Applications of diffusing wave spectroscopy to complex fluids in industry
Date
2020
Authors
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Publisher
University of Delaware
Abstract
Complex fluids experience a variety of environmental conditions that impact a fluid’s rheology and microstructural formation or breakdown. However, conditions relevant to the material’s application are often outside the operating regime of mechan- ical rheometers. Light scattering microrheology increases the experimentally accessible conditions and can serve as a complementary technique to mechanical rheology. In a microrheology approach, probe particles are embedded into a sample and the mea- sured light intensity correlation arising from the Brownian motion is interpreted using the Generalized Stokes-Einstein Relation (GSER) to determine the viscoelastic re- sponse of the material. In this work, the application of diffusing wave spectroscopy (DWS) microrheology to hydraulic fracturing fluids and aqueous paints is discussed. Hydraulic fracturing fluids are polymer solutions and gels that are designed to trans- port and suspend solids, reduce friction, and prevent fluid loss. To address the need to characterize the viscoelasticity of fracturing fluids under high-temperature and high- pressure operating conditions, I developed a passive microrheology experiment capable of generating pressures up to 200 MPa. The apparatus incorporates a sealed steel alloy sample chamber with dual sapphire windows into a DWS experiment. This high- pressure microrheology instrument is validated by measuring the increase in viscosity of 1-propanol aqueous solutions and the measurement is extended to hydraulic fracturing fluids containing poly(vinyl alcohol) polymer and borate as a physical cross-linker. The linear viscoelasticity of the cross-linked network decreases with increasing pressure, and demonstrates the pressure dependence of the borate crosslinking chemistry—an effect that reduces the fluid capabilities in down-hole conditions. ☐ In a second application area, DWS is used to characterize in situ paint dry- ing dynamics, structure development, and particle interactions. Paints are aqueous suspensions of pigments, binders, and rheology modifiers that are designed to be cost effective, stable, and have good flowability while imparting important aesthetic and protective properties. As the suspension dries, it must form a uniform film that is non- cracking and has good self-leveling properties. Critical final film properties including opacity, rub and stain resistance, and film integrity are linked to the structure devel- oped during drying. To build the framework for paint drying, DWS is performed on concentration series of model silica and industrial titanium dioxide suspensions and the mean-squared displacements of the particles are correlated with the particle interac- tion potentials. This technique is accessible with the recent development of commercial instruments, and particle interactions can be measured in a fraction of the time that it would take for other specialized high-frequency rheology instruments. Additionally, techniques commonly used in industry are unable to measure in situ drying dynamics. A novel DWS paint drying setup is developed to quantitatively characterize differences in drying dynamics for low and semi-gloss paints. The sensitivity of DWS to concen- tration and structural fluctuations makes this technique an excellent tool for assessing paint performance.
Description
Keywords
Aqueous paints, Concentrated suspensions, Diffusing wave spectroscopy, Fracturing fluids, Microrheology