Transition to the viscoelastic regime in the thinning of polymer solutions

TL;DR

This paper investigates the transition from Newtonian to viscoelastic behavior during droplet pinch-off in polymer solutions, revealing how polymer properties influence the critical strain rate and thinning dynamics.

Contribution

It provides empirical scaling laws for the critical strain rate based on polymer concentration, molar weight, solvent viscosity, and nozzle size, and analyzes the self-similar thinning dynamics at the transition.

Findings

Critical strain rate depends on polymer and solvent properties.

Thinning dynamics follow a self-similar pattern controlled by the inverse of the critical strain rate.

The characteristic time at transition is shorter than the polymer relaxation time.

Abstract

In this study, we investigate the transition between the Newtonian and the viscoelastic regimes during the pinch-off of droplets of dilute polymer solutions and discuss its link to the coil-stretch transition. The detachment of a drop from a nozzle is associated with the formation of a liquid neck that causes the divergence of the local stress in a vanishingly small region. If the liquid is a polymer solution, this increasing stress progressively unwinds the polymer chains, up to a point where the resulting increase in the viscosity slows down drastically the thinning. This threshold to a viscoelastic behavior corresponds to a macroscopic strain rate $\dot{\varepsilon}_{\rm c}$. In the present study, we characterize the variations of $\dot{\varepsilon}_{\rm c}$ with respect to the polymer concentration and molar weight, to the solvent viscosity, and to the nozzle size, i.e., the weight of the drop. We provide empirical scaling laws for these variations. We also analyze the thinning dynamics at the transition and show that it follows a self-similar dynamics controlled by the time scale ${\dot{\varepsilon}_{\rm c}}^{-1}$. This characteristic time is different and always shorter than the relaxation time of the polymer.