Torque Measurements in Viscous Ferrofluids

Ferrofluids are a class of magnetically-active complex fluids consisting of suspensions of magnetic nanoparticles in Newtonian carrier fluids. We will report measurements of torque in medium to high viscosity ferrofluids in a uniform rotating magnetic field created by a two-pole magnetic induction motor stator, as a function of: geometry and applied field amplitude and frequency. The objective of these experiments is to assess the adequacy of the commonly accepted balance and constitutive equations for the magnetic relaxation under situations in which the product of Brownian relaxation time and frequency is of order unity, ôÙ~1. Previously reported work has focused solely in low viscosity fluids, for which ôÙ~0. We measured the torque needed to restrain a cylindrical spindle immersed in the fluid and to restrain a cylindrical container filled with the fluid. Various geometric aspect ratios and fluid viscosities were used in order to test recently derived analytical theories for the flow of ferrofluids in these situations.