Water droplets tend to bead up on rough or textured hydrophobic surfaces by trapping air on the crevices underneath resulting in “Cassie” state of wetting. When a textured hydrophobic surface is immersed in water, the resulting underwater “Cassie” state can lead to significant drag reduction. The entrapped air pockets act as shear free regions and the composite interface consisting of alternate no slip and no shear regions thus formed can deliver substantial drag reduction during flow. The magnitude of drag reduction depends not only on the fractional coverage of air on the surface, but also on the size of the air pockets, with larger sized air pockets facilitating larger drag reduction. It is a common observance that Lotus leaf when kept immersed in water for a few minutes loses its water repellency due to the loss of entrapped air on the surface. Underwater Cassie state on textured hydrophobic surfaces is also not sustainable because of the depletion of air pockets caused by the diffusion of trapped air into water. This causes the drag reduction to diminish with time. Rate of diffusion of air across the water–air interface depends on the concentration gradient of air across the interface. Under flow conditions, removal of entrapped air is further enhanced by convection, leading to more rapid shrinkage of the air pockets. In order to sustain the Cassie state, it is thus necessary to continuously supply air to these air pockets. In this work, we explore the possibility of supplying air to the cavities on the textured surface inside a microchannel by controlling the solubility of air in water close to the surface. The solubility is varied by i) Controlling the absolute pressure inside the channel and ii) Localized heating of the surface To trap uniform air pockets, a textured surface containing a regular array of blind holes is used. The textured surface is generated by photo etching of brass and is rendered hydrophobic through a self-assembled monolayer. The sustainability of the underwater Cassie state of wetting on the surface is studied at various flow conditions. The air trapped on the textured surface is visualized using total internal reflection based technique, with the pressure drop (or drag) being simultaneously measured. Water which is initially saturated with air at atmospheric conditions, when subjected to sub-atmospheric pressures within the channel becomes supersaturated causing the air bubbles to grow in size. Further growth causes the bubbles to merge and eventually detach from the surface. The growth and subsequent merging of the air bubbles leads to a substantial increase in the pressure drop because as the air pockets grow in size, they project into the flow and start obstructing the flow. On the other hand, a pressure above the atmospheric pressure within the channel makes the water undersaturated with air, leading to gradual shrinkage and eventual disappearance of air bubbles. In this case, the air bubbles do cause reduction in the pressure drop with the minimum pressure drop (or maximum drag reduction) occurring when the bubbles are flush with the surface. The rate of growth or decay of air bubbles is found to be significantly dependent on the absolute pressure in the channel. Hence by carefully controlling the absolute pressure, the Cassie state of wetting can be sustained for extended periods of time. A drag reduction of up to 15% was achieved and sustained for a period of over 5 hours. Temperature of water also influences the solubility of air in water with higher temperatures resulting in reduced solubility. Thus locally heating the textured hydrophobic surface causes the air bubbles to grow, with the rate of growth being dependent on the heat input. The effect of trapped air bubbles on thermal transport is also determined by measuring the heat transfer rate through the surface in the presence and absence of trapped air bubbles. Even though the trapped air bubbles do cause a reduction in the heat transfer coefficient by about 10%, a large pressure drop reduction of up to 15% obtained during the experiments helps in circumventing this disadvantage. Hence for the same pressure drop across the channel, the textured hydrophobic surface helps to augment the heat transfer rate. The experiments show that, by varying the solubility of air in water either by controlling the pressure or by local heating, underwater Cassie state of wetting can be sustained on textured hydrophobic surfaces, thus delivering up to 15% drag reduction in both cases for extended periods of time. The results obtained hold important implications towards achieving sustained drag reduction in microfluidic applications.

