase in the ferrofluid viscosity. An additional resistance to the flow occurs due to the field oriented magnetic particles impeded by free rotation in a vortex flow. It is shown that in an alternating, linearly polarized magnetic field the additional viscosity is positive at low frequencies of the field and negative at high frequencies. The point is that an alternating field induces rotatory oscillations of the particles, but does not single out any direction of their rotation. One can say that half of the particles rotate clockwise and the other half counterclockwise. Hence, the macroscopic angular velocity of the particles equals zero. However, this corresponds only to fluid at rest. Any shear (i.e., any vorticity) is sufficient to break down the degeneracy of the direction of rotation, which results in the nonzero angular velocity of the particles. The occurring ‘‘spin up’’ of the flow by the rotating particles leads to the decrease of the effective viscosity,...

Ferrohydrodynamics: Retrospective and Issues

Two basic sets of hydrodynamic equations for magnetic colloids (so-called ferrofluids) are reviewed. Starting from the quasistationary ferrohydrodynamics, we then give a particular attention to an expanded model founded on the concept of internal rotation. A specific relation between magnetic and rotational degrees of freedom of suspended grains provides a coupling of the fluid magnetization with the fluid dynamics. Hence a complete set of constitutive equations consists of the equation of ferrofluid motion, the Maxwell equations, and the magnetization equation. There are three kinds of the latter. Two of them were derived phenomenologically as a generalization of the Debye relaxation equation in case of spinning magnetic grains, while one of them was derived microscopically from the Fokker-Planck equation. Testing the magnetization equations, we compare their predictions about the dependence of the rotational viscosity on the magnetic field and the shear rate.

Ferrohydrodynamics: testing a third magnetization equation.

A new magnetization equation recently derived from irreversible thermodynamics is employed to the calculation of an increase of ferrofluid viscosity in a magnetic field. Results of the calculations are compared with those obtained on the basis of two well-known magnetization equations. One of the two was obtained phenomenologically, another one was derived microscopically from the Fokker-Planck equation. It is shown that the third magnetization equation yields a quite satisfactory description of magnetiviscosity in the entire region of magnetic-field strength and the flow vorticity. This equation turns out to be valid-like the microscopically derived equation but unlike the former phenomenological equation-even far from equilibrium, and so it should be recommended for further applications.

Frequency dependence and long time relaxation of the susceptibility of the magnetic fluids

Abstract A simple dynamic theory of nonequilibrium phenomena in magnetic fluids is suggested. It is shown that temperature maximum of the magnetic susceptibility and special character of its dispersion (1/ω−noise) are explained by the one-particle effects. A conception which links these phenomena with transition of the magnetic fluid into the spin-glass state is criticized.

Convective instability of magnetized ferrofluids

Abstract Convective instability in a flat ferrofluid layer subject to a transverse uniform magnetic field is investigated theoretically. A temperature gradient imposed across the layer induces a concentration gradient of magnetic grains owing to the Soret effect. Both these gradients cause a spatial variation in magnetization that establishes a gradient of magnetic field intensity within the fluid layer. The field gradient induces in its turn a redistribution of magnetic grains due to magnetophoresis. The resulting self-consistent magnetic force tries to mix the fluid. Linear analysis performed for the case of realistic boundary conditions on confined horizontal planes predicts double-diffusive oscillatory instability in a certain region of parameters, whereas if the particle diffusion had been not operative, then only stationary instability would occur.

Magnetic properties of ferrocolloids: The effect of interparticle interactions

Abstract Several new experimental results on the initial susceptability of magnetic fluids are presented and discussed from the theoretical point of view.

Comment on "Magnetoviscosity and relaxation in ferrofluids".

It is shown and discussed how the conventional system of hydrodynamic equations for ferrofluids was derived. The set consists of the equation of fluid motion, the Maxwell equations, and the magnetization equation. The latter was recently revised by Felderhof [Phys. Rev. E 62, 3848 (2000)]. His phenomenological magnetization equation looks rather like our corresponding equation, but leads to wrong consequences for the dependence of ferrofluid viscosity and magnetization relaxation time on magnetic field.

Magnetic Fluid as an Assembly of Flexible Chains

Dipolar chains formed in magnetic fluids out of colloidal magnetic grains have much in common with polymer molecules. An investigation of spatial and orientational intrachain correlations and elucidation of an important role of the chains flexibility lead us to natural and fruitful extension of basic concepts of polymer physics to the case of dipolar chains. Conformations of the chains (statistical coil, globule) in zero and infinitely strong external magnetic field are studied and the possible coil-globule phase transition is predicted and discussed.

Ferrofluids: flexibility of magnetic particle chains

Conformational properties of dipolar chains ,a nd spatial and orientational intrachain correlations in zero and infinitely strong external fields are investigated theoretically. A striking similarity and essential distinctions between the chains and polymer molecules are revealed an dd iscussed. The main attention is given to the chain flexibility. The coil–globule phase transition in dipolar chains is predicted.

Rotational viscosity of magnetic fluids: contribution of the Brownian and Néel relaxational processes

Abstract The general Fokker-Planck equation is derived, which takes into account two simultaneous diffusion processes: rotation of the magnetic moment of the particle around its crystal axes and rotation of the particle together with its magnetic moment with respect to the liquid. The influence of the joint diffusion on additional (rotational) viscosity arising in the flow of magnetic fluid in the presence of magnetic field is studied.