Rheological properties of polydisperse magnetic fluids. Effect of chain aggregates

A theoretical model of medium-density polydisperse magnetic fluids is propo sed. The model takes into account that the major fraction of particles in t ypical ferrofluids is characterized by a magnetic core diameter of about 10 nm. In addition, there is a certain proportion of large particles with a c ore diameter of about 16 nm. As a result of the magnetic dipole interaction , the large particles form chain aggregates. Small particles, for which the magnetic dipole interaction energy (both with each other and with large pa rticles) is smaller than the thermal energy, remain in the individual nonag gregated state. The distribution of chains with respect to the number of (l arge) particles and some rheological characteristics of the ferrofluids are determined. The proposed model is capable of explaining, in principle, the giant magnetoviscosity effect and a strong dependence of the rheological p roperties of ferrofluids on the shear rate observed in some recent experime nts. (C) 2001 MAIK "Nauka/Interperiodica".