SPURT PHENOMENA OF THE JOHNSON SEGALMAN FLUID AND RELATED MODELS

We examine the behavior of viscoelastic fluid models which exhibit local extrema of the steady shear stress. For the Johnson-Segalman and Giesekus models, a variety of steady singular solutions with jumps in shear rate are constructed and their stability to one dimensional disturbances analyzed. It is found that flow-rate versus imposed stress curves in slit-die flow fit experimental observation of the 'spurt' phenomenon with some precision. The flow curves involve a linearly stable singular solutions, but some assumptions on the dynamics of the spurt process are required. These assumptions are tested by a semi-implicit finite element solution technique which allows solutions to be efficiently integrated over the very long time-scale involved. The Johnson-Segalman model with added Newtonian viscosity is used in the calculations. It is found that the assumptions required to model spurt are satisfied by the dynamic model with added Newtonian viscosity is used in the calculations. It is found that the assumptions required to model spurt are satisfied by the dynamic model. The dynamic model also displays a characteristic 'latency time' before the spurt ensues and a characteristic 'shape memory' hysteresis in load/unload cycles.