Ce travail de thèse décrit la préparation de deux types de films de polymères : i) des films nanocomposites à base de silice pyrogénée aux propriétés superhydrophobes et ii) des films LbL auto-assemblés, incluant tous deux des dérivés de poly(2,3,4,5,6-pentaflurostyrene) (PPFS), utilisés comme briques élémentaires. La stratégie utilisée ici consiste à exploiter les nombreux avantages que présente la réaction de substitution du fluor en position para du PPFS avec un thiol, pour générer de nouveaux dérivés aux propriétés ajustables. Ainsi, le premier volet de la thèse a consisté à introduire des chaînes de PPFS de façon covalente à la surface de silice pyrogénée par une stratégie dite de «grafting through» en utilisant la polymérisation radicalaire contrôlée par le voir nitroxydes, en présence de PS-DEPN comme macroamorceur. La cinétique de polymérisation du PFS avec et sans particules silice a été étudiée dans divers solvants, différentes particules hybrides de silices modifiées en surface par une couronne de PPFS ont été préparé. Ensuite, un thiol perfluoré (perfluorodecanethiol:PFDT) a été utilisé pour modifier le PPFS, considéré dans ce cadre comme matrice hôte pour la préparation des nanocomposites et pour modifier le PPFS présent à la surface des particules de silice. A partir de là, un large panel de films nanocomposite a été préparé à partir des différentes combinaisons possibles de polymère hôte (PPFS ou PPFS-PFDT) et de charges inorganiques de silice (modifiées par le PPFS ou par le PPFS-PFDT). Les propriétés de mouillabilité ainsi que la morphologie de surface de chaque film ont été analysées et il en résulte que certains films présentent un caractère superhydrophobe. Le deuxième volet de la thèse a porté sur la modification du PPFS par des thiols porteurs de fonctions acide carboxylique, toujours par la réaction de substitution décrite précédemment. Différents dérivés de PPFS carboxylés de DS variés ont été synthétisés. Leur habilité à développer des liaisons hydrogène avec un polymère modèle accepteur de liaison H (la poly(4-vinyl pyridine) (P4VP)) a été étudiée. Il en ressort que dépendamment de la nature du solvant, des mélanges miscibles ou des complexes interpolymères ont ensuite été formés. Des solutions de complexes préformés ont été successivement déposées par spin-coating pour construire des films. De plus, des films multicouches LbL stabilisés par des liaisons H entre le PPFS carboxylé et la P4VP ont été élaborés and il a été démontré que la nature du solvant de dépôt, ainsi que le taux de modification du PPFS, impactent fortement le mécanisme de croissance, l’épaisseur du film et les caractéristiques de surface, en termes de topologie et de mouillabilité. / This work describes the preparation of two kinds of thin polymer films : i) self-cleaning silica-based (nano)composites films and ii) LbL self-assembling films, both including poly(2,3,4,5,6-pentaflurostyrene) (PPFS) derivatives, as building blocks. The cornerstone of the approach is to exploit the thiol-para fluoro substitution reaction to PPFS chains in order to generate derivatives with tailored properties. In this frame, PPFS chains were anchored onto the surface of vinyl-functionalized fumed silica nanoparticles by nitroxide-mediated polymerization (NMP) in presence of PS-DEPN as macro-initiator via a “grafting through” strategy. The kinetics of NMP of PFS were investigated in presence and without silica in various solvents and well-characterized hybrid silica particles containing different polymer grafting weight were declined. Then, perfluorodecanethiol (PFDT) was employed to modify PPFS, considered as the host polymer matrix, and to functionalize PPFS chains tethered to silica particles. A large panel of (nano)composite films from the different possible host matrix/silica particles combinations was prepared. The wettability and the surface morphology of each film were discussed, as a function of the host structure (PPFS or PPFS-PFDT with different DS) and silica (modified with PPFS or PPFS-PFDT), as well as the silica content. It results that superhydrophobic features can be reached. Subsequently, PPFS was modified by using carboxylic acid mercapto modifier via the thiol-para fluoro coupling. Various carboxylated PPFS derivatives differing in the degree of substitution (DS) were prepared and their ability to develop H-bonds in solution with a model strong H-bond acceptor partner (poly(4-vinyl pyridine) (P4VP)) was investigated. Dependently on the nature of the solvent, a miscible blend or interpolymer complexes (IPC) were achieved. IPC-containing solutions were used to successfully fabricate spin-assisted films. Furthermore, H-bonds mediated LbL self-assembly multilayer films involving carboxylated PPFS and P4VP were prepared and it was evidenced that the nature of the deposition solvent as well as the extent of the modification (quantified by the DS), impact the growth mechanism, the thickness and the surface features, in terms of topology and wettability.

