A mesoscopic approach to the "negative" viscosity effect in ferrofluids

We present a mesoscopic approach to analyse the dynamics of a single magnet ic dipole under the influence of an oscillating magnetic field, based on th e formulation of a Fokker-Planck equation. The dissipated power and the vis cosity of a suspension of such magnetic dipoles are calculated from non-equ ilibrium thermodynamics of magnetized systems. By means of this method we h ave found a non-monotonous behaviour of the viscosity as a function of the frequency of the field which has been referred to as the "negative" viscosi ty effect. Moreover, we have shown that the viscosity depends on the vortic ity field thus exhibiting non-Newtonian behaviour. Our analysis is compleme nted with numerical simulations which reproduce the behaviour of the viscos ity we have found and extend the scope of our analytical approach to higher values of the magnetic field. (C) 1999 Elsevier Science B.V. All rights re served.