New viscosity measurement: The oscillating magnetically suspended sphere

A vertically oscillating spherical buoy, magnetically suspended, is used in a new measurement of viscosity. Thus far it has been applied only to transparent, dilute aqueous biomolecular solutions of 0.5-ml volume. With a different position transducer and sufficient attention to the hydrodynamics, it is possible that these limitations could be transcended. In the method, the phase lag of the motion relative to a sinusoidal magnetic driving force is measured with a phase-lock amplifier. This has been effective with amplitudes of motion from 100 Å to 1.8×10−3 cm. At the lowest amplitude noise reduces the precision, but different position sensors could improve that. Motion with larger amplitude risks invoking more complicated boundary layer effects, but has not been necessary thus far for precision at the level of two or three parts in 103 relative viscosity. This level of precision has been confirmed with aqueous sucrose and propanol solutions, and the output versus viscosity is linear at least to this level. Practical difficulties in the design of an instrument with such small volume has been our present limitation, both in precision and in the lack of ease of data taking. In this paper we present a simplified theory of operation, the design of the instrument, results of calibration runs, noise behavior and estimated limitations, and a discussion of improvements which might increase the precision about a factor of 10.