Understanding the interactions between a body of liquid and a curvy surface is important for many applications such as underwater drag force reduction, droplet filtration, self-cleaning, and fog harvesting, among many others. This study investigates ways to predict the performance of granular and fibrous surfaces for some of the above applications. More specifically, our study is focused on 1) modeling the mechanical stability of the air-water interface over submerged superhydrophobic (SHP) surfaces and their expected drag reduction benefits, and 2) predicting the mechanical stability of a droplet on a fiber in the presence of an external body force. For the first application, we modeled the air–water interface over submerged superhydrophobic coatings comprised of particles/fibers of different diameters or Young–Laplace contact angles. We developed mathematical expressions and modeling methodologies to determine the maximum depth to which such coatings can be used for underwater drag reduction as well as the magnitude of the depth-dependent drag reduction effect of the surface. For the second application, we studied the force required to detach a droplet from a single fiber or from two crossing fibers. The results of our numerical simulations were compared to those obtained from experiment with ferrofluid droplets under a magnetic field, and excellent agreement was observed. Such information is of crucial importance in design and manufacture of droplet–air and droplet–fluid separation media, fog harvesting media, protective clothing, fiber-reinforced composite materials, and countless other applications.

Water repellent fabrics with superhydrophobic properties have been constructed during this diploma work. First the fabrics were woven using six different weft yarns creating micro roughness and then a nanoparticle and surface energy lowering treatment was made. Contact angle measurements, contact angle hysteresis measurements, roll-off angle measurements and spray tests were made on the fabrics to investigate the hydrophobicity and water repellence. Also the durability was tested to examine the fastness of the treatments. It was found that the nanoparticles boosted the hydrophobicity of the hydrophobic treatments. Also by varying the size of textile filaments in yarns, the hydrophobicity of the material was affected. In this study, it was found how small textile parameters in the fabric could be changed to increase both durability and water repellence. / Program: Textilingenjörsutbildningen

Neste trabalho, exploramos o uso de técnicas de microestruturação de materiais poliméricos a laser, visando a obtenção de superfícies superhidrofóbicas. Primeiramente, empregamos o fenômeno de transporte de massa em larga escala, obtido quando polímeros azoaromáticos são expostos a um padrão de interferência, para microestruturar a superfície dos filmes. Com esse método, produzimos uma morfologia superficial com formato semelhante ao de uma caixa de ovos, com espaçamento entre picos variando de 1,0 a 3,5 µm. Essa microestruturação permitiu a obtenção de amostras com distintas características hidrofóbicas; um ângulo de contato com água de 117º foi observado para a estrutura com período de 2 µm, o que corresponde a um aumento de 9º em relação à superfície não estruturada. Visando obter superfícies com características ainda mais hidrofóbicas, desenvolvemos uma técnica de microestruturação que utiliza pulsos laser para produzir micro-ranhuras por ablação na superfície polimérica. Neste caso, a morfologia da superfície produzida era composta por pilares, de seção reta quadrada, dispostos lado a lado, com distintos espaçamentos. Com esse tipo de microestruturação, e utilizando pulsos de 100 ps e 532 nm, observamos ângulos de contato com água acima de 150º, ou seja, o regime superhidrofóbico foi atingido Quando pulsos de femtossegundos foram utilizados, ranhuras com alta resolução lateral são obtidas, porém com pouca profundidade, o que não possibilitou a obtenção de um aumento significativo na hidrofobicidade. De maneira geral, nossos resultados mostram a viabilidade do uso de técnicas de microestruturação a laser para criar superfícies com propriedades de molhamento controlável, que podem ser exploradas para aplicações tecnológicas. / In this work, we investigate the use of laser microstructuring techniques to obtain superhydrophobic surfaces in polymeric materials. Initially, we employed the large-scale mass transport mechanism, obtained when azoaromatic polymers are exposed to an interference pattern, to structure the film surface. With this method, we produce an egg-crate-like surface morphology, with periods from 1.0 to 3.5 m that present distinct wetting properties. This method allowed obtaining samples with different hydrophobic characteristics; a contact angle with water of 117° was observed for the structure with period of 2 m, which corresponds to an increase of 9° with respect to the unstructured surface. Aiming at surfaces with higher hydrophobicities, we developed an ultrashort pulse micromachining technique to produce grooves, by ablation, on the polymer surface. In this case, square-shape pillar morphologies, with distinct periods, were fabricated. With this microstructuring method, and using 100 ps at 532 nm pulses, we observed contact angles with water above 150°, ie superhydrophobicity was achieved. When femtosecond pulses were used, although grooves with high lateral resolution are obtained, they are too shallow to produce a significant increase in hydrophobicity. Overall, our results indicate the use of laser microstructuring for the fabrication of surfaces with controllable hydrophobicity, which can be exploited for technological applications.

