A charged finitely extensible dumbbell model: Explaining rheology of dilute polyelectrolyte solutions

TL;DR

This paper develops a new kinetic theory-based model for dilute polyelectrolyte solutions, incorporating electrostatic interactions into a finitely extensible dumbbell framework to explain their complex rheological behavior.

Contribution

The paper introduces a novel charged dumbbell model that accounts for electrostatic repulsion and solvent salinity effects, advancing the understanding of polyelectrolyte rheology.

Findings

Model predicts rheological features consistent with experiments.

Electric-to-elastic energy ratio $E$ influences material functions.

Includes limiting cases of uncharged and rigid dumbbells.

Abstract

A robust non-Newtonian fluid model of dilute polyelectrolyte solutions is derived from kinetic theory arguments. Polyelectrolyte molecules are modeled as finitely elongated nonlinear elastic dumbbells, where effective charges (interacting through a simple Coulomb force) are added to the beads in order to model the repulsion between the charged sections of polyelectrolyte chains. It is shown that the relative strength of this repulsion is regulated by the electric-to-elastic energy ratio, $E$, which is one of the key parameters of the model. In particular, $E$ accounts for the intrinsic rigidity of polyelectrolyte molecules and can be used to explain the impact of solvent salinity on polyelectrolyte rheology. With two preaveraging approximations, the constitutive equations of the resulting fluid model are formulated in closed form. Material functions predicted by the model for steady shear flow, steady extensional flow, small-amplitude oscillatory shear flow, and start-up and cessation of steady shear flow are obtained and investigated using a combination of analytical and numerical methods. In particular, it is shown how these material functions depend on $E$. The two limiting cases of the model -- uncharged dumbbells ($E=0$) and rigid dumbbells ($E\to \infty$) -- are included in the analysis. It is found that despite its simplicity, the model predicts most of experimentally observed rheological features of polyelectrolyte solutions.