Paper Number
SC24 

Session
Suspensions and Colloids

Title
Orthogonal superposition to distinguish thixotropy, anti-thixotropy, and viscoelasticity

Presentation Date and Time
October 11, 2022 (Tuesday) 11:10

Track / Room
Track 1 / Sheraton 4

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, Mechanical Science and Engineering)

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

Speaker / Presenter 
Wang, Yilin

Keywords
experimental methods; theoretical methods; colloids; gels; suspensions

Text of Abstract
This study resolves a long-existing ambiguity of experimentally observed transient stress growth-then-decay of carbon black suspensions in step-down shear flow by theoretically considering canonical signatures of thixotropy, anti-thixotropy, and viscoelasticity in orthogonal superposition (OSP) rheometry. Using OSP, we experimentally reveal that anti-thixotropic shear-induced structuring of particles (Vulcan XC-72 in mineral oil) is responsible for the decay dynamics, ruling out a long list of other possible hypotheses including experimental artefacts. We find that anti-thixotropy and viscoelasticity cannot be distinguished from the step shear test alone, even though it is commonly used to distinguish between thixotropy and viscoelasticity. But a step change in rate combined with OSP provides more insights and is potentially a promising way to identify different causes of the transient dynamics. However, no systematic study of OSP for thixotropy, anti-thixotropy, and viscoelasticity is available. Therefore, to understand such signatures with OSP, we theoretically consider different canonical models of thixotropy and nonlinear viscoelasticity in OSP, from which we show that OSP can separate thixotropic and viscoelastic timescales. Combining this theoretical insight with experiments, we reveal how the long-time stress decay observed in CB suspensions is associated with anti-thixotropy, rather than viscoelasticity. We also experimentally measure the orthogonal moduli of different materials, including a colloidal suspension of Laponite and a polymer solution of PEO under different applied shear conditions, to verify the credibility of using OSP to distinguish between thixotropy, anti-thixotropy, and viscoelasticity.