Wave-induced motion of magnetic spheres

We report an experimental study of the motion of magnetized beads driven by a travelling wave magnetic field. For sufficiently large wave speed, we report the existence of a backward motion, in which the sphere can move in the direction opposite to the driving wave. We show that the transition to this new state is strongly subcritical and can lead to chaotic motion of the bead. For some parameters, this counter-propagation of the sphere can be one order of magnitude faster than the driving wave speed. These results are understood in the framework of a model based on the interplay among solid friction, air resistance and magnetic torque.