ORTHOGONAL VERSUS PARALLEL SUPERPOSITION MEASUREMENTS

The non-linear flow behaviour of viscoelastic fluids can be studied in detail by means of a perturbation analysis. For that purpose small am plitude oscillations can be superimposed on the steady-state shear flo w. In this manner, detailed information about the spectral content of the material under the non-linear steady flow is obtained. The oscilla tory flow can be either parallel or perpendicular to the main shear fl ow. Devices for both types are becoming readily available and apparent ly are being used without realizing the intricate nature of these flow s. An analysis shows that linear superposition moduli do not obey the basic rules of linear viscoelasticity. This includes deviations from t he Kramers-Kronig relation and from the usual relation between steady- state and dynamic viscosities. This is demonstrated on the basis of a Wagner I model for which analytical solutions of the superposition mod uli can be derived. Other models give different results, consequently superposition flows could be used for the critical evaluation of rheol ogical models. Preliminary data for both parallel and orthogonal super position flows on a polyisobutene solution illustrate the potential of this technique. The relation between parallel and orthogonal superpos ition moduli derived by Bernstein for the K-BKZ model seems to be in a greement with the data. The results offer a potential for further theo retical work. The data also suggest that a physical interpretation of superposition moduli is not straightforward. (C) 1998 Elsevier Science B.V. All rights reserved.