Drag enhancement due to macro-chains in uniformly magnetized ferrofluids

Abstract We report on experiments and simulations performed on small non-magnetic glass balls and nearly spherical gas bubbles moving through a uniformly magnetized ferrofluid. Use of the Advanced Photon Source X-ray beamline at Argonne National Laboratory permitted sufficient spatial and temporal resolution to accurately track the dynamics of these 500 μ m diameter spheres simultaneously with an array of magnetic particle macro-chains —agglomerations each of several mm long and 4 – 20 μ m thick. The enhanced drag induced by the macro-chains is substantial: we infer viscosity coefficients up to four times larger than for unmagnetized fluid. We provide direct visualization of a possible mechanism by which macro-chains mechanically impede the transverse motion of spheres, enhancing the drag and effecting an anisotropic viscosity. Direct numerical simulations of spheres falling through magnetic fluid can reproduce the observed dynamics by means of a phenomenological magnetization-dependent viscosity model with one free parameter.