Frictional torque on a sphere rotating in a uniformly magnetized ferrofluid

We analyze the frictional torque experienced by a macroscopic sphere rotating steadily in a uniformly magnetized magnetic colloid. The magnetoviscous increment in the apparent shear viscosity is predicted to be some five times smaller than the corresponding increment when the sphere translates through the colloidal suspension. In each case the external magnetic field is parallel to the angular or linear velocity of the sphere.