Droplet dynamics, encompassing relatively static processes such as evaporation to more vigorous phenomena like self-propelled jumping, are of considerable interest due to their significance in both natural phenomena and practical applications. These behaviors are pivotal in facilitating mass, momentum, and energy transfer between droplets and their surroundings, with applications spanning phase-change heat transfer, material transport, surface engineering, and energy optimization. While droplet dynamics have been extensively studied over the past several decades, advancements in surface engineering, such as the development of functional surface materials, have introduced novel mechanisms governing droplet behavior. These complex droplet-substrate interactions exhibit intricate dynamics that transcend conventional understanding and remain inadequately explored. This dissertation investigates the intricate dynamics of droplet-substrate interactions, spanning processes from evaporation to out-of-surface jumping, offering insights into the interplay of thermal, capillary, and inertial forces that govern these phenomena. The evaporation of sessile water droplets on heated microstructured superhydrophobic surfaces is experimentally and theoretically explored across a temperature range of 20 °C to 120 °C. A thermal circuit model is developed to decouple heat and mass transfer contributions from the droplet cap and base. The findings reveal that substrate roughness and temperature significantly influence evaporation behavior, with suppressed boiling observed due to evaporative cooling. The results elucidate the role of substrate microstructures in modulating heat transfer pathways, advancing the understanding of evaporation dynamics on non-wetting surfaces. As the substrate temperature increases, vapor bubbles form at the droplet base, transitioning the droplet into the nucleate boiling regime. At relatively low temperatures, droplets exhibit versatile jumping behaviors similar to the high-temperature Leidenfrost effect. Unlike the traditional Leidenfrost effect, which occurs above 230 °C, fin-array-like micropillars enable water microdroplets to levitate and jump off the surface within milliseconds at just 130 °C, triggered by the inertia-controlled growth of individual vapor bubbles at the droplet base. The droplet jumping, driven by momentum interactions between the expanding vapor bubble and the droplet, can be modulated by adjusting the thermal boundary layer thickness through pillar height. This allows for precise control over bubble expansion, switching between inertia-controlled and heat-transfer-limited modes. These two modes lead to distinct droplet jumping behaviors: one characterized by constant velocity and the other by constant energy. Bubble expansion provides an effective method for achieving droplet out-of-surface jumping. To better understand the gas-liquid-substrate three-phase interactions, we inject an air bubble into a sessile droplet to explore the bubble burst-induced droplet jumping process. Upon bubble bursting, the surface energy released from both the inner and outer surfaces of the bubble drives the droplet jumping. Specifically, the bursting bubble generates capillary waves that propagate nearly vertically towards the substrate, causing the droplet to retract with minimal spreading upon impact with the capillary waves. When sufficient surface energy is released, this bubble burst-based strategy facilitates efficient momentum transfer through direct and localized capillary wave-solid surface interactions, enabling the lifting of large puddle droplets on the centimeter scale. / Doctor of Philosophy / Have you ever noticed how water droplets can sit on a surface, evaporate into thin air, or even jump away on their own? These fascinating behaviors might seem simple, but they play a big role in how heat, energy, and even materials move around in nature and in technology. My research looks at how droplets behave in different situations, from slowly disappearing through evaporation to suddenly jumping off a surface, and what makes these behaviors possible. One part of my work explores how tiny droplets evaporate on special surfaces designed to repel water. I found that the texture of the surface and how hot it is can change how quickly droplets evaporate. Interestingly, the surfaces I studied keep water from boiling, even at high temperatures, because the droplets cool themselves as they evaporate. Another part of my research investigates something even more dramatic: droplets jumping off surfaces. By using surfaces covered in tiny structures, I discovered that droplets can jump away at temperatures much lower than expected—around 130 °C instead of over 230 °C, which is typical in other scenarios. This happens because bubbles forming underneath the droplet give it a little "push" that helps it leap into the air. Finally, I studied how bursting a bubble inside a droplet can make the entire droplet jump. The burst sends out ripples, like waves in a pond, but these waves hit the surface below the droplet and bounce it upward. By tweaking the surface texture, we can control this jumping behavior and even lift very large droplets. These findings could help design surfaces that clean themselves or remove liquids quickly and efficiently.

No description available.

A theoretical model describing the motion of a small, fast rare gas atom as it passes over a liquid surface is developed and discussed in detail. A key feature of the model is its reliance on coarse-grained capillary wave and local mode descriptions of the liquid surface. Mathematically, the model is constructed with several concepts from probability and stochastic analysis. The model makes predictions that are quantitative agreement with neon-liquid surface scattering data collected by other research groups. These predictions include the dominance of single, rather than multiple, neon-liquid surface collision dynamics, an average of 60 % energy transfer from a neon atom upon colliding with a non-metallic surface, and an average of 25 % energy transfer upon colliding with a metallic surface. In addition to this work, two other investigations into the gas-liquid interface are discussed. The results of an experimental investigation into the thermodynamics of a gas flux through an aqueous surface are presented, and it is shown that a nitrous oxide flux is mostly due to the presence of a temperature gradient in the gas-liquid interface. Evidence for a reaction between a carbon dioxide flux and an ammonia monolayer on an aqueous surface to produce ammonium carbamate is also found. The second of these is an investigation into the mechanism of bromine production from deliquesced sodium bromide aerosol in the presence of ozone, and involves a sensitivity and uncertainty analysis of the computer aerosol kinetics model MAGIC. It is shown that under dark, non-photolytic conditions, bromine production can be accounted for almost exclusively by a reaction between gas-phase ozone and surface-bound bromide ions. Under photolytic conditions, bromine production instead involves a complicated interplay between various gas-phase and aqueous-phase reactions.