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"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.