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Randomly entangled fibers forming loosely bound nonwoven structures are evaluated for use in lightweight armor applications. These materials sacrifice volumetric efficiency in order to realize a reduction in mass versus traditional armor materials, while maintaining equivalent ballistic performance. The primary material characterized, polyester fiberfill, is shown to have improved ballistic performance over control samples of monolithic polyester as well as 1095 steel sheets. The response of fiberfill is investigated at a variety of strain rates, from quasistatic to ballistic, under compression, tension, and shear deformation to elucidate mechanisms at work during ballistic defeat. Fiberfill’s primary mechanisms during loading are fiber reorientation, fiber unfurling, and frictional sliding. Frictional sliding, coupled with high macroscopic strain to failure, is thought to be the source of the high specific ballistic performance in fiberfill materials. The proposed armor is tested for penetration resistance against spherical and cylindrical 7.62 mm projectiles fired from a gas gun. A constitutive model incorporating the relevant deformation mechanisms of texture evolution and progressive damage is developed and implemented in Abaqus explicit in order to expedite further research on ballistic nonwoven fabrics.

The movement of fluids has a significant impact on the biological world, from the transport of critical medications, to the shaping of cellular life. The presence of a fluid-fluid interface gives rise to regions where a fluid---and its contents---can be selectively transported or trapped, and where stresses from the rearranging interface can lead to damage or even death of nearby microorganisms. First, we examine the role of local displacement on network level transport. Multiphase fluid flow through small length-scale networks---such as porous rock or tumor vasculature---can be described by examining local interactions of two adjacent channels (pores) using a pore doublet model. However, the traditional pore doublet model does not take into account the region at the interface of the two fluids, and thus the applicability of this model for low aspect ratio pores is unknown. Here we show using computational fluid dynamics (CFD) that traditional pore doublet models begin to break down for lower channel aspect ratios due to increased energy dissipation in the fluid interface region. We also show that our pore doublet model is able to extend previous models, elucidating network level behavior from a local response. Second, we focus on the generation of highly uniform droplets. When air is blown in a straw near an air-liquid interface, typically one of two behaviors is observed: a dimple in the liquid's surface, or a frenzy of sputtering bubbles, waves, and spray. Here we report and characterize an intermediate oscillatory regime that can create monodisperse aerosols from periodic angled jets. The underlying mechanisms responsible for this highly periodic regime are not well understood. We present experimentally validated scaling arguments to rationalize the fundamental frequencies driving this system, as well as the conditions that bound the periodic regime. This mechanism has the potential to aerosolize microorganisms in the bulk fluid. Third, we look at the role of fluid stresses on nearby biological life. In the biotechnology industry mammalian cells are grown in aerated tanks where locally elevated stresses---created by bubbles rapidly changing shape---can be high enough to kill nearby cells; however the effect of elevated stresses on cells at the timescales of these bubble events is unclear. Here we investigate the effect on cell viability from fluid stresses created by a bubble undergoing topological change, using a combination of CFD, numerical particle tracking, and experimental microfluidics. Using this approach we elicit an overall bubble-induced effect on a cell population's viability. Finally, we examine the role stresses can have on bacterial aerosolization. A key component of the airborne infection pathway is the survival of the pathogen during aerosolization pinch-off processes. Due to a rapidly rearranging interface, pinch-off processes have the potential to generate an enormous amount of hydrodynamic stress in the surrounding fluid. However, the magnitude and duration of the hydrodynamic stresses in these droplets is unknown. We show using numerical simulations the spatial and temporal hydrodynamic stress history of microscale aerosol droplets produced by the central jet of collapsing bubbles. This stress history can then be linked to the stress tolerance of various bacteria allowing for the creation of a new stress-based metric for bacterial survival during aerosolization. / 2021-01-28T00:00:00Z

Air bubbles in liquids have complex interactions with their surroundings. A rising bubble not only mixes the surrounding fluid but also collects suspended particles such as bacteria or microplastics on its interface, transporting them to the liquid surface. When a bubble bursts, it releases droplets that carry sea salt, microorganisms, and chemicals into the air, affecting both human health and the climate. Through experiments and theory, this dissertation studies the underlying mechanisms behind bubble-mediated biofouling prevention, air-sea particle transport, and sea spray formation. Our first study examines the relationship between the flow fields created by rising bubbles and biofouling prevention on a submerged surface. Bubble aeration is a method for preventing biofouling organisms, such as barnacles, from growing on a surface without using environmentally harmful chemicals. We identify the critical flow characteristics of periodically rising bubbles that correlate with the prevention of multi-species biofouling over a seven-week period, offering a potential framework for studying and comparing flow fields that successfully inhibit biofouling. Our next study investigates how small bubbles concentrate particles adhered to the bubble interface, such as plastics or microorganisms, into highly-contaminated droplets during the bursting process. We reexamine the assumption that only particles small enough to fit within a thin microlayer around the bubble can be transported into the influential top jet drop, and demonstrate that larger particles can also be transported and exhibit higher enrichment levels than predicted. We combine experiments and theory to develop an analytical model estimating the expected enrichment based on the bubble size, particle size, and particle position on the bubble. We proceed by focusing on plastic particle transfer into the atmosphere via bursting bubbles from breaking ocean waves. Existing estimates of micro- and nanoplastic transport through this pathway have large uncertainties due to limited detection techniques and studies. We develop a modeling framework that examines the size-dependent transport of hydrophilic and hydrophobic plastic particles, revealing the dominance of jet drops over film drops and the potential for nanometer-sized plastics to become highly concentrated in the smallest drops. Finally, we explore the role of salinity on the bursting bubble production of submicron drops, which are critical to cloud formation and other atmospheric processes. It is well-known that bubbles bursting in saltier water will produce more submicron drops. However, previous studies have attributed this trend to the suppression of bubble coalescence with higher salinity, leading to more numerous bubbles and consequently more drops. We demonstrate that submicron drop production increases with salinity, even when using a salt that does not affect bubble coalescence behavior. This finding implies that salinity has a systematic effect on the physics of submicron drop formation, even at the scale of a single bubble.

109 leaves : illustrations.

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