Paper Number
SC27
Session
Suspensions, Colloids, and Granular Materials
Title
Thixotropic spectra and Ashby-style charts for thixotropy
Presentation Date and Time
October 13, 2021 (Wednesday) 11:05
Track / Room
Track 5 / Ballroom 6
Authors
- Sen, Samya (University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering)
- Ewoldt, Randy H. (University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering)
Author and Affiliation Lines
Samya Sen and Randy H. Ewoldt
Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801
Speaker / Presenter
Sen, Samya
Keywords
experimental methods; colloids; rheology methods
Text of Abstract
There is no universal model for thixotropy, so comparing thixotropic effects between different fluids is difficult. Specific model fits provide thixotropic timescales, but these alone are insufficient: a long recovery time suggests significant thixotropy, but it may be negligible if the rheological change due to thixotropy is small. We thus introduce a generalized (model-insensitive) framework for comparing thixotropic properties to assess its importance across different systems. The approach is based on thixotropic spectra, a superposition of exponential stress modes distributed over thixotropic timescales, to quantify buildup and breakdown times and mode strengths in response to step-change of shear rate tests. This mathematical framework is tested with several experimental step-shear datasets on colloidal suspensions, including yield-stress fluids such as fumed silica and carbon black. Discrete and continuous spectra obtained from regularized data fits are considered as a function of initial and final shear rates probed, including parameterized continuous spectra and the use of credibility metrics (such as BIC) to assess model credibility based on a balance between goodness of fit and the number of model parameters involved. Common models, such as stretched exponential fits, are a subset of this generalized thixotropic spectra framework. Low-dimensional metrics based on moments of the distribution serve as general thixotropic yardsticks, including average timescales, dispersity of timescales, and amount of thixotropic stress change, all of which are visualized on Ashby-style diagrams. These methods establish a framework to describe thixotropy across a diverse range of microstructures, validated here for aggregating colloidal suspensions but more broadly applicable, supporting scientific studies as well as material selection for engineering design applications.