Effect of flow kinematics on extensional viscosity of dilute polymer solutions

TL;DR

This study uses simulations to explore how different flow kinematics influence the extensional viscosity of dilute polymer solutions, revealing the roles of polymer conformation and flow structure in strain hardening behavior.

Contribution

It introduces an analytical relation linking extensional viscosity to polymer conformation and flow kinematics, clarifying their individual impacts.

Findings

Polymer solutions show strain hardening at high extension rates.

Differences in viscosity are mainly due to flow kinematics when polymers are unperturbed.

Polymer stretching significantly affects viscosity variations.

Abstract

We investigate the effect of flow kinematics on the extensional viscosity of dilute polymer solutions by conducting dissipative particle dynamics simulations under uniaxial, planar, and biaxial extensional flows. At high extension rates, dilute polymer solutions exhibit strain hardening under these flows, while the quantitative behavior depends on the flow type. To elucidate the physical origin of this flow-kinematics dependence, we relate the extensional viscosity to polymer conformation using an analytical expression derived from a single-chain model. The resulting relation allows us to separate the contribution of flow-induced polymer conformational changes and the purely kinematic contribution determined by the structure of the velocity gradient tensor. When polymers remain almost unperturbed by extensional flows, differences in the extensional viscosity are governed primarily by the purely kinematic effects. In contrast, as polymers are stretched, the gyration radius in the extensional direction becomes the dominant factor, and differences in the stretching degree in this direction lead to corresponding variations in the extensional viscosity.