Superhydrophobicity refers to surfaces with extremely large water droplet contact angles (usually greater than 150°). This phenomenon requires a hydrophobic material with micro or nano-scale roughness. Superhydrophobic surfaces exist in nature (e.g. the lotus leaf) and can be produced synthetically. This project focuses on the development and characterization of superhydrophobic materials with tunable wettability (i.e. smart superhydrophobic materials). In this study, surfaces were prepared by electrospinning thin, aligned polystyrene fibers onto a piezoelectric unimorph substrate. Results showed electric field induced changes in substrate curvature, which produced corresponding changes in surface wettability. From experiments, an average change in water contact angle of 7.2° ± 1.2° with 90% confidence was observed in ~2μm diameter fiber coatings electrospun for 5 minutes with applied electric field. In addition, fiber coatings electrospun with equivalent deposition showed average electric field induced changes in WCA of 2.5° ± 0.92° for lower diameter fibers (~1μm) and 3.5° ± 1.37° for higher diameter fibers (~2μm) with 90% confidence.

Aqui apresentamos um modelo teórico para superfícies super-hidrofóbicas que são formadas por superfícies contendo padrões periódicos na forma de microcavidades. Com este modelo obtivemos a relação ideal entre profundidade e diâmetro das cavidades para que a superfície manifeste seu caráter super-hidrofóbico. Assim, fabricamos superfícies em PDMS (popular silicone) capazes de produzir ângulos de contato elevados. Produzimos amostras contendo microcavidades específicas (paralelepípedas, hexagonais e cilíndricas) as quais foram microfabricadas por litografia de feixes de elétrons e caracterizadas por Microscopia Eletrônica de Varredura (MEV), Microscopia de Força Atômica (AFM), e medidas de ângulo de contato. Os padrões das microcavidades das superfícies produzidas seguiram as considerações da teoria e as medidas dos ângulos de contato de avanço e recesso mostram boa concordância com as previsões do modelo. Portanto, podemos afirmar que a teoria aqui descrita permite projetar superfícies altamente hidrofóbicas. / Here we present a theoretical model for super-hydrophobic surfaces formed by surfaces containing periodic patterns in the form of microcavities. With this model we obtained the ideal relationship between depth and diameter of the cavities so that the surface expresses a super-hydrophobic character. Thus manufacture of PDMS surfaces (with known silicone) is capable of producing high contact angles. We produced samples containing specific microcavities (parallelepipeds, hexagonal and cylindrical) which have been microfabricated by electron beam lithography and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurements. The patterns of the surfaces of the cavities produced following the considerations of theory and measurements of advancing and recending contact angles show good agreement with the model predictions. Therefore, we can attest that the theory described here allows the design of highly hydrophobic surfaces.

Superhydrophobic (SH) surfaces are super water repellent surfaces on which a droplet of water will bead up like a marble and roll off the surface with minimal tilting of the surface. This is caused by the combination of a hydrophobic coating and a rough surface structure. To achieve thermodynamic stability, surface tension of the water pulls the droplet into this shape to minimize the contact area between the droplet and the surface. This creates a high contact angle (CA) between the droplet and the surface and a low sliding angle (SA) of which the droplet begins to roll off the surface. SH surfaces have a variety of potential applications such as drag reduction, anti-icing, improved heat transfer through condensation, and self-cleaning. Numerous reports have been dedicated to exploring the fluid dynamic behavior of water droplets on SH surfaces. This thesis focuses on exploring the self-cleaning properties of SH surfaces. Surfaces contaminated with salt, tobacco, and pollen are cleaned by rolling water droplets over the surface or condensing water on the surface such that when large enough, these droplets roll away due to gravity. SH surfaces explored here are composed of micro-scale or nano-scale rib and cavity structures and are compared with smooth, hydrophobic surfaces with a similar hydrophobic coating. To determine the self-cleaning efficiency of each surface, the CA and SA were measured before and after each surface was cleaned. In this study, it was observed that the presence of each of the three contaminates considered greatly affects the overall hydrophobicity of the surface, as indicated by the CA and SA. Ideally, as the contaminates are removed from the surface, the hydrophobicity of the surface improves to match the hydrophobicity of a clean surface. This is best seen on surfaces contaminated with salt as the CA and SA match that of a clean surface after only two to three water droplets roll over the surface or after the first condensed water droplets roll off the surface. This implies that all the salt particles are removed from the SH surface. Surfaces contaminated with tobacco showed that the hydrophobicity improves to a limited extent when cleaned with rolling water droplets or condensation but never is capable of matching the hydrophobicity of a clean surface. This suggests that only a portion of the tobacco residue is capable of being removed from the surface by either of the two cleaning methods considered in this thesis. Finally, when water came in contact with pollen on the surfaces, it experienced hydrodynamic osmosis leading to cellular bursting. After cellular bursting, the surface behaves as a hydrophilic surface and selfcleaning properties were never observed on any surface contaminated with pollen. Thus, overall this study shows that rolling water droplets over a contaminated surface and condensing water droplets on a contaminated surface are both viable means of utilizing the self-cleaning properties on SH surfaces. For the contaminates considered in this study, the efficiency of the self-cleaning surfaces is shown to be the same for both micro-structured and nano-structured surfaces.

