Remarks on two available extensional viscosity measurement techniques

Two different experimental methods for measuring extensional viscosity are examined, the entrance pressure drop method and the opposed-nozzle approach. The velocity and stress fields for the flows that occur in these geometries have been obtained by integrating the corresponding governing equations. The Galerkin method with finite element basis functions was employed in the integration. An inelastic constitutive relation with a new viscosity function was used to relate the extra stress with the flow field. The pressure drop and force that are measured in the experimental techniques are evaluated here by using the calculated pressure and stress fields. These quantities are then used in conjunction with the working equations of each technique to determine apparent extension viscosities and extension rates. The thus obtained extensional viscosity functions are compared with the ones used in the constitutive equation, to give an assessment of the accuracy of each technique. Results show that the entrance pressure drop method underestimates the extension viscosity of extension-thinning fluids and overestimates this material function when the fluid displays extension-thickening. The opposed-nozzle method is quite reliable at low to moderate extension rates. At high rates, shear thinning becomes important and affects the measured force.