Magnetoviscosity and relaxation in ferrofluids

The increase in viscosity of a ferrofluid due to an applied magnetic field is discussed on the basis of a phenomenological relaxation equation for the magnetization. The relaxation equation was derived earlier from irreversib le thermodynamics, and differs from that postulated by Shliomis. The two re laxation equations lend to a different dependence of viscosity on magnetic field, unless the relaxation rates are related in a specific field-dependen t way. Both planar Couette flow and Poiseuille pipe flow in parallel and pe rpendicular magnetic field are discussed. The entropy production for these situations is calculated and related to the magnetoviscosity.