Konstantin I. Morozov

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,...

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.

Long-range order of dipolar fluids

The general theory of the isotropic–ferromagnetic transition of dipolar hard sphere fluids is considered on the base of the formalism of the direct correlation function. The equation of the Lovett–Mou–Buff–Gubbins type, described the one-particle orientational distribution, is generalized on a case of long-range interparticle potential and ellipsoidal sample shape. The exact criterion of the isotropic–ferromagnetic transition is found as a result of bifurcation analysis. It is shown that the formalism reduces to all known approximations in partial cases. A new expression for the susceptibility is obtained analytically within the framework of the generalized mean-spherical approximation. It is demonstrated that the susceptibility diverges at appropriate values of short-range correlations. In this way, the decisive role of short-range correlations outside the core in the transition appearance is established whereas the long-range correlations are found to be of secondary importance. The qualitative estimations show that the short-range pair correlations in dipolar fluids are most likely antiferromagnetic in character so as the transition to ferromagnetic liquid becomes questionable.

On the Theory of the Soret Effect in Colloids

Movement of colloidal particles can be driven by the inhomogeneous temperature field. In this paper the relation between the phenomenon of the migration of colloidal particles and the characteristics of the protective layer around the grain is established. Two cases of surfacted and ionic colloids are considered. The temperature gradient in the fluid induces the redistribution of surfactant molecules or ions near the particle. The redistribution leads to a loss in the balance of the stress on the particle surface, and that results in the grain movement along the temperature gradient. The values of the Soret coefficient of thermodiffusion motion in colloids turn out to be extremely high—two-four orders larger than the values of the Soret coefficient for molecular systems. The analytical solution of the problem of thermodiffusion motion in surfacted colloids is obtained using an assumption for the solute-particle interaction. The particles of these colloids always migrate towards colder temperature regions of the fluid. In the case of ionic colloids the direction of the particle movement depends on the magnitude of the double layer thickness and the value of the electric potential of the particle surface. The slowing-down of the thermodiffusion motion and the changing of its direction as a result of an increase in the ionic force of the solution are predicted.

The dielectric virial expansion and the models of dipolar hard-sphere fluid.

The virial expansion technique to determine the dielectric constant epsilon of dipolar hard-sphere fluid is developed. It is shown that the formalism allows to bring into agreement the results of Debyes, Onsagers, and Langevins to the problem. The third virial coefficient of epsilon is considered as a series over dipolar parameter lambda=m(2)d(3)kT. The terms up to O(lambda(11)) are calculated analytically providing a correct description of the third virial coefficient for small and intermediate values of lambda (0<or=lambda<or=4). The results of the dielectric virial series are compared with the Monte Carlo data for epsilon found by Matyushov and Ladanyi [J. Chem. Phys. 110, 994 (1999)]. The theory is in agreement with simulations only at small values of lambda<or=2. At higher polarities, the virial series diverges. Realization of the renormalization procedure permits to enlarge the range of applicability of the virial series. In this way, the new expression for the dielectric constant as a function of two dipolar parameters, lambda and y=4 pi nm(2)9kT, has been found explicitly. The expression gives a perfect upper bound of the dielectric constant and is more reliable for determination of epsilon than the previously known ones.