Shear History Effects on Extensional Flow of Non-Newtonian Fluids in Filament Stretching Rheometers

Modifications to filament stretching rheometers have been used by several researchers in recent years to study the effect of preshear on the transient evolution of the micro structure and viscoelastic tensile stresses for moderately viscous polymer solutions. A typical preshear/extensional experiment usually includes three well-defined stages, namely a preshearing step, followed by exponential axial stretching and then stress relaxation and filament thinning. Measurements collected from these three stages provide a complete quantitative characterization of the effect of preshear history on the extensional rheological properties of the test fluid. In the present work, we simulate all the three stages of the preshearing/filament-stretching experiment using a transient free surface finite element approach. The 'swirl' velocity component due to the preshear is efficiently handled by a 2.5-dimensional axisymmetric finite element formulation, which incorporates the effects of viscoelasticity, surface tension, fluid inertia and a deformable free surface. The rheology of the Boger fluid used in the experiments is simulated by a single-mode FENE-P model with strong strain-hardening corresponding to large values of the extensibility parameter L. During the filament stretching stage, we consider the simplest 'type II' kinematics, with a simple exponential separation of the two plates. The simulation results demonstrate the strong effect of preshear on the evolution of the filament profile and the resulting extensional viscosity (Trouton ratio). The predicted effect of pre-shear on the transient extensional viscosity agrees qualitatively well with the available experimental data.