  Measurements of the capillary-driven thinning and breakup of fluid filaments
are widely used to extract extensional rheological properties of complex
materials. For viscoelastic (e.g., polymeric) fluids, the determination of the
longest relaxation time depends on several assumptions concerning the polymeric
response to the flow that are derived from constitutive models. Our capillary
thinning experiments using polymeric fluids with a wide range of extensibility,
suggest that these assumptions are likely only valid for highly extensible
polymers but do not hold in general. For polymers with relatively low
extensibility, such as polyectrolytes in salt-free media, conventional
extrapolation of the longest relaxation time from capillary thinning techniques
leads to a significant underestimation.
