How do polymers stretch in capillary-driven extensional flows?

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.