The capillary-driven thinning and breakup of polymeric fluids has captured the attention of scientists for decades due to the puzzling polymer dynamics ruling the thinning of fluid filaments. During filament thinning, stretching of the polymer chains leads to the elasto-capillary (EC) regime where the diameter at the neck of the filament, D, decays exponentially over time (t) as D~exp(-t/EC). The most controversial aspect is how the fluid self-selects the elasto-capillary time scale EC based on the polymer conformation and concentration. Complexities arise from (i) the use of polydisperse polymers in most experimental studies and (ii) the strong dependence of EC on polymer flexibility (e.g., flexible vs inelastic polymers).

In this work we overcome these challenges by using mono-, bi-, and polydisperse DNAs as model semi-flexible polymers to investigate the polymer dynamics governing the EC regime. By measuring EC as a function of DNA concentration c, we show that both mono- and polydisperse DNAs follow the same scaling ECc0.7 across the dilute, semi-dilute and entangled regimes. This scaling ECc0.7 persisting over the three concentration regimes leads us to hypothesize that DNA polymers in the EC regime are highly stretched and insensitive to inter-polymer interactions. For the polydisperse DNA solutions, the observed scaling is explained by a simple summation model that accounts for the time scale contribution from each DNA size. This summation model is validated using well-defined bi-disperse DNA solutions and proposed as a suitable framework for analyzing semi-flexible polymers, including most biopolymers and polyelectrolytes.