Aqui apresentamos um modelo teórico para superfícies super-hidrofóbicas que são formadas por superfícies contendo padrões periódicos na forma de microcavidades. Com este modelo obtivemos a relação ideal entre profundidade e diâmetro das cavidades para que a superfície manifeste seu caráter super-hidrofóbico. Assim, fabricamos superfícies em PDMS (popular silicone) capazes de produzir ângulos de contato elevados. Produzimos amostras contendo microcavidades específicas (paralelepípedas, hexagonais e cilíndricas) as quais foram microfabricadas por litografia de feixes de elétrons e caracterizadas por Microscopia Eletrônica de Varredura (MEV), Microscopia de Força Atômica (AFM), e medidas de ângulo de contato. Os padrões das microcavidades das superfícies produzidas seguiram as considerações da teoria e as medidas dos ângulos de contato de avanço e recesso mostram boa concordância com as previsões do modelo. Portanto, podemos afirmar que a teoria aqui descrita permite projetar superfícies altamente hidrofóbicas. / Here we present a theoretical model for super-hydrophobic surfaces formed by surfaces containing periodic patterns in the form of microcavities. With this model we obtained the ideal relationship between depth and diameter of the cavities so that the surface expresses a super-hydrophobic character. Thus manufacture of PDMS surfaces (with known silicone) is capable of producing high contact angles. We produced samples containing specific microcavities (parallelepipeds, hexagonal and cylindrical) which have been microfabricated by electron beam lithography and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurements. The patterns of the surfaces of the cavities produced following the considerations of theory and measurements of advancing and recending contact angles show good agreement with the model predictions. Therefore, we can attest that the theory described here allows the design of highly hydrophobic surfaces.

No description available.

Surfaces with static contact angle greater than 150 degrees are typically classified as superhydrophobic. Such coatings have been inspired by the lotus leaf. As water flows over a superhydrophobic surface, "slip effect" is produced resulting in a reduction in the skin-friction drag exerted on the surface. Slip flow is caused by the entrapment of a layer of air between water and the surface. Superhydrophobicity could be utilized to design surfaces for applications such as energy conservation, noise reduction, laminar-to-turbulent-transition delay, and mixing enhancement. A popular method of manufacturing a superhydrophobic surface is microfabrication in which well-designed microgrooves and/or poles are placed on a surface in a regular configuration. This method is a costly process and cannot easily be applied to large-scale objects with arbitrary shapes. In this work, we fabricated and characterized simpler low-cost superhydrophobic coatings based on controlling the volume of entrapped air in order to enhance durability (longevity) and the properties of the coating bringing the technology closer to large-scale submerged bodies such as submarines and ships. Two different low-cost fabricating techniques have been utilized: (i) random deposition of hydrophobic aerogel microparticles; and (ii) deposition of hydrophobic polymer micro- and nanofibers using DC-biased AC-electrospinning. The present study is aimed at providing experimental, numerical, and analytical models to characterize the superhydrophobicity and longevity of the coatings depending on the morphology of the surfaces and the concentration of the hydrophobic materials. The surface's micro/nanostructure were observed by field emission scanning electron microscopy. The degree of hydrophobicity of the coatings was estimated using drag-reduction and contact-angle measurements using a rheometer and a goniometer respectively. Furthermore, We have advanced and calibrated a novel optical technique to noninvasively measure the longevity of submerged superhydrophobic coatings subjected to different environmental conditions. We have also modeled the performance of superhydrophobic surfaces comprised of randomly distributed roughness. The numerical simulations are aimed at improving our understanding of the drag-reduction effect and the stability of the air–water interface against pressure in terms of the microstructure parameters. Moreover, we have experimentally characterized the terminal pressure (i.e. the pressure at which the air–water interface completely fails) of aerogel coatings with different morphologies.

