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Coatings play a pivotal role in everyday life and across various industries. They offer protection, corrosion resistance, insulation, optical improvements, aesthetics, etc. This study investigates the design, fabrication, characterization and evaluation of surface coatings in two areas: antimicrobial activity and fast evaporation. The COVID-19 pandemic underscored the necessity for coatings that mitigate microbial transmission through surfaces, alleviating both contagion and personal fears. The first part of this study presents the design, development, and evaluation of antimicrobial coatings that efficiently inactivate 99.9% of SARS-CoV-2 virus and kill more than 99.9% of pathogenic bacteria such as Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, and Pseudomonas aeruginosa within one hour. Prioritizing rapid infectivity reduction, we designed and fabricated several coatings using silver oxide (Ag2O), cupric oxide (CuO), and zinc oxide (ZnO) particles as active ingredients. Applying small quantities of micron-sized opaque particles onto a surface yields a transparent film. Although Ag2O particles are inherently opaque, they possess potent antimicrobial properties. Consequently, incorporating small quantities of Ag2O into the coating results in the desired antimicrobial activity while maintaining transparency. Transparent antimicrobial coatings are a necessity for applications such as touchscreens, offering the benefit of reducing disease transmission while maintaining the aesthetic appeal of surfaces. We employed a variant of the Stöber process to bind Ag2O particles to the substrate using a silica matrix. To improve this coating method, we employed room-temperature spin-coating of a suspension of Ag2O/sodium silicate solution on the substrate, eliminating reactions with toxic chemicals in Stöber process and subsequent heat treatment. Two key features of the improved coating are its high robustness and its capability to kill 98.6% of Clostridioides difficile endospores in 60 minutes. On the other hand, CuO and ZnO particles exhibit mild antimicrobial properties; thus, their activity could be enhanced by a porous coating. When an infected droplet lands on such a coating, it is imbibed into the porous structure, where diffusion distances are smaller, and there is a larger active area to inactivate the virus or kill the bacteria. Furthermore, porosity facilitates faster droplet drying, leading to the concentration of cupric and zinc ions in the droplet, which are designed to be toxic to microbes. The second major topic of this thesis is the development, and evaluation of porous coatings for fast evaporation. At low Bond numbers, droplet evaporation is slow on an impermeable surface. We investigated whether application of a thin, porous coating leads to faster droplet evaporation. The droplet will imbibe quickly, but progress normal to the interface will be limited to the thickness of the coating. Therefore, the liquid will spread laterally into a broad disk to expose a large liquid–vapor interface for evaporation. As a result, the evaporation of a droplet is enhanced by a factor of 7–8 on the thin porous coatings. Factors such as coating thickness, pore size and distribution, and the contact angle of the coating, as well as ambient conditions like temperature and relative humidity, could affect the droplet evaporation rates by modifying the droplet's imbibition process and the evaporation driving force. While decreasing the coating thickness and increasing pore size and distribution promoted evaporation, the impact of contact angle is insignificant. Confocal microscopy observations of a coating composed of particles with varying sizes depicted liquid migration along the top of the coating and the edges of the interface. We developed and validated an equation to estimate the rate of evaporation. The rate correlated with the radius of the imbibition area, with higher temperatures and lower humidity further augmenting evaporation. / Doctor of Philosophy / Coatings serve as integral components in various industries and everyday settings, offering multifaceted benefits such as protection, aesthetic enhancement, and functional properties. This study investigates the design, fabrication, and evaluation of two types of surface coatings; coatings that reduce microbes transmission (antimicrobial coatings) and coatings that expedite evaporation. The COVID-19 pandemic underscored the necessity for coatings that mitigate microbial transmission through surfaces, alleviating both contagion and personal fears. The first part of this study presents the design, development, and evaluation of coatings that efficiently reduce 99.9% of COVID-19 virus and kill more than 99.9% of dangerous bacteria that can be found in hospital settings. Prioritizing rapid killing of bacteria, we designed and fabricated several coatings using metal oxides. In particular, we