Influence of superimposed steady shear flow on the dynamic properties of non-Newtonian fluids

Abstract

Experiments in which an oscillatory shear flow is superimposed on a steady-state circular shear flow between a cone and a plate were performed on non-Newtonian solutions by means of aWeissenberg Rheogoniometer. The steady-state shear stress and in a first approximation also the normal stress difference arising from the steady shear flow appear not to be influenced by the superimposed oscillatory flow. On the other hand, the dynamic moduli as obtained from the oscillatory parts of shear stress and shear flow are highly dependent on the superimposed steady rate of shear. The absolute value of the complex shear modulus decreases and the phase difference between oscillatory shear stress and shear flow increases in all cases and for all frequencies if the superimposed shear rate is increased. Consequently, this phase difference can become equal to and even larger than π/2. Between the angular frequency ω0 at which the phase difference is π/2 and the steady shear rateq the relation ω0= 1/2,q was experimentally found to exist in most cases. These dynamic results cannot be described by the current theories of viscoelasticity. The large and fast deformations imposed on the material should explicitly be taken into account.