Magnetic field induced rotations in ferrofluids

An experimental study has been made of the fluid rotations observed at the surface of a water-based ferrofluid (μ₀M_s=0.006 T) when the fluid is in a rotating magnetic field with values up to 0.03 T and a frequency of 50 to 500 Hz. It is shown that the fluid rotation is surface-driven and that the rotation rate increases with both frequency and field. The direction of rotation is determined by the surface curvature. The study has been extended to include analysis of the rotational behavior of micron-size nonmagnetic particles immersed in ferrofluid in the same rotating field conditions. In this case, the torque at a sufficiently high frequency acting at the particle-ferrofluid interface produces a rotation of particles counter to the field direction. The corotation with field of small particles, <30 μm in diameter, is attributed to an opposing torque which is the dominant torque at low frequencies. The experimental observations are taken as conclusive evidence of the presence of a torque due to nonequilibrium tangential surface stress