Geometrical effects in the measurements of mechanical properties in parallel superposed flows

The shear‐dependent complex viscosity η*12, which governs the linear time‐dependent stresses in a viscoelastic fluid undergoing infinitesimal shearing oscillations in‐line with a viscometric motion, has been investigated. Formulas relating η*12 to measured quantities have been derived for the cone‐and‐plate and parallel‐plate geometries. In the former case, the formula has the same form as that found when there is no steady shearing present. For the parallel‐plate geometry, the formula is different because of the radial variation in the viscometric shear rate. In order to determine the empirical significance of this formula, η*12 data were obtained for two fluids, NBS nonlinear sample No. 1 and a 4.3 g/dl aqueous polyethylene oxide (molecular weight, 5 X 106) solution using a Weissenberg rheogoniometer Model R‐18. Data were acquired and analyzed using a minicomputer on‐line with the rheogoniometer. The obtained data indicate that good agreement between the η*12 measured using the two geometries is obtained only if the derived formulas are used. Also, it is shown that theoretical results of simple‐fluid theory are valid for these data.