Transient Flow Viscometry

Theoretical relations are derived between material properties and the torque measured in transient flow experiments using the parallel‐plate and the cone‐and‐plate geometries. The formulas are similar to well‐known expressions from viscometric flow theory, except in the parallel‐plate case where the differentiation with respect to edge shear rate is taken at a fixed time. Shear stress growth $σ+$ and shear stress relaxation $σ−$ functions were measured for two aqueous polyethylene oxide solutions and the NBS fluid 40 using both geometries attached to a Weissenberg rheogoniometer fitted with a piezoelectric load cell and interfaced to a microcomputer. The results show that $σ+$ and $σ−$ can be measured in the parallel‐plate geometry if the appropriate formulas are used to transform measured torques to material properties. Data from both geometries clearly indicate the existence of primary overshoot and undershoot and that the strain at which the primary overshoot maximum is obtained is the same for all three fluids over the range of shear rates studied.