This dissertation focuses on the effects of superhydrophobic (SHPo) surfaces on thermal transport. The work is divided into two main categories: thermal transport without phase change and thermal transport with phase change. Thermal transport without phase change is the topic of four stand-alone chapters. Three address jet impingement at SHPo surfaces and the fourth considers natural convection at a vertical, SHPo wall. Thermal transport with phase change is the topic of a single stand-alone chapter exploring pool boiling at SHPo surfaces. Two chapters examining jet impingement present analytical models for thermal transport; one considered an isothermal wall and the other considered an isoflux wall. The chapter considering the isothermal scenario has been archivally published. Conclusions are presented for both models. The models indicated that the Nusselt number decreased dramatically as the temperature jump length increased. Further, the influence of radial position, jet Reynolds number, Prandtl number and isoflux versus isothermal heating become negligible as temperature jump length increased. The final chapter concerning jet impingement reports an experimental exploration of jet impingement at post patterned SHPo surfaces with varying microfeature pitch and cavity fraction. The empirical results show a decrease in Nusselt number relative to smooth hydrophobic surfaces for small pitch and cavity fraction and the isoflux model agrees well with this data when the ratio of temperature jump length to slip length is 3.1. At larger pitch and cavity fractions, the empirical results have higher Nusselt numbers than the SHPo surfaces with small pitch and cavity fraction but remain smaller than the smooth hydrophobic surface. We attribute this to the influence of small wetting regions. The chapter addressing natural convection presents an analytical model for buoyant flow at a vertical SHPo surface. The Nusselt number decreased dramatically as temperature jump length increased, with greater decrease occurring near the lower edge and at higher Rayleigh number. Thermal transport with phase change is the topic of the final stand-alone chapter concerning pool boiling, which has been archivally published. Surface heat flux as a function of surface superheat was reported for SHPo surfaces with rib and post patterning at varying microfeature pitch, cavity fraction, and microfeature height. Nucleate boiling is more suppressed on post patterned surfaces than rib patterned surfaces. At rib patterned surfaces, transition superheat decreases as cavity fraction increases. Increasing microfeature height modestly increases the transition superheat. Once stable film boiling is achieved, changes in surface microstructure negligibly influence thermal transport.

Dans cette thèse, nous avons travaillé sur la mise au point de surfaces superhydrophobes modèles dont la mouillabilité peut être contrôlée par un stimulus externe. Composées de forêts de piliers micrométriques élastomères à forts rapports d'aspect dans lesquels sont incorporées des particules magnétiques, les surfaces présentent, via l'application d'un champ magnétique externe, une orientation modulable des piliers, donc une rugosité de surface adaptable. En faisant varier la géométrie, l'élasticité et l'aimantation de ces derniers, nous avons pu mettre en évidence les points suivants. Nous avons vu dans un premier temps qu’en accord avec la littérature, et en l’absence de champ magnétique, l’hystérèse de mouillage augmente avec la fraction de surface. Cependant, elle reste constante lorsque l’élasticité des piliers varie. Résultat déroutant, car à l’échelle du pilier, il existe bel et bien une différence de mobilité des piliers entre les piliers les plus rigides et les plus complaisants qui subissent la traction de la ligne triple.Nous avons ensuite montré que l’orientation des piliers changeait significativement l’angle de glissement via l’application d’un champ magnétique. De plus, le glissement de la goutte sur la surface est favorisé lorsque les piliers sont orientés à l’opposé de la pente. Enfin, nous avons pu contrôler la façon dont une goutte d’eau se déplace sur une surface inclinée en deçà de l’angle de glissement, puisqu’elle n’avance vers le bas de la surface que si une actuation magnétique est appliquée. Ces surfaces seront une source d’étude intéressante pour comprendre comment moduler le mouillage ou l’écoulement de liquide en état fakir. / During this thesis, we have developped superhydrophobic surfaces whose wettability can be controlled by an external magnetic stimulus. Formulating a network of elastomeric and magnetic micro-pillars with high aspect ratio allows the orientation of the pillars through magnetic forces, hence an adaptable surface roughness. Moreover, modulating the geometry, elasticity and magnetization of pillars allowed us to highlight the following conclusions.We have seen first that in agreement with the literature, without magnetic field, the wetting hysteresis increased with the surface fraction. However, it remains constant varying the elasticity of pillars. This conclusion is confusing, because at the pillar scale, there is indeed a difference of mobility between rigid and flexible pillars due to the force exerted by the triple line.We then demonstrated that the deflexion of the pillars can change significantly the sliding angle due to the applied magnetic field. Moreover, sliding of the droplet on such a surface is promoted when pillars are deflected against the slope.Finally, we managed to control the displacement of a droplet on a surface which is tilted with an angle below the sliding angle : it moves forward from the surface only if magnetic actuation is applied. This surfaces will be an attractive source of study in order to understand how to modulate wetting and liquid flow in fakir state.

