The influence of magnetic separation on the magnetoviscous behaviour of ferrofluids.

S. Thurm - S. Odenbach

ZARM, University of Bremen, Am Fallturm, 28359 Bremen

Abstract
The size-distribution of nanosized magnetic particles in ferrofluids is of fundamental significance if the fluid is subjected to shear-flow under the influence of an applied magnetic field. By magnetic separation it is possible to divide a ferrofluid into two fractions which differ in mean particle size. During the separation-process large particles and so-called primary agglomerates diffuse to regions which are located in the stronger field, whereas this effect is very weak on smaller particles. After the experiment there are two different fractions: one fraction with lower, and another fraction with higher percentage of bigger particles. Rheological investigations after the separation-process show the important effect of bigger particles on the magnetoviscous behavior of the fluid. It has been found that the fraction with high percentage of big particles and primary agglomerates shows a strong, field dependent increase of viscosity, which in turn is dependent on the shear-rate. The other fraction shows a very weak increase of viscosity, and the shear-rate dependence vanishes. Tables 1, Figs 5, Refs 9.