Dynamic properties of non-dilute ferrofluids

L. Yu. Iskakova - A. Yu. Zubarev

Urals State University, 620 083 Ekaterinburg, Russia

Abstract
Functions of dynamic response to external magnetic field as well as rheological properties of moderately concentrated magnetic colloid consisting of identical spherical single-domain ferromagnetic particles have been derived by methods of statistical physics. Hydrodynamic and magnetodipole interactions between the par-ticles were taken into account. The ferroparticle magnetic moment has been sup-posed to be constant in absolute value and to be frozen into the particle body. Our analysis has shown that the magnetodipole interaction increases both the functions of magnetic response and rheological parameters of the colloid as well as character-istic times of relaxation of both the colloid magnetization and hydrodynamic stress. We have studied some peculiarities of hydrodynamic behaviour of the ferrocolloid in oscillating magnetic field. If the shear rate is large enough, the effective viscosity is nonmonotonic (with minimum) function of the field frequency w; the rotational effec-tive viscosity is negative provided that this frequency is large enough. For the given w, the rotational viscosity is nonmonotonic function of the shear rate W. It is nega-tive if W is small and positive if W is large enough. The interparticle interaction in-creases the effective rotational viscosity in oscillating field. Figs 12, Refs 31.