The ultimate goal of this project has been to develop a computational model for quantifying the interactions between of a body of fluid and a fibrous surface. To achieve this goal, one has to develop a model to create virtual structures that resemble the morphology of a fibrous surface (Objective-1) as well as a model that can simulate the flow of a fluid over these virtual surfaces (Objective-2). To achieve the first objective, we treated fibers as an array of beads interconnected through viscoelastic elements (springs and dampers). The uniqueness of our algorithm lies in its ability to simulate the curvature of the fibers in terms of their rigidity, fiber diameter, and fiber orientation. Moving on to Objective-2, we considered woven screens for their geometric periodicity, as a starting point. We studied how fiber diameter, fiber spacing, and contact angle can affect the skin-friction drag of a submerged hydrophobic woven screen, and how such surfaces resist against water intrusion under elevated hydro-static pressures (a requirement for providing drag reduction benefits). We also studied the impact of surface geometry and wetting properties on droplet mobility over these surfaces. Laboratory experiment was conducted at various stages throughout this investigation, and good agreement was observed between the experimental data and the results from our numerical simulation.

Self-cleaning, anti-corrosion, anti-icing, dropwise-condensation, and drag-reduction are some applications in which superhydrophobic surfaces are implemented. To date, all the studies associated with superhydrophobic surfaces have been dedicated to understanding the liquid interaction with surfaces that are macroscopically smooth. The current study investigates the solid-liquid interaction of such surfaces which are fully decorated with macroscopic ridges (ribbed surfaces). In particular, the drop motion and impact on our newly designed non-wettable ribbed surface have been investigated in this work. Our experimental investigations have shown that liquid drops move faster on the ribbed surfaces due to lower friction induced by such a surface pattern. Moreover, an impacting droplet shows shorter contact time on ribbed surfaces. This concludes that ribbed surface pattern can be an efficient alternative design for the related applications. Besides the experimental studies, the theoretical analyses done in this work have led to, firstly a scaling model to predict descent velocity of a rolling viscous drops on an inclined non-wettable surface more accurately. Secondly, for curved superhydrophobic surfaces a scaling model which correlates the contact time of the impacting drop to its impact velocity has been developed. At the end, the knowledge obtained from this work has led to a special surface design which exhibits a contact time shorter than the inertial-capillary time scale, an unprecedented phenomenon.

This study proposes a method to enhance the anti-icing capabilities of superhydrophobic surfaces by utilizing vibration to further reduce contact time of an impacting droplet in addition to keeping the droplet in the Cassie-Baxter regime, where surface adhesion is lower than the opposing Wenzel regime. We tested this with two methods: by investigating the effects of vibration normal to the plane of a superhydrophobic surface being impacted by water droplets in a room temperature environment, with the surface horizontal in a room temperature environment and tiled in a subzero degree environment. The amplitude and frequency of the vibration were varied in our experiments. Our results show that the mean contact time of a 10µL droplet consistently decreased linearly as the vibration frequency increased, though the standard deviations drastically increased. The ice accretion in the second phase of the testing also had significant variance, which obfuscated any reliable trend from the introduction of vibration.

