Magnetoviscosity of Magnetic Colloids

Abstract

We have observed that the viscosity of a dilute (≈ 1015 particles cm−3) colloidal suspension of single domain ferromagnetic particles is a marked function of both the magnitude and direction of an externally applied magnetic field. For particles with dimensions of a few hundred angstroms, the viscosity saturates in a field of order 1 kG. The additional viscosity is greater by a factor of 2 for the field parallel to the flow than with it perpendicular. This magnetoviscous effect is qualitatively accounted for by the hindrance to particle rotation caused by the magnetic torque.