Distinguishing thixotropy from viscoelasticity

TL;DR

This study evaluates existing methods to distinguish thixotropy from viscoelasticity in materials, proposing a new criterion based on step-strain or step-stress tests that effectively differentiates the two behaviors.

Contribution

It introduces a novel experimental criterion using step-strain or step-stress tests to reliably distinguish thixotropic from viscoelastic materials, validated through experiments and modeling.

Findings

Step-strain or step-stress tests after preshear effectively differentiate thixotropy from viscoelasticity.

Thixotropic materials show slowing relaxation dynamics with waiting time.

Viscoelastic materials exhibit a shift in relaxation modulus to lower values over time.

Abstract

Owing to nonlinear viscoelasticity, materials often show characteristic features that resemble those of thixotropy. This issue has been debated in the literature over the past several decades, and several experimental protocols have been proposed to distinguish thixotropy from viscoelasticity. In this work, we assess these protocols by carrying out experiments using polymer solutions, thixotropic clay dispersions, and modeling their behavior, respectively, using the FENE-P constitutive equation and a viscoelastic aging model. We find that the criteria proposed in the literature, such as a step down jump in the shear rate and shear startup at different waiting times elapsed since preshear, are inadequate to distinguish thixotropy from viscoelasticity. In marked contrast, we show that the application of step-strain or step-stress after cessation of the preshear serves as a useful discriminant between thixotropy and viscoelasticity. In thixotropic materials, we observe that the application of step strain (or step stress) after cessation of the preshear eventually leads to slowing down of relaxation dynamics as a function of waiting time. However, for viscoelastic materials, the relaxation modulus (creep compliance) curve shifts to lower modulus (higher compliance) values as a function of waiting time until equilibrium is reached. While the proposed criterion offers a robust distinction between viscoelasticity and thixotropy for the systems studied here, further experimental investigations based on other systems are needed to establish its versatility and will lead to a greater insight into this long-standing issue in rheological categorization.