Transition to the viscoelastic regime in the thinning of polymer solutions

When a dilute polymer solution is extruded through a nozzle, the initial thinning of the liquid shows a Newtonian behavior, where the time evolution of the neck diameter can be fitted with a power-law.  The presence of polymer, however, inhibits the singularity expected in Newtonian fluids and the fluid transitions to a viscoelastic regime, where a long and slender cylindrical filament is formed and thins exponentially. The time scale of thinning of the filament is associated with the relaxation time of the dilute polymer. The present study focuses on the intermediate regime where the fluid undergoes a dynamic transition from the Newtonian to the viscoelastic behavior. The short time scales associated with the transitional regime require high-resolution imaging at large frame rates. We characterize here the influence of the polymer concentration, solvent viscosity and molecular weight on the length scale and timescale associated with the transition. We report a self-similar behavior for the instantaneous strain rate at the neck, where the critical strain rate is the relevant scale that allows us to rescale the experimental data. Such an approach is a useful tool to predict the thinning and drop formation of a non-Newtonian liquid, from the initial Newtonian thinning to the viscoelastic behavior.