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Dynamic behavior of impacting water drops on superhydrophobic solid surfaces provides important details on the stability/durability of such solid surfaces. Multi-scale surface roughness combined with a layer of low energy chemical is an essential surface modification process followed to create superhydrophobic capabilities on solid surfaces. The present work aims at studying the effect of low energy surface coating on droplet impact dynamics by carrying out experiments of water drop impacts on rough solid surfaces with and without chemical modification. A group of six aluminium alloy (Al6061) surfaces (three pairs) are prepared. Roughness, characterized in terms mean surface roughness, Ra, is introduced to these metallic surfaces using sand-paper polishing, electric discharge machining (EDM), and chemical based surface etching process. Low energy surface layer is laid on the rough surfaces by coating NeverWet hydrophobic solution, octadecyl-trichloro-silane (OTS), and perfluorodecyltricholorosilane (FAS-17). The impact dynamics of water drops is analyzed by capturing high speed videos for a range of drop Weber number from 1 to 570 and the salient features of drop impact process on the coated rough surfaces are compared with the corresponding uncoated rough surfaces. A one-to-one comparison on the spreading, fingering, receding, and final equilibrium of impacting drops on the coated and uncoated target surfaces is presented. Upon coating NeverWet, the original surface features of the base aluminium surface are completely covered by the hydrophobic coating material resulting in a fresh top surface layer. The outcomes as well as the bounce-off characteristics of impacting water drops on the coated surface are comparable to those observed on lotus leaf. The surface morphology features of rough aluminium surfaces coated with OTS and FAS-17 are comparable to those of the corresponding uncoated surfaces. The quantitative measurements on primary spreading and maximum spread factor of impacting drops are largely unaffected by the presence of low energy chemical coating. The dominant effect of surface coating is seen on the receding of impacting drops and hence the final drop configuration. This behavior is more prominently seen on EDM fabricated rough surface (larger Ra) combined with OTS coating than that on etching based rough surface (smaller Ra) combined with FAS-17 coating highlighting the dependence of coating effect with roughness features.

Lors de la polymérisation en suspension du chlorure de vinyle, il se forme sur les parois un dépôt de polychlorure de vinyle (PVC). Ce phénomène, nommé encroûtement, génère des problèmes car il limite la production de PVC et affecte la qualité du produit final. Dans ce contexte, un projet FUI (Fond Unique Interministériel) intitulé «Ecoating», a été financé dans le cadre d’une collaboration entre plusieurs partenaires industriels et universitaires (INEOS ChlorVinyls, Mäder Research, Avenir Group, LPIM, ESPCI-ParisTech). Deux thèses ont vu le jour au LPIM, avec pour but de développer un revêtement (photo)polymère aux propriétés anti-encroûtement durables qui permettrait d’améliorer la qualité du PVC produit, d’augmenter les quantités produites et ainsi d’améliorer la compétitivité des usines de PVC. Cette thèse s’inscrit dans le développement d’un vernis photopolymère répondant au cahier des charges. Pour éviter l’encroûtement des réacteurs, il est nécessaire de stopper une étape du mécanisme d’encroûtement comme l'adsorption sur les parois du réacteur d’un copolymère nommé Acvagen Graft Copolymer (AGC). Ce copolymère est très actif dans le phénomène d’encroûtement (site de nucléation) et se trouve principalement dans la phase aqueuse du milieu réactionnel. La stratégie de recherche élaborée dans ce projet a été basée sur le développement d'un revêtement photopolymère présentant une faible affinité pour l'eau et devant adhérer à la surface des réacteurs pour éviter la formation de croûte. Les polymères à base de fluoroacrylates ont été les premiers candidats choisis dans cette étude du fait