Shear dependence of field-induced contributions to the viscosity of magnetic fluids at low shear rates

Viscoelastic properties of ferrofluids are an upcoming field of scientific interest, since the magnetic control of the related fluid behavior would give rise to new applications as well as for new possibilities in basic research concerning viscoelasticity. We have constructed a specialized rheometer for the investigation of fluids under the influence of magnetic fields, to examine such effects in stable suspensions of magnetic particles. In particular we will report the change of field-induced increase of viscosity due to variation of the shear rate applied to the fluid. The results show that the available theoretical approach, namely the concept of rotational viscosity, is not valid for the description of the field-induced increase of viscosity in concentrated fluids at low shear rates. (C) 1998 Elsevier Science B.V. All rights reserved.