used silver oxide (Ag2O), cupric oxide (CuO), and zinc oxide (ZnO) particles as active ingredients. Applying small quantities of fine-sized opaque particles onto a surface yields a transparent film. Although Ag2O particles are inherently opaque, they possess potent antimicrobial properties. Consequently, incorporating small quantities of Ag2O into the coating results in the desired antimicrobial activity while maintaining transparency. Transparent antimicrobial coatings are a necessity for applications such as touchscreens, offering the benefit of reducing disease transmission while maintaining the aesthetic appeal of surfaces. A chemical reaction was used to produce a glass matrix to bind Ag2O particles to the solid, but this method required heating and toxic chemicals. So we developed a second methods that eliminated these two disadvantages. On the other hand, CuO and ZnO particles exhibit milder antimicrobial properties; thus, their activity could be enhanced by a porous coating. These coatings function as large reservoirs of antimicrobial agents for trapping and deactivating pathogens, while facilitating rapid droplet evaporation through enhanced wicking and porous structure. The second part of this study elucidates the mechanisms underlying accelerated droplet drying as a result of the application of thin, porous coatings. The speed of drying is slow for small droplets on flat surfaces. However, when a droplet is placed on a porous coating, it will be wicked quickly and spread through the porous coating to create a large area for evaporation. As a result, the speed of drying was increased by a factor of 7–8 on the thin porous coatings. Coating parameters such as thickness, pore size, and distribution, surface energy, as well as environmental factors like temperature and humidity could influence the droplet drying from porous surfaces. Decreasing the coating thickness and increasing pore size and variation in pore size promoted droplet evaporation, whereas the impact of surface energy was found to be insignificant. The rate of drying correlated with the radius of the wetted area, with higher temperatures and lower humidity further augmenting evaporation.

Droplet evaporation and condensation are two important topics of interest, since these two phase-change phenomena not only occur in the cycle of global water, e.g., the formation of rain, fog, dew, and snow in nature, but also play a critical role in a variety of applications including phase-change heat transfer enhancement, surface chemistry and energy system optimization. Especially, in the past two decades, the rapid development of the nature-inspired non-wetting surfaces has promoted the applications of droplet-based phase change phenomena in various scenarios. However, most previous studies focused on the sessile droplets on one flat surface in the open space, and the effects of surface topography, i.e., surface curvature or configurations, on droplet evaporation and dropwise condensation are still elusive. This dissertation aims to explore droplet-based evaporation and condensation in more complex spaces and to elucidate how the surface topography affects the evaporating or coalescing droplet dynamics during these phase-change processes. The coalescence-induced jumping of nanodroplet on curved superhydrophobic surface is modeled via molecular dynamic simulations. As the surface curvature increases from 0 to 2, the corresponding energy conversion efficiency of jumping droplet during the coalescence process could be significantly improved about 20 times. To explain this curvature-enhanced jumping effect, the contact line dissipation, i.e., an important source of energy dissipation in nanoscale, is considered in our scaling energy analysis. And this energy-effective jumping of coalesced droplet could be mainly attributed to the reduction of contact line dissipation due to the decrease of contact line length and contact time on curved surface. As the droplets are confined between two parallel or non-parallel low-energy surfaces, i.e., hydrophobic or superhydrophobic surfaces, with a narrow gap, the total evaporation time of the squeezed droplets would be dramatically prolonged about two times. An ellipsoidal segment diffusion-driven model is established to successfully predict the evolution of contact radius and volume of the squeezed droplets during the evaporation process and to clarify it is the vapor enrichment inside the confined space giving rise to the mitigated evaporation. If two hydrophobic surfaces are configured as non-parallel, the confined droplet inside the V-shaped grooves would be self-transported towards the cusp/corner during the evaporation. Based on our energy and force analyses, the asymmetrically confined droplet would move towards an equilibrium location le, where the Laplace pressure induced force is balanced with normal adhesion force, to minimize its Gibbs surface energy. As le decreases during the