Investigating and modeling the thixotropic behavior, microstructure, and rheology of complex material
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
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.