Field-induced force-suppression in ferromagnetic colloids

We have analyzed the behavior of a colloidal ferromagnetic particle in a rotational flow and found a dynamic transition upon variation of a static applied magnetic field. Beyond the transition point the force exerted by the field on the particle does not increase, as usual, but decreases as the strength of the field is increased. Macroscopically, this behavior results in an effective viscosity that decreases as a function of the strength of the field.