evaporation, this equilibrium location would directionally shift towards the cusp, which could be regarded as the origin of this evaporation-triggered unidirectional motion. For the first time, the solvent transport and colloidal extraction could be accurately controlled in a combined manner. / Doctor of Philosophy / Droplet evaporation and condensation are two important topics of interest, since these two phase-change phenomena not only occur in the global cycle of water including the formation of rain, fog, dew, and snow in nature, but also play a critical role in a variety of applications including heat transfer enhancement, surface chemistry, and the energy system optimization. Generally, the droplets in these scenarios are deposited on one flat surface opened to the atmosphere. and the effects of surface topography on droplet evaporation and dropwise condensation are still elusive. This dissertation aims to explore droplet-based evaporation and condensation in more complex spaces and to clarify how the surface curvature or configurations affects evaporating or condensing droplet dynamics accompanying these phase change processes. As the coalesced droplet jumps off the curved superhydrophobic surfaces during dropwise condensation, the corresponding energy conversion efficiency would be significantly improved about 20 times due to the increases of curvature. It is demonstrated that the decrease of contact line length and contact time would give rise to the reduction of contact line dissipation, which should be the main factor driving this energy-effective jumping of the coalesced droplets. As the droplets are confined between two parallel or non-parallel low-energy surfaces, i.e., hydrophobic or superhydrophobic surfaces, with a narrow gap, the total evaporation time of the squeezed droplets would be dramatically prolonged about two times in the small space. An ellipsoidal segment diffusion-driven model is established to successfully predict the evolution of contact radius and volume of the squeezed droplets during the evaporation and to clarify it is the vapor enrichment in the confined space giving rise to the mitigated evaporation. If two hydrophobic surfaces are configured as non-parallel, the confined droplet inside the V-shaped grooves would be self-transported towards the cusp/corner of the structure during evaporation. Based on our energy and force analyses, the asymmetrically confined droplet would move towards an equilibrium location le, where the Laplace pressure induced force is balanced with normal adhesion force, to minimize its Gibbs surface energy. As le decreases in the scale of during the evaporation, this equilibrium location would directionally shift towards the cusp, which could be regarded as the origin of this evaporation-triggered unidirectional motion.

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.

Evaporators employed in the distillation of sea water have scale form on their heat transfer surfaces as calcium carbonate, magnesium hydroxide, and calcium sulfate in 300 to 500 hours of operation. Although the measures directed to overcome the formation of scale in sea water evaporators have been varied, none of these measures have been as successful as desired, and the problem is, therefore, still a major one. In 1947, however, C. A. Hempel, Armour Research Foundation, Chicago, Ill., approached the scale problem of sea water evaporators rationally by saying that if the carbon dioxide content and the pH of sea water could be controlled by either physical or chemical means, that the scale deposition on the heat transfer surfaces would be reduced. By experimentation, Hampel developed a process whereby sea water is heated under pressure for a definite period of time, and then it is released to atmospheric pressure with aeration. This physical pretreatment process removes the carbon dioxide that is evolved from the decomposition and hydrolysis of the carbonate and bicarbonate content of sea water, and thus prevents the formation of insoluble calcium carbonate. The change in alkalinity does, however, cause insoluble magnesium hydroxide to form, but this insoluble material can be readily removed by filtration followed by acidification. Therefore, two of the scale forming salts, calcium carbonate and magnesium hydroxide, have been eliminated by this process. In 1948, both the U. S. Coast Guard and the Bureau of Ships, Navy Department, became interested in this development of Hampel's, and they agreed to design a pretreatment plant for a 4000-gallon per day evaporator in order to evaluate further this process on a large scale basis. Such a plant was designed and constructed at the Norfolk Naval Shipyard, Portsmouth, Va., with the Coast Guard providing the necessary materials and equipment for the pretreatment plant, and the Navy furnishing the materials and equipment for the distilling unit. The Army Engineer's building and sea water facilities that were available at Fort