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 magnetic field balances vorticity and impedes the rotation of individual magnetic particles. Conversely, an alternating magnetic field helps vorticity and favors this rotation: The magnetic energy is partially transformed into the angular momentum of the particles, which in turn is converted into a hydrodynamic motion of the fluid. It manifests itself in a decrease of the total viscosity: Its rotational part becomes negative. The theory developed here corroborates the experimental results.