Browsing by Author "Dennis, Kimberly A."
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Item Applications of diffusing wave spectroscopy to complex fluids in industry(University of Delaware, 2020) Dennis, Kimberly A.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.Item High-pressure linear viscoelasticity measurements(University of Delaware, 2018) Dennis, Kimberly A.Complex fluids from biological systems to polymeric solutions and gels experience elevated pressures due to environmental and processing conditions, which may impact the fluid performance. Tunable pressure-dependent fluid behavior is desirable for oilfield applications to optimize hydrocarbon recovery. Oilfield fluids are used to help transport and suspend solids, reduce friction pressure, and prevent fluid loss. Key to these fluid performance metrics is the fluid rheology. Depending upon the composition and flow conditions, the fluid can behave as a purely viscous or viscoelastic fluid. By selecting the composition, the flow properties can be optimized for specific functions, such as, suspending proppants to keep fractures open or retaining fluid downhole. ☐ High-pressure measurements may be performed using falling body, pressure-driven, and rotational devices. Falling body rheometers use a stationary object in a moving fluid or a stationary fluid with a mobile object to obtain viscosity measurements. Pressure-driven devices force a fluid through a capillary and obtain pressure drop and volumetric flow rate to obtain the viscosity. These techniques are restricted in the material properties that may be obtained and their application to non-Newtonian fluids. Rotational rheometers apply a shear or oscillatory stress or strain to the fluid to obtain viscoelastic properties, however, this technique is often pressure-limited. Overall, high-pressure viscoelastic measurements can be challenging for mechanical rheometers. ☐ To address these shortcomings, a passive microrheology experiment has been designed and validated to measure the linear viscoelasticity of complex fluids at high pressures. The apparatus incorporates a steel alloy sample chamber with dual sapphire windows into a simple diffusing-wave spectroscopy (light-scattering) device and is capable of both transmission and backscattering geometries. The measured light intensity correlation from the Brownian motion of polystyrene probe particles dispersed in the sample is interpreted using the Generalized Stokes-Einstein Relation to determine the material linear viscoelasticity. This high-pressure microrheology instrument is validated by measuring the viscosity change of water and 1-propanol over pressures from 0 to 172.4 MPag at ambient temperature. ☐ Complimentary mechanical and microrheology measurements are performed at ambient pressure on stimulation fluids containing a crosslinked guar gum biopolymer before the measurement is performed at elevated pressures. We investigate the effect of crosslinker density on rheological properties at frequencies up to 1 MHz and pressures of 200 MPag, expanding the accessible range of experimental conditions beyond those of existing rheological measurement techniques.