title: Role of interactions in ferrofluid thermal ratchets.
abstract: Orientational fluctuations of colloidal particles with magnetic moments may be rectified with the help of external magnetic fields with suitably chosen time dependence. As a result a noise-driven rotation of particles occurs giving rise to a macroscopic torque per volume of the carrier liquid. We investigate the influence of mutual interactions between the particles on this ratchet effect by studying a model system with mean-field interactions. The stochastic dynamics may be described by a nonlinear Fokker-Planck equation for the collective orientation of the particles which we solve approximately by using the effective field method. We determine an interval for the ratio between coupling strength and noise intensity for which a self-sustained rectification of fluctuations becomes possible. The ratchet effect then operates under conditions for which it was impossible in the absence of interactions.
authors.affiliation: Charite, Augustenburger Platz 1, 13353 Berlin, Germany. volker.becker@charite.de
authors.forename: Volker
authors.initials: V
authors.lastname: Becker
authors.forename: Andreas
authors.initials: A
authors.lastname: Engel