Experiments were performed in which liquid droplets were videographically recorded impacting horizontal superhydrophobic surfaces. The superhydrophobic surfaces were micropatterned with alternating ribs and cavities and coated with a hydrophobic coating. The following surface types were also tested for comparison: smooth uncoated, micropatterned uncoated, and smooth coated surfaces. Three liquid types were used: pure water, ethanol, and a 50/50 water/glycerine mixture. Acquired data demonstrated that the maximum droplet spread diameter exhibited a greater Weber number dependence than that previously reported in the literature. The time delays between impact and maximum spread and between impact and ejection of a vertical jet were characterized, and it was found that experiments with hydrophilic surface behavior follow somewhat different trends than those with hydrophobic behavior, and that there are modest differences between superhydrophobic and hydrophobic surfaces. When analyzing the velocity of the issuing vertical jet, a region of micro-jets was observed with velocities that, under certain conditions, can exceed 15 times the impact velocity. The experimental data acquired were also compared to two recent models from the literature and it was determined that the models do not adequately account for surface anisotropy or apparent slip at the solid-liquid interface. The experiments also showed that instabilities resulting in fingering are dependent upon surface and fluid type, but not contact angle. The onset of peripheral splashing was observed, in general, to occur at a lower Weber number as contact angle increased for the differing surfaces. For surfaces with rib and cavity features, the droplet spread and retraction were generally observed to be asymmetric with spread and retraction faster along the length of the ribs. The occurrence of two-pronged and oscillating jets for water/glycerine tests was also observed for all patterned surfaces. Lastly, an interesting spread pattern with four liquid droplets clustered at about 30° from the perpendicular direction was observed for all fluid types on patterned surfaces for high Weber numbers.

The hydrophobicity of a surface is defined as the degree to which it repels water molecules, and the internal contact angle that the droplet makes with the surface is a measure of the hydrophobicity. Contact angles less than 90° occur on hydrophilic surfaces, while contact angles greater than 90° occur on hydrophobic surfaces. If a surface's contact angle is greater than 120° the surface is commonly defined as superhydrophobic (SH). Superhydrophobicity is accomplished through a combination of microscale surface roughness and water repellant surface chemistry. The roughness creates cavities, or pockets, of vapor underneath the droplet which act to increase the effects of surface tension and lead to increased contact angles. The cavity fraction, F_c, of a surface is a measure of the surface roughness and is defined as the ratio of the projected cavity area to the projected total area of the surface. This thesis investigates the effects of varying cavity fraction, F_c, and substrate temperature, T_s, on heat transfer to evaporating water droplets. Distilled water droplets of nominally 3 mm in diameter were placed on heated SH substrates of varying F_c (0.5, 0.8, and 0.95). A smooth hydrophobic surface was included in the experiments for comparative purposes. The temperature of the surface was held constant at temperatures ranging from 60 to 230°C while the droplet evaporated. Measurements of droplet temperature and size were taken throughout the evaporation process using CCD and infrared camera images. These images were analyzed to yield heat transfer rates for the various surface types and surface temperatures studied. At temperatures below the saturation point of water, average droplet temperatures and heat transfer rates decrease with increasing cavity fraction. Differences in heat transfer rate between substrates increase with substrate temperature. Nusselt number decreases as cavity fraction is increased. Cavity fractions less than about 0.5 show only modest differences in Nusselt number between surfaces. As cavity fraction approaches unity, differences in Nusselt number become amplified between surfaces. At temperatures above the saturation point of water, boiling behavior on SH surfaces deviates dramatically from that of smooth untextured surfaces. Average heat transfer rates decrease with increasing cavity fraction. Nucleate boiling is delayed to highter superheats than normal or is not observed. The Liedenfrost point is advanced to lower superheats as cavity fraction is increased. Similar heat transfer rates are observed beyond the Leidenfrost point.

