Field-induced force-suppression in ferromagnetic colloids

We have analyzed the behavior of a colloidal ferromagnetic particle in a ro tational flow and found a dynamic transition upon variation of a static app lied magnetic field. Beyond the transition paint the force exerted by the f ield on the particle does not increase, as usual, but decreases as the stre ngth of the field is increased. Macroscopically, this behavior results in a n effective viscosity that decreases as a function of the strength of the f ield. (C) 2001 Elsevier Science B.V. All rights reserved.