Browsing by Subject "Emulsions"

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    Chemical Thermodynamics of Water Soluble Organic Compounds Found in Aqueous Atmospheric Aerosols: Modeling and Microfluidic Measurements
    (2018-06) Nandy, Lucy
    Atmospheric aerosols, suspensions of tiny particulates in atmosphere, are known to have a major impact on Earth’s climate. Due to the highly chemically and physically complex nature of aerosol particles, large uncertainty in climate modeling arises when attempting to predict the aerosol effect. This dissertation comprises of (1) development of thermodynamic statistical mechanics models to predict solute and water content in aqueous aerosols, and (2) development of an experimental microfluidics approach to measure water loss and study liquid-liquid phase separation. The research effort will significantly advance understanding of aerosol particle thermodynamics by assessing the water content of multiphase particles containing soluble organic compounds, and reduce uncertainty in climate modeling associated with aerosol properties and dynamics. The specific objectives attained in this dissertation research are as follows. I. Aqueous Solution Thermodynamic Model Development: Thermodynamic analytic predictive models using statistical mechanics were developed for multicomponent systems across the entire range of equilibrium relative humidity (RH - 0 to 100%). The models predicted solute activity for a wide range of compounds consisting of partially dissociating organic and inorganic acids, fully dissociating symmetric and asymmetric electrolytes, and neutral organic compounds to capture their chemical behavior. II. Model Applications: (1) pH of aerosols was evaluated in a collaborative work, which is of significant interest due to its effect on the environment. (2) Hygroscopicity was estimated in a collaborative work, which has effects on the optical properties of aerosol particles. III. Experimental Microfluidics: The thermodynamic model was parameterized and validated with measurements of water uptake of multicomponent aerosol particles. The influence of relative humidity on phase behavior to assess the effects on water loss properties was studied for improved understanding of liquid-liquid morphologies. Hydrodynamic trapping of atmospheric aerosol chemical mimics in microfluidic channels was used to perform the experiments, that also represented supersaturated solutions. The efforts in this dissertation together will enhance understanding of atmospheric aerosol phase, solid/liquid/gas partitioning, and liquid-liquid morphologies found in the troposphere. Additionally, the measurements and modeling performed here are useful to any application that requires thermodynamic predictions of water content in complex fluids, like emulsions.
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    Complex droplet interfaces at the microscale: Surfactant and hydrodynamic effects in the separation of water-in-oil emulsions
    (2020-08) Narayan, Shweta
    Complex, surfactant-stabilized emulsions are relevant to various technological applications, such as the removal of dispersed water from diesel fuel in engines. Due to chemical stabilization of micrometer-sized dispersed droplets by surfactant molecules, emulsions can be challenging to separate, especially because surfactant transport to the interface is enhanced by the small droplet size and large interfacial curvature. The main goal of this work is to measure fundamental emulsion properties affecting their stability, such as dynamic interfacial tension and interfacial rheological properties, on the microscale, and relate these properties to droplet dynamics and coalescence behavior in water-in-fuel emulsions. First, dynamic interfacial tension (IFT) of water-in-diesel fuel systems containing surface-active additives such as monoolein and poly(isobutyl) succinimide (PIBSI), relevant to fuel filtration, is measured using a microfluidic tensiometer with contraction-expansion geometries. Microfluidic dynamic IFT measurements are compared with pendant drop tensiometry measurements employing millimeter-sized droplets. It is found that the dynamic interfacial tension decreases on orders of magnitude faster timescales on the microscale due to enhanced diffusive flux to curved microscale interfaces. This result has implications for fuel-water separation testing in the filtration industry. Next, a microfluidic hydrodynamic ‘Stokes’ trap is used to trap droplets in a cross-slot geometry. A four-channel hydrodynamic trap is applied towards studying drop shape relaxation as well as binary droplet coalescence of water droplets in mineral oils, stabilized by SPAN 80. It is found that the film drainage time for coalescence increases with droplet radius and surfactant concentration, while it decreases with incoming drop velocity. Critical conditions for flocculation and rebound of droplets are identified in terms of the capillary number. Finally, interfacial dilatational rheological properties of water-in-diesel fuel systems are measured using a capillary pressure microtensiometer. PIBSI and monoolein are added to the diesel fuel, and the dependence of the dilatational modulus on oscillation frequency and surfactant concentration is investigated. The dilatational modulus is found to increase with oscillation frequency and decrease with surfactant concentration. PIBSI-laden interfaces have higher modulus than monoolein-laden interfaces. Collectively, these experiments enhance our understanding of the intricate relationship between surfactant transport on the microscale, and droplet coalescence leading to emulsion separation.