Comment on ``Tangential stresses on the magnetic fluid boundary and rotational effect''

C. Rinaldi - A. Chaves

Department of Chemical Engineering, University of Puerto Rico, Mayagüez, PO Box 9046, Mayagüez, Puerto Rico USA

Abstract
We demonstrate that the expression for the torque on the inner cylinder surrounded by a ferrofluid in an annular gap given by Pshenichnikov and Lebedev [1] is incorrect because their integration constants for the magnetic field and magnetization are erroneous. The general three-region cylindrical problem is solved to obtain correct expressions for the magnetic field and general expressions for the torque on the inner cylinder and on the outer cylinder walls enclosing the ferrofluid. These results are shown to agree with the expression reported by Rinaldi et al. [4]. Finally, we demonstrate that the expression of Pshenichnikov and Lebedev for the torque on the inner cylinder has the same qualitative features as the correct expression derived here, and is quantitatively correct in the limit of small Ωτ observed in their experiments, hence, the interpretation of their experimental observations and the resulting conclusions are unaffected by the error in their derivation. Refs 4.