Oscillatory Extensional Rheometry*

We continue to analyze a promising method to measure fluid parameters kinematically with oscillatory extensional rheometry. The axisymmetric free surface is optically determined and its shape is fit to linear theory to determine viscosity, surface tension and relaxation and retardation times.  Now we time march the 1D nonlinear equations to determine the limits of linearity, challenges in reaching a steady limit cycle, symmetry and effect of perturbations.  Our 1D Crank-Nicolson model is second-order in space and time and often requires very small time steps and sometimes backward differencing. We also extend our linear model by using multiple frequencies to obtain multiple relaxation times.