Physica A 293, 51 (2001)

Field-induced force-suppression in ferromagnetic colloids,

Leonor Saiz(a), J. M. G. Vilar(b), and J.M Rubí(c)

(a) Center for Molecular Modeling, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323  USA

(b) Department of Molecular Biology, Princeton University, Princeton, NJ 08544  USA

(c) Departament de Física Fonamental, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona,  Spain

We have analyzed the behavior of a colloidal ferromagnetic particle in a rotational flow and found a dynamic transition upon variation of a static 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.