On the dynamics of magnetic colloidal particles and holes

Having characterized the macroscopic behavior of a ferrofluid, giving expressions for the viscosities, as well as for the average of its magnetization, we analyze the dynamics of a non-magnetic particle (magnetic hole) suspended in a ferrofluid. Although the particles are not magnetic, when they are suspended in a carrier ferrofluid they acquire an induced magnetic moment equal to the magnetic moment of the ferrofluid volume they displace. The rotational dynamics of the particle is strongly influenced by the presence of non-uniform or rotating magnetic fields. For instance, it turns out that the holes rotate in the opposite direction to that of the field. This result is corroborated by recent experiments. We have compared the dependence of the angular velocity of the hole on the field frequency and strength to these experimental data, obtaining quite a good agreement in the frequency range we are considering.