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 the extensional rheological properties of complex materials. For viscoelastic fluids, such as polymer solutions, the longest relaxation time of the polymer is inferred from the decay rate of the filament diameter in the elastocapillary thinning regime. However, this determination relies on assumptions from constitutive models that are challenging to validate experimentally. By comparing the response of fluids in capillary thinning with that in a microfluidic extensional flow (in which the polymeric dynamics can be readily assessed), we show experimentally 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 polyelectrolytes in salt-free media, the conventional extrapolation of the longest relaxation time from capillary thinning techniques leads to a significant underestimation. We explain this discrepancy by considering the macromolecular dynamics occurring in the initial Newtonian-like thinning regime prior to the onset of elastocapillarity.