How to obtain the elongational viscosity of dilute polymer solutions?

Measurement of the resistance to a stretching motion, the so-called elongational viscosity ηe, has proven to be difficult for dilute polymer solutions, although it is important for a large number of applications. We attempt to deduce ηe of dilute solutions of flexible polymers in two different ways. First, ηe can be inferred from measurements of the shear viscosity η and the first normal stress difference N1, invoking an appropriate constitutive equation. Second, it is measured in the opposing jet elongational viscometer. Although the results of the latter method have been much debated they agree reasonably well with those from the first method. The combination of the two methods thus allows to obtain the correct order of magnitude of ηe.