Rotational viscometry by torque relaxation at low shear rates with application to biological systems

A technique for obtaining viscometric data at very low shear rates from rotational viscometer torque decay is presented. It consists of measuring torque as a function of time and involves the assumption that the instantaneous shear rate is proportional to the rate of change of angular position. A perturbation scheme is used to show that the assumption introduces negligible error if the rate of decay is slow enough. Advantages of the technique are that (1) viscometric data at shear rates less than 1 sec⁻¹ can be obtained easily with the simplest of equipment, and (2) readings are obtained at once, no time delay to reach steady-state being involved. The scheme is illustrated with sample data for Newtonian and non-Newtonian fluids.