Numerical approaches have been conducted to investigate the effect of droplet deformation and internal circulation on droplet dynamics. Although droplet drag is a classical area of study, there are still theoretical gaps in understanding the motion of large droplets. In applications such as spray combustion, droplets of various sizes are generated and move with the flow. Large droplets tend to deform in the flow, and they have complex interactions with the flow because of this deformation. To better model spray, the physical understanding of droplets needs to be improved. Under spray conditions, droplets are subjected to a high-temperature-and-pressure environment, and the coupling between liquid and gas is enhanced. Therefore the deformation and internal circulation will affect the droplet drag coefficient more significantly than they would under atmospheric conditions. To study the mechanism of how droplet shape and internal circulation influence droplet dynamics, we have used direct numerical simulation (DNS) to simulate a droplet falling at its terminal velocity in high-pressure air. An in-house code developed for interface-capturing DNS of multiphase flows is employed for the simulation. The drag coefficient is calculated, and the results are consistent with the existing literature for slightly deformed droplets. The results show that the drag coefficient is directly related to the droplet deformation and droplet internal circulation. This paper also develops an analytical theory to account for the effect of the Weber number and fluid properties on droplet deformation. / Master of Science / This study investigates how larger droplets interact with airflow in spray conditions. Classical droplet drag models are not accurate under extreme conditions due to the neglect the droplet deformation and droplet internal circulation. To better understand droplet dynamics and to improve the accuracy of droplet models, direct numerical simulations were conducted. In our simulations, a non-evaporating falling droplet in high-pressure air was modeled. Results show a direct link between drag coefficient and droplet shape and internal flow. We also derived an analytical scaling law to explore the parameters related to droplet deformation. This research enhances our understanding of droplet dynamics in spray conditions.

An analytical model is developed to quantify the heat transfer to droplets impinging on heated superhydrophobic (SH) surfaces. Integral analysis is used to incorporate an apparent temperature jump at the superhydrophobic surface as a boundary condition. This Thesis considers the scenario of both isotropic and anisotropic slip, as would be realized on post-cavity style and rib-cavity style SH surfaces. This thermal model is combined with a hydrodynamic model which incorporates velocity slip at the surface. Use of the two models allows determination of the overall cooling effectiveness, a metric outlined in contemporary work. The effect of varying velocity slip and temperature jump is determined for impact Weber numbers ranging from 20 to 150 and surface temperatures ranging from 60 to 100°C. The model results are compared to experiments and good agreement is shown. Heat transfer to a drop impacting superhydrophobic surfaces is decreased when compared to conventional surfaces. A correlation function for the total heat transfer (cooling effectiveness) as a function of relevant parameters is found for isotropic surfaces with a good fit. Anisotropic rib-cavity surfaces are compared to isotropic surfaces to explore the impact of anisotropic slip on the cooling effectiveness, with similar trends seen to that for isotropic surfaces. It's determined that anisotropic surfaces can be modeled with minimal error as an isotropic surface with a temperature jump length equal to the anisotropic surface's average temperature jump length.

<p> The major part of the present work dealt with adapting the droplet technique to the study of nucleation from solution of some analytically important metal chelates. Precipitation from homogenous solution was introduced as the means of gradually increasing the supersaturation in the droplets. This new method of producing supersaturation enabled the extention of the droplet technique to nucleation studies of sparingly soluble substances. </p> <p> In addition, a second novel way of achieving supersaturation in the droplets was devised. In contrast to the first method, which increased the amount of solute at constant droplet volume, the second method maintained the amount of solute constant, while gradually reducing the volume of the droplet. This permitted studies on nucleation from solution of soluble substances to be carried out isothermally. </p> <p> The two techniques mentioned above were applied successfully to the study of nucleation from solution of four analytically important metal chelates and to several inorganic salts, respectively. </p> / Thesis / Doctor of Philosophy (PhD)

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Most aircraft accidents are caused by technical problems or weather-related issues. One cause of weather-related incidents is inlight icing, which can induce negative performance characteristics and endanger the operation of an airplane. Various researchers investigating the problem of inlight icing have proposed ice-phobic coatings as one viable solution. For this purpose, it is critical to study the behavior of a droplet impact on different types of surfaces. As an alternative to physical testing, three-dimensional numerical simulation using computational fluid dynamics offers a promising strategy for evaluating the effects of surface characteristics. Using the volume of fluid method, three simulations of high-speed droplet impact on superhydrophobic surfaces with and without micro-scale roughness elements, were generated. The simulations showed that, for the roughness configurations considered, the superhydrophobic surfaces with micro-scale roughness elements were significantly less effective at repelling the droplet than the smooth superhydrophobic surfaces.

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.