Investigating and modeling the thixotropic behavior, microstructure, and rheology of complex material

Date
2015
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University of Delaware
Abstract
Thixotropic materials can be found everywhere around us, in the manufacturing industry, as well as in everyday life. This includes the petroleum industry, the food industry, personal care and soap industry, pharmaceuticals and paints, as well as highly radioactive, transuranic waste in several multi-billion dollar Superfund cleanup sites across the country. In addition many biological materials have been shown to exhibit thixotropic properties, like blood, offering the potential for another gateway into blood pathology diagnosis. To properly understand, and be able to predict the rheological behavior of these thixotropic materials, better models connecting to the underlying microstructure are required. Rheological and microstructural information can be gained in many ways, beginning with more elaborate experiments, both rheological and scattering, while better predictions must come from the development of better modeling frameworks and more accurate parameter estimations. This is exactly the objective of the present thesis. We first constructed two model thixotropic systems, following protocol from literature, a 2.9vol% fumed silica in paraffin oil and polyisobutylene, and a 3.23vol% carbon black in naphthenic oil. Both systems have been well characterized in literature to provide a solid basis for further investigation. Both systems have then been methodically tested subject to several linear and nonlinear rheological tests with the ARES G2 strain controlled, and DHR-3 stress controlled rheometers. Those included steady state, small amplitude oscillatory shear (SAOS), transient step-up and step-down in shear rate experiments. We have then extended the rheological testing of these model thixotropic systems to large amplitude oscillatory shear (LAOS) to gain additional information and obtain both systems rheological fingerprints. Two additional tests have been conducted to further investigate the material response and compare with model predictions: the flow reversal, and a novel unidirectional LAOS (developed here for the first time), or UD-LAOS. All of the experiments together have been conducted on the same samples allowing for the first time for such an extensive complete set of rheological data, spanning a full spectrum of linear, nonlinear, static, and dynamic tests. This provided for a unique test bed for thixotropic models development and validation. In parallel, a robust, parametric determination procedure has been developed and extensively validated that accurately determines, based on a global optimization process, the parameters of various user-defined models. Moreover, we developed a new structural parameter thixotropic model, the Modified Delaware Thixotropic Model (MDTM). The MDTM is based on previous work at Delaware, and incorporated some of the best thixotropic modeling features from contemporary literature. Using the parametric determination procedure, we then thoroughly tested the MDTM against three other representative models from literature. We showed the new model to be superior overall, over a wide variety of data. Still the predictions of LAOS of all the models were relatively poor at low strain amplitudes under conditions under which the structural contributions are especially important. To further probe the underlying physical reasons behind the current model inefficiencies in capturing LAOS, flow reversal and a newly developed UD-LAOS experiments have been used. With their help a hypothesis has been put forward that it is the strong microstructural rearrangement caused by the flow reversals that is not captured by the current scalar structural parameter-based models. In particular, it is conjectured that the current single scalar thixotropic models fail to capture flow reversal and LAOS experiments because of the extensive aggregate anisotropicity and structure breakdown caused by changes in the direction of flow deformation. Lastly, we show key structural differences between the steady state flow and LAOS rheological experiments performed on our model thixotropic systems against optical scattering experiments under flow, correlating two key structural metrics, the structural scalar parameter, lambda, from the model predictions, and the alignment factor, Af, from the scattering experiments. This information opens the door to a lot of opportunities for new model developments in the near future. With a better model, the MDTM, a better, generic and user-friendly parameter fitting framework, along with key correlations drawn from scattering experiments and structural model predictions, we successfully opened previously closed doors to the prediction and the understanding of microstructure of complex, thixotropic materials under flow. In particular, we are finally in a position to understand why an entire class of scalar, structure parameter, thixotropic models cannot simultaneously fit onedirectional flow, and LAOS experiments. Those observations led to our recommendation of the appropriate direction to take in future thixotropic modeling. This is the direction towards a tensorial-conformation-based tensor framework, with better ties to the underlying microstructure and developed in a thermodynamically consistent manner.
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