EXTENSIONAL DEFORMATION OF NON-NEWTONIAN LIQUID BRIDGES

A numerical method for simulating the extensional dynamics of elongati ng filaments of non-Newtonian fluids in a filament stretching rheomete r is presented. The boundary element method, in conjunction with eithe r the Oldroyd-B or the generalized multimode Upper-Convected Maxwell c onstitutive model, is used to calculate the transient evolution of the liquid interface, the applied force on the stationary end plate and t he polymeric stresses. The numerical results are compared to experimen tal results and are in excellent agreement at low Hencky strains (Newt onian response) but provide less accurate modeling of the stress growt h observed in experiments at higher strains. A comparison of different methods for measuring the apparent extensional viscosity from global measurements of the net force and the mid-point radius of the filament is presented. At large strains calculations show that the fluid motio n in these devices closely approximates ideal uniaxial elongation.