Dynamic behavior of simple magnetic hole systems

Abstract

Experiments on a system of two magnetic holes (nonmagnetic microspheres in ferrofluid) subject to a rotating magnetic field show various types of behavior depending on the driving frequency. For two spheres (holes) mechanically bound together the stable rotation mode at low frequencies is replaced above a critical frequency by a mode with alternating rotation directions. This is described by simple nonlinear equations, and simulations show good quantitative agreement with the experiments. For two free spheres not bound together one observes a transition to a mode in which the spheres undergo both angular and radial motion around the center of mass of the system. Depending on the frequency and the anisotropy of the rotating magnetic field the motion passes through a sequence of states with sphere rotation mode-locked to the driving frequency at different ratios in good qualitative agreement with simulations. Adding a constant magnetic field normal to the plane causes the minimum sphere separation to increase from contact to a finite value. At high frequencies the spheres are influenced by an effective magnetic potential, giving rise to a static particle separation proportional to the normal component of the field.