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Title:Towards a deeper understanding of thixotropy via transient nonlinear rheology and scattering
Author(s):Choi, Jiho
Director of Research:Rogers, Simon A.
Doctoral Committee Chair(s):Rogers, Simon A.
Doctoral Committee Member(s):Kenis, Paul J.A.; Kong, Hyunjoon; Juarez, Gabriel
Department / Program:Chemical & Biomolecular Engr
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Sequence of physical processes
Data reduction scheme
Recoverable strain
Shear rate reduction
Transient elastic stress
Parallel superposition
Two-time correlation function
Box-counting method
Microstructural mechanism
Abstract:Thixotropy is exhibited by a wide range of soft materials that are important in a large number of applications, and yet remains incompletely understood. Central to the issue of understanding thixotropy is an under-development of suitable analytical and experimental tools, and a series of vague definitions. Without addressing methods of characterization, numerous models have been introduced to describe a limited set of experimental results. In search of a more profound understanding of thixotropy, in this dissertation a series of analytical, experimental, and structural tools is developed for thixotropic phenomena. Using these new tools, enhanced insights regarding conceptual and structural aspects are obtained. The experimental development of rheological protocols and in-situ rheology and x-ray photon correlation spectroscopy (rheo-XPCS) techniques are presented. A general framework for understanding transient rheological responses of soft materials is presented, based on the sequence of physical processes (SPPs) analysis method. Expressions of the SPP metrics under arbitrary deformation are presented in terms of time derivatives of deformation variables. The transient viscoelastic parameters for arbitrary deformation or loading protocols are significantly advantageous in the understanding of the time-dependent nature of thixotropy. Due to the importance of oscillatory shearing in transient nonlinear rheology, the unification of disparate rheological measures in oscillatory shearing is presented, such as the relation between the SPP metrics and Fourier-based metrics. The translation leads to the development of a data reduction scheme of SPP framework based on center, size, and orientation of a deltoid in a transient Cole-Cole plot and an enhanced way of displaying traditional amplitude sweep data. Rigorous experimental protocols for studying thixotropy in soft materials are also presented. Crucial to any study of thixotropy is the ability to reproducibly erase the shear history of the material via a procedure commonly referred to as ‘pre-shearing’. Success criteria for a good pre-shear protocol are presented with an experimental way of inspecting a biased state of a thixotropic material. Conditions of optimal pre-shearing are suggested and used to define a pre-shear protocol which is subsequently shown to lead to structural isotropy when applied. The bias that results from a commonly-used one-directional high rate protocol is removed by the addition of a step strain opposing the initial direction of shearing. Based on this insight regarding the bias formed by shear history, we also suggest an advanced experimental methodology for commonly-used rheological experiments. The convolution of material response and measurement protocol in those tests is resolved by separating every single point with our new pre-shearing method. A comprehensive series of stress jump tests is performed on a thixotropic colloidal suspension to determine thixotropic behaviors. The series of stress jumps tests is the most complete performed to date, encompassing jumps between three well-defined flow states, as well as incorporating the possibility of flow reversal. The elastic stress during the stress jumps is measured with utilization of the flow cessation method, providing a detailed data set that shows the importance of elasticity in thixotropy due to the resemblance between total stress and elastic stress. We further investigate the elastic contribution during the shear rate reduction tests with superimposed oscillation by integrating the time-dependent SPP modulus. The shape of the SPP elastic stress also mirrors the total stress in parallel superposition tests, as observed in the transient elastic tress measurement with flow cessation method. The SPP framework therefore offers a more efficient way of identifying elastic stress during the stepwise reduction in shear rate. Besides, it is a great advantage to obtain accurate and physically-meaningful viscoelastic metrics for parallel superposition tests through SPP analysis in comparison with traditional parallel superposition moduli acquired from subtracting the steady components from the total response. The results regarding parallel superposition tests further highlights the utility of the SPP approach to the study of thixotropy. To unveil how microscopic changes are linked to thixotropic behaviors, we use a hybrid rheo-XPCS experimental method for studying the weakly-flocculated fumed silica suspension. The structural correlation is seen to deviate significantly from the continuous exponential decay that is typically observed in complex fluids, and displays discrete decorrelating events that can be characterized by a fractal dimension. The fractal dimension of the structural correlation at the particle length scale is determined via a box-counting method. Flow cessation tests are performed with rheo-XPCS and fractal analysis to investigate the build-up process and quiescent state of thixotropic materials. In the quiescent state, stationary fractal dimensions are observed with respect to time. The increase of the fractal dimension corresponds the build-up of microstructure in the thixotropic dispersion such as flocculation and network formation. The use of fractal dynamics is a perfect fit for interpreting two-time correlation functions of thixotropic materials, in that it reflects the microstructural change not only in quiescence, but also from bulk-rheology. The rheo-XPCS technique and fractal analysis fulfill the need for establishing the microstructural technique which can study the structural dynamics of thixotropic materials at the particle length scale. Overall, this dissertation describes the development of useful tools such as the enhanced SPP framework, the pre-shearing protocol with recovery step and rheo-XPCS with fractal analysis, and direct experimental evidences underpinning the existence of thixoelasticity and microstructural mechanism of thixotropy. These robust foundational works pave the way for advancement of knowledge regarding thixotropy, therefore facilitating the availability to control, predict, and design of thixotropic materials.
Issue Date:2020-11-16
Rights Information:Copyright 2020 by Jiho Choi. All rights reserved.
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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