Normal stresses in semiflexible polymer hydrogels

TL;DR

This paper develops a microscopic model for normal stresses in semiflexible polymer hydrogels, explaining their inverse relation to nonlinear shear strain and transient behavior, validated through experiments on fibrin gels.

Contribution

It extends existing models by providing a microscopic understanding of normal stresses in semiflexible hydrogels, linking stress components to strain and transient dynamics.

Findings

Normal stress magnitude inversely related to nonlinear shear strain onset

Model accurately predicts transient normal stress behavior

Experimental validation on fibrin gels confirms model predictions

Abstract

Biopolymer gels such as fibrin and collagen networks are known to develop tensile axial stress when subject to torsion. This negative normal stress is opposite to the classical Poynting effect observed for most elastic solids including synthetic polymer gels, where torsion provokes a positive normal stress. As recently shown, this anomalous behavior in fibrin gels depends on the open, porous network structure of biopolymer gels, which facilitates interstitial fluid flow during shear and can be described by a phenomenological two-fluid model with viscous coupling between network and solvent. Here we extend this model and develop a microscopic model for the individual diagonal components of the stress tensor that determine the axial response of semi-flexible polymer hydrogels. This microscopic model predicts that the magnitude of these stress components depends inversely on the characteristic strain for the onset of nonlinear shear stress, which we confirm experimentally by shear rheometry on fibrin gels. Moreover, our model predicts a transient behavior of the normal stress, which is in excellent agreement with the full time-dependent normal stress we measure.