Bubbles battling biofouling, dewetting dynamically, and persisting with volatility
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Bubbles are commonly found in the world around us, from industrial products to carbonated beverages. This thesis will discuss three processes involving of bubbles, from applications to fundamental phenomena. In the first portion of this thesis, I describe the use of bubbles to prevent the formation of marine biofilms and other colonizing organisms onto built structures, collectively referred to as biofouling. Biofouling detrimentally affects the structures upon which they grow, increasing drag and fuel consumption of moving vessels, reducing performance of acoustic sensors, and enhancing degradation of static structures. With recent international bans placed on common biocidal coatings, there is a demand for environmentally friendly antifouling technologies with strong performance. Bubbles rising along a submerged surface have been shown to inhibit biofouling growth, but little work has been done to determine the primary mechanisms responsible for their antifouling behavior. In this thesis I discuss a combination of field and laboratory experiments as well as a theoretical approach used to gain insight into the dominant mechanisms at play, thus laying a foundation for optimization of this antifouling technique. We find that biofouling is inhibited by shear stresses generated throughout the flow, and the degree of biofouling prevention relates to the distribution of bubbles which locally alters the shear stress. Inspired by the potential for direct interactions between bubbles and biofouling, the second topic of this thesis considers the process by which a bubble dewets, or "sticks to", a solid surface. As a bubble approaches a solid surface, the liquid between the gas and solid begins to drain until it resembles a thin film. Upon rupture of this thin film, the air dewets the surface as a contact line is formed and expands. Previous work regarding this contact line motion assumes viscous effects dominate the spreading dynamics while inertial effects are neglected. Studying the early-time dynamics of dewetting bubbles, we find viscosity to be negligible while inertia and capillarity govern the motion of a newly established contact line, suggesting early stages of dewetting are more rapid than anticipated. In the final portion of this thesis, I discuss the fundamental stability of bubbles in volatile liquids. When a bubble arrives at a free surface, we typically expect the film of the bubble cap to thin over some period of time until it ruptures. Traditionally, the drainage of this film has been considered inevitable with evaporation only hastening the film rupture. Here I show air bubbles at the free surface of liquids which appear to defy traditional drainage rules and can avoid rupture, persisting for hours until dissolution. Using pure, volatile liquids free of any surfactants, we highlight and model a thermocapillary phenomenon in which liquid surrounding the bubble is continuously drawn into the bubble cap, effectively overpowering the drainage effects.
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