Magnetoviscosity and thread-like agglomerations in ferrofluids

We report on experiments and simulations performed on small non-magnetic glass balls falling under gravity through a magnetized ferrofluid. The applied magnetic field is oriented horizontally, normal to the fall, and is uniform but its magnitude can be adjusted over a wide range. Using the Advanced Photon Source x-ray beamline at Argonne, we were able to achieve sufficient spatial and temporal resolution to track the dynamics of these $500 \mu$m diameter spheres simultaneously with an array of magnetic particle {\em macro-chains} -- thread-like agglomerations each several mm long and $2-10 \mu$m thick. The enhanced drag induced by the macro-chains is enormous: up to four times larger than for unmagnetized fluid, a value greater than is predicted by the prevailing magneto-viscosity model. We provide direct visualization of a possible mechanism by which macro-chains impede the transverse motion of spheres. Numerical simulations can reproduce the observed drag, without modeling it physically, by implementing a simple magnetization dependent anisotropic viscosity.