Story, Va., made it a highly desirable location for the testing of the decarbonation and distilling units. So, after all the equipment had been fabricated and hydrostatically tested, it was shipped to Distillation Test Station at Fort Story where it was erected and made operational by Naval Shipyard personnel. In September, 1949, a series of tests were started on this equipment at Fort Story to evaluate the design and construction of the pretreatment plant to decarbonate sea water es a means of reducing the scale in a 4000-gallon per day Grissom-Russell low pressure, double effect Soloshell evaporator. The first test of decarbonation calibration was made to determine the optimum operating conditions of the pretreatment plant which would give the maximum degree of carbon dioxide removal from sea water. The second test was a blank determination of evaporator scale, using untreated feed, by which a comparison could be made with all subsequent tests. The pretreatment plant involved the operation of two pieces of equipment, the feedwater holding tank and the aerator tank. The procedure that was followed in this calibration was essentially this: the sea water was heated to a definite temperature at a specific feed rate, pumped to the feedwater holding tank and retained in this tank for definite periods of time; then the sea water was released to the aerator tank, again held for definite periods of time while being aerated with air, and a sample of sea water analyzed to determine the degree of carbon dioxide that was removed by these conditions. The variables that had their effect on the degree of decarbonation were, therefore, feed temperature, feed rate, feedwater tank holding time, aerator tank holding time, and air rate. The effect of temperature on the degree of decarbonation was that the greater the temperature, the greater the carbon dioxide removal; 250 °F removed 47.5 to 50 per cent, 240 °F removed 38 to 42 per cent, and 228 °F removed 25.2 to 25.8 per cent. The effect of feed rate at 250 °F on the degree of decarbonation was that the smaller feed rate, the greater the carbon dioxide removal; a feed rate of 5.25 gallons per minute removed 42.1 to 52.4 per cent whereas a feed rate of 10.5 gallons per minute only removed 36.6 to 47 per cent. The effect of feedwater tank holding time on the degree of decarbonation et 250 °F and 5.25 gallons per minute feed was negligible since holding times of 25, 45, 66, 86, and 106 minutes removed 50.2 to 54 per cent of the carbon dioxide. In the determination of the effect of aerator holding time on the degree of decarbonation a 250 °F and 5.25 gallons per minute feed, the greater the holding time, at 14 and 28 minutes, the greater the carbon dioxide removal, i.e., 46 end 50 per cent, respectively. The effect of air on the degree of decarbonation at 250 °F, 5.25 gallons per minute feed, and 28 minutes aerator tank holding time, was that it gave the greater degree of carbon dioxide removal, but without air, the effect was that the greater the feedwater tank holding time, the less the carbon dioxide removal. The difference between the two air rates tried was negligible; 5 cubic feet per minute removed 51 to 54 per cent, whereas 20 cubic feet per minute only removed 44 to 52 per cent. With no air, however, a feedwater tank holding time of 25 minutes removed 50.8 per cent, 45 minutes removed 43.5 per cent, 66 minutes removed 39.5 per cent, and 106 minutes removed 38.5 per cent. Therefore, from the results of the calibration, it is concluded that the optimum operating conditions for the pretreatment plant are a feed temperature of 250 °F, a feed rate of 5.25 gallons per minute, a feedwater tank holding time of 25 minutes, an aerator tank holding time of 28 minutes, and an air rate of 5 cubic feet per minute which will remove 50.8 per cent of the carbon dioxide in the sea water. In the blank determination, the feed by-passed the pretreatment plant and went directly to the evaporator, which was operated at a feed rate of 5 gallons per minute, 150 °F, 2.5 pounds per square inch, gage of steam to the first effect, 97 pounds per square inch, gage of steam to the air ejector, 26.5 inches of vacuum, 70 gallons per minute of circulating water, and 1.5-thirty seconds overboard brine density. In 135 hours of operation, 19,400 gallons of fresh water were produced which had a salinity of less than 0.5 grain per gallon. Scale was formed at a rate of 0.23 pound per 1000 gallons of distillate produced, and whose composition wan 91.1 per cent calcium carbonate, 2.6 per cent magnesium hydroxide, 2.7 per cent calcium sulfate hemihydrate, 2.7 per cent silica dioxide, and 1.1 per cent ferric oxide. From the operation of these tests, it is concluded that both the pretreatment plant and the distilling unit operated satisfactorily within the limits of their design. / Master of Science

Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Cavitation is the formation of vapor bubbles in a liquid that is a consequence of tensions acting on the liquid. It is of great interest to lots of different scientific fields such as fluid mechanics, acoustics, hydraulic engineering and biology. Although widely studied in macroscale and microscale confined liquids, heterogeneous cavitation at the nanoscale has only been experimentally observed recently in 2-D nanofluidic channels during an evaporation process, where vapor bubbles form and expand inside the nanochannels instead of menisci receding along the channels. Such evaporation-induced cavitation shows a strong correlation with the nanochannel cross-section non-uniformity and exhibited lots of interesting phenomena, including fast evaporation rate and self-controlled bubble dynamics. In this work, we further investigated this new cavitation phenomenon using a series of specially designed 2-D multi-section nanochannels. Each of these channels includes two or three sections of nanochannel with heights of 25 and/or 35 nm and the same width of 3 μm. A modified sacrificial layer etching method was developed to fabricate these nanochannel devices. Water evaporation processes in these channels were recorded using a high-speed camera mounted on an inverted microscope. We observed that cavitation only occurred in multi-section nanochannels with a “Low to High” channel design. In such nanochannels, when menisci receded to the “Low to High” step, bubbles occurred in the higher channel section and started expansion until they occupied the whole section. We explored the origin of these cavitation phenomena and discovered that that initial bubbles were formed during a snap-off process, where meniscus curvature difference induced reverse liquid flows cause air trapping right at the step. The following bubble expansion is a result of evaporation-induced negative pressure (up to -58 bars) as water inside the nanochannels is in a metastable state. We also analyzed water evaporation rates (bubble growth rates) in these nanochannels in the presence of cavitation. While most evaporation rates can be explained by classic vapor diffusion theories or the kinetic limit of evaporation, water evaporation rates in nanochannels with a Low-High-Low design in the presence of cavitation were as high as 630 μm/s, which is even much higher than the kinetic limit of evaporation and cannot be explained by any current theories. This study further expands our understanding of cavitation and provides new insights and explanations for phase-change phenomena at the nanoscale, including cavitation in plants and quick drying process in nanoporous media. The discovered ultra-high evaporation rates in the Low-High-Low nanochannels also offer a new solution to address thermal management needs for next generation electronic devices. / 2999-01-01

Dans la littérature, que ce soit pour un explosif secondaire ou un matériau composite, une balance en oxygène (BO) proche de 0% est assimilée à de bonnes performances énergétiques (vitesse de détonation, chaleur de décomposition, etc…). L’objectif majeur de cette thèse est d’améliorer les performances énergétiques d’explosifs secondaires courants (RDX, HMX, CL-20) par l’ajout d’un oxydant (DNA) afin d’obtenir un matériau composite avec une BO de -1%. Le mélange intime de ces deux composés est permis par un procédé d’évaporation flash de spray, utilisé habituellement pour réduire la taille de particules des explosifs. Les matériaux composites ont été cristallisés dans les trois cas avec succès, avec la présence d’explosif submicrométrique et de DNA nanostructuré. Un tel résultat a été permis grâce à une meilleure compréhension du procédé, et en conséquence l’ajustement des conditions expérimentales. L’étude de la réactivité de ces matériaux composites montre dans certains cas une désensibilisation, une diminution de la distance de la déflagration à la détonation, ou encore une augmentation de la vitesse de détonation, comparée aux explosifs correspondants. / In literature, for secondary explosive or composite material, an oxygen balance (OB) close to 0% is often linked to good energetic performances (detonation velocity, heat of decomposition, etc.). The main objective of this thesis is to enhance energetic performances of current secondary explosives (RDX, HMX, CL-20) by adding oxidizer (ADN) to obtain a composite material with an OB of -1%. The spray flash evaporation process, usually used for particle size reduction of explosives, enables to obtain an intimate mixture of these two compounds. Composite materials were successfully crystallized in three cases, resulting of submicrometric explosives and nanostructured ADN particles. These results were obtained thanks to a preliminary study for better process understanding and the optimization of experimental conditions. Reactivity studies show some desensitization, shorter distance from deflagration to detonation, and/or higher detonation velocity, compared to corresponding explosives.