Shear free measurement of the rotational viscosity of ferrofluids with a forced torsional pendulum

J. Embs^1,2 - H. W. Müller³ - M. Lücke¹ - K. Knorr²

¹ Theoretische Physik, Universit?t des Saarlandes, Postfach 151150, D-66041 Saarbr?cken
² Technische Physik, Universit?t des Saarlandes, Postfach 151150, D-66041 Saarbr?cken } {Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz

Abstract
The rotational viscosity of different ferrofluids undergoing shear free solid-body rotation is determined directly by measuring the associated magneto-viscous dissipation rate in a resonance experiment with a torsional pendulum. The data are fitted to a simple mesoscopic model for the related transverse magnetic relaxation time. In this way the average size of the magnetic grains and their hydrodynamic diameter are deduced from in situ measurements of undiluted samples. Tables 2, Figs 4, Refs 16.