This dissertation explores the physics of droplet impingement on superhydrophobic surfaces. The research is divided in three categories. First, the effect of a slip boundary condition on droplet spreading/retracting is considered. A model is developed based on energy conservation to evaluate spreading rates on surfaces exhibiting isotropic and anisotropic slip. The results show that larger slip causes the droplet to spread out farther owing to reduced friction at the interface for both slip scenarios. Furthermore, effects of slip become magnified for large Weber numbers due to the larger solid-liquid contact area during the process. On surfaces with anisotropic slip, droplets adopt an elliptical shape following the azimuthal contour of the slip on the surface. It is common for liquid to penetrate into the cavities at the superhydrophobic interface following droplet impact. Once penetrated, the flow is said to be in the Wenzel state and many superhydrophobic advantages, such as self-cleaning and drag-reduction, become negated. Transition from the Wenzel to the Cassie state (liquid resides above the texture) is referred to as dewetting and is the focus of the second piece of this dissertation. Micro-pillar pitch, height and temperature play a role on dewetting dynamics. The results show that dewetting rates increase with increasing pillar height and increasing surface temperature. A scaling model is constructed to obtain an explanation for the experimental observations and suggests that increasing pillar height increasing the driving dewetting force, while increasing surface temperature decreases dissipation. The last piece of work of this dissertation entails droplet impingement on superheated surfaces (100°C - 400°C). We find that the Leidenfrost point (LFP) occurs at a lower temperature on a hydrophobic surface than a hydrophilic one, where the LFP refers to the lowest temperature at which secondary atomization ceases to occur. This behavior is attributed to the manner in which vapor bubbles grow at the solid-liquid interface. Also in this work, high-speed photographs reveal that secondary atomization can be significantly suppressed on a superhydrophobic surface owing to the micro-pillar forest which allows vapor to escape hence minimizing bubble formation within the droplet. However, a more in-depth study into different superhydrophobic texture patterns later reveals that atomization intensity can significantly increase for small pitch values given the obstruction to vapor flow presented by the increased frequency of the pillars.

The primary focus of my work has been to prepare new solid adsorbents for solid phase microextraction (SPME) via sputtering of silicon. The orientation of the silica substrates/fibers and the sputtering pressure induced the formation of porous and columnar structures. Sputtering was performed for different times to yield fibers with different thicknesses. Piranha treatment of the surface increased the concentration of silanol groups, which underwent condensation with vapor deposited octadecyldimethylmonomethoxy silane to incorporate octadecyl chains onto the fiber surfaces. Silanized, sputtered fibers were preconditioned for 3 h at 320 °C to remove the unreacted chains. Comparison of the extraction efficiencies of 1.0 and 2.0 µm sputtered, silanized fibers with a commercial fiber (7 µm PDMS) for a series of analyte mixtures, which included alkanes, alcohols, aldehydes, esters, and amines, was demonstrated. The silanized, sputtered fiber performed better than the commercial fiber in extraction of most of the compounds. These fibers demonstrated long life as no degradation was seen even after 300 extractions. Carry-over between runs was not observed. The repeatability of the sputtered fibers was similar to commercial ones. The extraction of more than 50 compounds from a real world botanical sample using the 2.0 µm sputtered, silanized fiber was also demonstrated. In my second project, a facile method for the preparation of superhydrophobic surfaces (SHS) on glass and silicon surfaces was developed. A two-tier topography (needed for an SHS) was created in 60 min by the aggregation of nanosilica during in situ urea-formaldehyde polymerization. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) demonstrated rough topography. Vapor deposition of a low surface energy silane imparted hydrophobicity, which was confirmed by the presence of an F 1s signal in X-ray photoelectron spectroscopy (XPS). The prepared surfaces exhibited water contact angles (WCA) of greater than 150 °C with very low sliding angles. In my third project, a multilayer assembly of nitrilotris(methylene)triphosphonic acid, a corrosion inhibitor, and zirconium was constructed on alumina at room temperature. Attempts to prepare a layer-by-layer assembly at higher temperature (70 °C) was unsuccessful due to etching of the alumina surface. A suite of analytical techniques, XPS, AFM, time-of-flight secondary ion mass spectrometry, and spectroscopic ellipsometry was used to characterize these surfaces. This thesis also contains appendices of tutorial articles I wrote on modeling in ellipsometry, and data analysis tools (classical least squares and multivariate curve resolution).