que leurs propriétés exceptionnelles (faible énergie de surface, stabilité chimique et haute hydrophobicité...) pouvaient éviter l'adsorption de l'AGC sur les parois du réacteur, et par conséquent le développement de la croûte. Une recherche bibliographique a été réalisée pour comprendre le comportement particulier de ces molécules qui migrent vers la surface du film et s’organisent en surface pour donner des surfaces hydrophobes. Des mélanges de résines fluoroacryliques modèles ont été testés pour évaluer le caractère hydrophobe du revêtement, comprendre la migration des molécules de fluor vers l’interface en fonction de la nature de substrat et aussi déterminer l’influence de l’ajout d’additifs fluorés au mélange sur les propriétés globales du film. Cette étude nous a permis de comprendre l’influence de l’additif fluoré sur les propriétés chimiques et physiques du film. À l’échelle du laboratoire des tests d’immersion de ces revêtements déposés sur l’acier inoxydable ont étés réalisés dans l’eau chaude (80°C) afin de caractériser leur caractère hydrophobe en fonction du vieillissement dans l’eau chaude ainsi que l’adhésion du film au substrat. Nous avons observé une diminution de l'hydrophobicité de la surface du film au cours du temps lors d’une immersion. [...] / Our scientific approach has explored different strategies to develop a durable UV-cured coating with antifouling properties to prevent the crust formation. Firstly, the potential of fluoroacrylate photocurable coatings was exhaustively investigated. Indeed, their outstanding properties (low surface energy, chemical stability and high hydrophobicity...) could limit the adsorption of the AGC on the reactor walls and further encrusting. A bibliographic research highlighted the behavior of fluorinated monomers on film surface and the parameters affecting the hydrophobic properties. Different fluorinated monomers were selected. At low concentration, they provide hydrophobic surfaces on 316L stainless steel, the reference substrate. However, a decrease of the films surface hydrophobicity in hot water was observed with time, and was attributed to a disorganization of the fluorinated chains on the coating surface. An optimization of the amount of fluoroacrylate monomer was performed by confocal Raman microscopy (CRM) to promote the fluorinated chains stability on the surface before and after immersion in hot water at 80°C. The beneficial effect was found maximal at a concentration ranging from 1 to 1.8 wt%. However, even after this optimization, a decrease of the film surface hydrophobicity was observed for increased immersion time in hot water. Therefore, optimized fluoroacrylate monomer concentration was combined with alternated thermal/immersion post-treatment and has conducted to more stable photocured films. This result was attributed to a rigidification of the fluorinated chains on the film surface limiting thus, the extent of their disorganization. After this study realized at a laboratory scale, we tested the photocured coating in the VCM pilot reactor. A surface cleaning, an increase of the stainless steel roughness by shot blasting and the use of alkoxysilanes as coupling agents were implemented in order to enhance the adhesion properties of the photopolymer film on stainless steel. In addition, the use of a fluorinated monomer containing a heteroatom improved the rigidification when associated with the alternated thermal/immersion post-treatment. The crust formation was limited during four successive polymerizations in the VCM pilot reactor. A durable anti-fouling UV-coating could be not obtained due to some swelling phenomena resulting from the lack of coating adhesion or some abrasion occurring from small PVC pellets during the PVC polymerization.A second part of this project was dedicated to superhydrophobic coatings. Indeed, reducing interaction with water should lead to a better protection of the substrate. A literature review on the superhydrophobic surfaces has shown that the contact with hot water generally strongly affects their antiwetting properties and induces a large contact angle decrease. [...]