Study of Ordered Macroporous Polymer Films by Templating Breath Figures Lulu Song 193 pages Directed by Dr. Mohan Srinivasarao Macroporous films with highly ordered pore patterns have many potential applications. Some examples include microstructured electrode surfaces, photonic band gap materials and filters for cell sorting and bio-interfaces. In this dissertation we discuss a moist-casting method to prepare hexagonally-ordered macroporous films with pore sizes in the range of sub-micron to several microns, where condensed water droplets (breath figures) work as templates. Compared with other templating methods, this one is fast and simple. Well-ordered porous films can be obtained in tens of seconds and the pore size can be easily tailored and dynamically controlled by adjusting the casting conditions. More importantly, there is no need to remove the templates; water droplets just evaporate when the casting processes are finished. This study was carried out with the intention of characterizing the structures, understanding film-formation processes and exploring special properties and possible applications. For the structural characterization, film morphology was studied in detail by normal optical microscopy and laser scanning confocal microscopy (LSCM). Several interesting features have been revealed. Meanwhile, the degree of the order of the porous structures were characterized both in real space via Voronoi diagram and bond-orientational correlation function, and in reciprocal space via Fraunhofer diffraction pattern. To further understand the mechanism, the evaporation of the polymer solutions during the film formation was studied by monitoring their mass over time. Besides, the evolution of breath figures formed on the evaporating polymer solutions was in-situ recorded via a high-speed camera coupled to an optical microscope. Combined with the information on the film structures obtained via LSCM, explanations for some detailed features have been attempted. Wetting property of these films was studied in some detail. The films exhibited lotus effect, mimicking natural non-wetting surfaces. To improve the solvent stability and mechanical properties of the macroporous films for possible applications, crosslinking of the polymer matrix was tried by heating. Crosslinked structures with hexagonal arrays of cone-like air holes were obtained, which might find use as micron-sized beakers for small-quantity analysis.

No description available.

Le contrôle de la mouillabilité de surface est un enjeu majeur pour le développement de matériaux innovants liés aux nano, bio et smart technologies. La mouillabilité est fonction de deux paramètres majeurs : l’énergie de surface du matériau et sa morphologie. La combinaison de ces deux paramètres est à la base de phénomènes tels que la super/parahydrophobie ou la superoléophobie. Ces capacités extrêmes à repousser les liquides avec soit une forte ou faible adhésion sont des propriétés de surface très intéressantes pour de multiples applications industrielles. La présente thèse propose l’étude d’une série de dérivés du polypyrrole élaborés par électrodéposition permettant d’influencer les paramètres régissant la mouillabilité de surface. Par cette approche, il a été possible d’élaborer des surfaces aux morphologies diverses avec une gamme de mouillabilité complète. Les différentes fonctionnalisations par des groupements hydrophobes greffés sur différentes positions préférentielles du monomère ont conduit à l’élaboration de surfaces para et superhydrophobes mettant en évidence l’impact de l’énergie de surface et de la morphologie sur la mouillabilité. Des études préliminaires ont mis en évidence la possibilité d’obtenir des morphologies variées allant de sphères jusqu’à des fibres à l’échelle du micro/nanomètre. Finalement, ces travaux contribuent à un contrôle en amont de la mouillabilité et de la morphologie de surface pour de nombreuses applications potentielles comme les matériaux collecteurs d’eau, les membranes séparatrices de liquide ou bien les revêtements auto nettoyant. / The control of the surface wettability is a key point for the development of innovative materials in several domains such as nano-, bio- and smart-technologies. The wettability is a function of two main parameters of the materials, such as the surface energy and the surface morphology. The combination of these two parameters allows to observe wetting phenomena as super/parahydrophobicity and superoleophobicity. These extreme abilities to repel liquids with different adhesion behaviors are very interesting properties for several industrial applications. This work presents a series of polypyrrole derivatives elaborated by electrodeposition allowing to influence the parameters driving the surface wettability. Following this approach, it was possible to develop surfaces with several types of morphology and different wetting behaviors from a low to high wettability. The different functionalizations using hydrophobic compounds grafted on various preferential positions on the monomer core yielded to para and superhydrophobic surfaces showing the impact of the surface energy and morphology on the wettability. Thanks to preliminary studies, it was showed the possibility to obtain several morphologies from spherical aggregates to fibers at the micro/nano scale. Finally, this work contributes to an upstream control of the surface wettability and morphologies for many potential applications such as water harvesting, separation membranes and self-cleaning coatings.