Distinguishing thixotropy from viscoelasticity

Abstract

Owing to non-linear 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 modelling 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 shear rate and shear start-up at different waiting times elapsed since pre-shear, are inadequate to distinguish thixotropy from viscoelasticity. In marked contrast, we show that the application of step-strain or step-stress after cessation of pre-shear 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 pre-shear 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.

New insights on carbon black suspension rheology -- anisotropic thixotropy and anti-thixotropy

“…step-down in shear rate, and the same is true for the step-up test. This results in an indistinguishable stress response for thixotropic, anti-thixotropic and viscoelastic materials 47 .…”

The Role of Structure in Polymer Rheology: Review

“…Formally, “pure” thixotropy is observed if a time-dependent viscous stress only takes place with stress relaxation or recovery (elastic) strain [ 26 ]. One can find a detailed discussion concerning the difference between thixotropy and other time-dependent processes in [ 27 , 28 ].…”

“…In this review, we do not list a great number of experimental studies because one can find many references in publications [ 26 , 27 , 28 , 29 , 30 , 31 , 34 , 36 , 44 ], thus, there is no sense in repeating them here. The flood of studies of thixotropic behavior in various technological liquids continues, due to the necessity to classify and standardize these objects.…”

Viscoelasticity and Rheological Hysteresis

“…In the literature, the subject of hysteresis in viscoelastic materials has been studied in detail both experimentally and theoretically. On an independent but related note, it has been well established that thixotropy, which concerns increase in viscosity under no flow or weak flow conditions and decrease in viscosity under strong flow conditions, shows many behavioural similarities with viscoelasticity [1][2][3][4]. In particular, thixotropic materials also show hysteresis in cyclic shear rate sweep experiment.…”

“…Specifically, the area enclosed by a hysteresis loop as a function of step time in a stepwise down-up ramp has been proposed to show some unique features for a thixotropic material. In our earlier study [4], we assessed four different experimental protocols that have been proposed to distinguish thixotropy and viscoelasticity. We found that stress relaxation and/or creep experiments at different waiting times lead to unique signatures for thixotropic vis-a-vis viscoelastic materials.…”