Paper Number
AR17 

Session
Applied Rheology and Rheology Methods

Title
Fingerprints of thixotropy, anti-thixotropy, and viscoelasticity: A comparison of protocols

Presentation Date and Time
October 12, 2021 (Tuesday) 10:40

Track / Room
Track 2 / Ballroom 7

Authors (Click on name to view author profile)

1. Wang, Yilin (University of Illinois at Urbana Champaign, Mechanical Science and Engineering)
2. Ewoldt, Randy H. (University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering)

Author and Affiliation Lines (in printed abstract book)
Yilin Wang and Randy H. Ewoldt
Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801

Speaker / Presenter 
Wang, Yilin

Keywords
experimental methods; colloids; rheology methods; suspensions

Text of Abstract
We explore different ways to distinguish between thixotropy, anti-thixotropy, and viscoelasticity. A step shear rate test is typically used to differentiate between thixotropic and viscoelastic fluids; however, it cannot differentiate between anti-thixotropy and viscoelasticity. Orthogonal superposition (OSP) combined with a step change in shear rate can uniquely identify anti-thixotropy, but cannot distinguish thixotropic and viscoelastic signatures. Here we show that hysteresis loops, which are obtained by continuously ramping shear rate up-then-down, or down-then-up, are another promising way to contrast the three dynamics. While there is no universal model for each of the three dynamics, we explore signatures of the most basic thixotropic, anti-thixotropic, and viscoelastic fluid models: a thixotropic visco-plastic kinetic model, an anti-thixotropic visco-plastic kinetic model, and the viscoelastic Jeffreys model. We describe first how establishing an equilibrium at high shear rates, then ramping down-then-up, simplifies features of the hysteresis loops. From these, we identify two distinguishing features. The first is a binary feature of clockwise versus counterclockwise loops: clockwise for thixotropy, but counterclockwise for viscoelasticity and anti-thixotropy. A second feature is therefore required to distinguish the latter two dynamics, and this is achieved at high ramping rates, which we interpret as either a high viscoelastic Deborah number for which elastic stress does not have time to relax, or a high thixotropic Deborah number for which the thixotropic structure does not have time to change. The distinguishing fingerprint features are observed independent of the model details. We establish further evidence for these hysteresis loop signatures by experimentally measuring different materials covering the range of behavior. The protocols explored here can be used to distinguish thixotropic, anti-thixotropic, and viscoelastic dynamics under shear, which have different microstructural origins.