On Anisotropic Mobility of Non-magnetic Microspheres in Ferrofluid

This article reports results demonstrating that inhomogeneities in the ferrofluid caused by the presence of tiny non-magnetic inclusions can change this behavior not only quantitatively, but qualitatively as well. The effect is studied here by observing Brownian motion of non-magnetic polystyrene microspheres confined to thin layers of ferrofluid (EMG 707, Ferrotech, 1% and 0.5% iron-oxide by volume) between two glass slides. Results show that microspheres´ mobility decreased with the applied field significantly. However, the observed mobility of the non-magnetic microspheres is up to 20% lower in the field direction compared with the direction perpendicular to the field. which contradicts the macroscopic Poiseuille flow.