Browsing by Subject "Complex fluids"
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Item Chemical Thermodynamics of Water Soluble Organic Compounds Found in Aqueous Atmospheric Aerosols: Modeling and Microfluidic Measurements(2018-06) Nandy, LucyAtmospheric 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.Item Swimming Despite Obstacles: Bacterial Swimming as an Evolution-selected Feature(2022-08) Kamdar, ShashankIn the 1670s, Leeuwenhoek used a single-lens microscope to bring the unfamiliar microscopic world of bacteria to human attention. In this research work, we use biophysical tools of quantitative microscopy and fluid dynamics to revisit the same world of microbes and shed light on the intricate yet fascinating motion of microbes. In particular, this thesis details two fundamentally significant problems related to microbial locomotion: 1) motility of microbes in complex fluids, and 2) impact of multiflagellarity on bacterial motility. Locomotion of flagellated microorganisms is of great importance for a wide range of biological processes from disease infection, to reproduction, and to ecosystem health. Bacterial swimming in simple Newtonian fluids is well understood; however, our understanding of their motion in their natural habitats comprising of microscopic particles and polymers is still far from complete. Even after six decades of research, whether bacteria show motility enhancement in polymer solutions and what is the origin of this enhancement remain under debate. We tackled this problem from a new perspective: we studied bacterial locomotion in dilute colloidal suspensions, which do not exhibit complex rheological behaviors such as shear thinning, thickening, etc. Surprisingly, we found that all the measurable swimming features of bacteria in colloidal suspensions are quantitatively the same as those in polymer solutions. This suggests a common origin of bacterial motility enhancement in all complex fluids and challenges all the existing theories which exclusively used polymer dynamics to explain this behavior. We subsequently developed a simple hydrodynamic model considering the colloidal nature of complex fluids, which predicted bacterial motility enhancement in both colloidal suspensions and polymer solutions. We also propose a new mechanism of bacterial wobbling that shows the enhancement and also reproduced bacterial helical trajectories with large pitches—another puzzling behavior of bacterial locomotion. Thus, our study combining experiments and theory unambiguously resolved the long-standing controversy of two problems at once, i.e., the origin of bacterial motility enhancement in complex fluids and the mechanism of bacterial wobbling in Newtonian fluids. Bacterial species also show variations in their flagellar architecture and adapt two common arrangements: monotrichous or uniflagellar bacteria possess a single flagellum at the pole of their body and peritrichous bacteria grow multiple flagella over their body, which form a helical rotating bundle propelling bacterial swimming. Although the cellular features of bacteria are under strong evolutionary selective pressures, extensive studies suggest that multiflagellarity confers no noticeable benefit to bacterial motility. These findings pose a long-standing question: why does multiflagellarity emerge in bacteria given the tremendous metabolic cost of flagellar synthesis? Here, contrary to common views that seek the answer beyond the basic function of flagella in motility, we show that multiflagellarity indeed provides a significant selective advantage in bacterial motility, allowing bacteria to maintain a constant swimming speed over a wide range of body sizes. Through experiments of immense sample sizes and detailed hydrodynamic modeling and simulations, we quantitatively reveal how bacteria utilize the increasing number of flagella to regulate the flagellar motor load, which leads to faster flagellar rotational speeds balancing the higher hydrodynamic drag on the bacterial body of larger sizes. Without such an elegant mechanism, the swimming speeds of uniflagellar bacteria decrease with increasing body sizes. This stark difference between the two swimming modes provides a novel fluid dynamic insight into the crucial role of multiflagellarity in maintaining optimum motility for navigation and survival in their native habitats. Beyond, the ecological implications, results, and insights from this thesis serve as guidelines for devising artificial swimmers that efficiently navigate complex biological environments for drug delivery.