Elongational flow behavior of viscoelastic liquids: Part II. Definition and measurement of apparent elongational viscosity

The collapse of a single spherical gas bubble within a large body of fluid creates a uniaxial elongational flow in the surrounding fluid. Collapse under constant bubble pressure is observed to produce nearly constant strain rate kinematics. If the steady state stress at the bubble/fluid boundary could be measured, it would be possible to estimate the elongational viscosity of the fluid. Unfortunatley the experiment does not yield the desired stress data, except in the special case of the Newtonian fluid. We are led to define an apparent elongational viscosity, and for specific constitutive models we can evaluate the deviation between the apparent and true viscosity. The experimental and theoretical work indicate that the apparent elongational viscosity provides a good estimate of true elongational viscosity for the tow viscoelastic solutions studied so far.