NEGATIVE-VISCOSITY EFFECT IN A MAGNETIC FLUID

The first experimental evidence of a ''negative-viscosity'' effect in a magnetic fluid is presented here. In a Poiseuille flow, a constant m agnetic field balances vorticity and impedes the rotation of individua l magnetic particles. Conversely, an alternating magnetic field helps vorticity and favors this rotation: The magnetic energy is partially t ransformed into the angular momentum of the particles, which in turn i s converted into a hydrodynamic motion of the fluid. It manifests itse lf in a decrease of the total viscosity: Its rotational part becomes n egative. The theory developed here corroborates the experimental resul ts.