Fabrication and characterisation of Slippery Icephobic Coatings for application to High Voltage Insulators

Slippery liquid-infused porous surface (SLIPS) based on Nepenthes pitcher plant has attracted increasing interest in many applications, particularly for mitigating icing hazard. Aside from exhibiting a water repellency, such surfaces have low contact angle hysteresis of <2.5° and low tilt angle of <5°. Nonetheless, SLIPS still suffer from challenges related to the oil depletion that reduces their service life. Herein, two different approaches, namely, microtexturing and the use of oil absorbent, are presented to effectively produce the slippery surfaces for the enhanced durability of engineered lubricant-infused materials. In the first approach, the replication method was employed to produce liquid-infused textured surfaces (LITS) through the chemical etching technique. Analysis such as lubricant depletion/recovery, confirmed that the presence of microtextures and lubricant viscosity can have significant roles in controlling the oil migration rate. Furthermore, merging microtexturing and slipperiness in LITS can enhance the icephobic performance. Such surface demonstrated an ice adhesion strength of less than 20 kPa, which is four orders of magnitude lower than the pristine surface. Furthermore, LITS can offer more long-lasting icephobic properties compared with pristine surface lacking microtextures. For the second strategy, the lubricant-loaded carriers were used to prohibit the rapid oil consumption, thus prolonging the life service of the prepared coatings. Accordingly, the negative pressure was applied to promote the incorporation of the lubricant within carrier pores and increase the carrier loading capacity. For this purpose, the thermogravimetric analysis (TGA), and the BET (Brunauer, Emmett and Teller) method have been used to evaluate the lubricant infusion quantitively. High oil content (around 50 wt. %) has been commonly used to enhance icephobic performance, but the presence of excessive amount of lubricant within the matrix can result in decreased mechanical characteristics of the prepared surfaces. The combination of different anti-icing mechanisms, such as stress localization, slipperiness, and the formation of nonfrozen molecules can be helpful to obtain desirable icephobic properties at low oil contents. These mechanisms were achieved using blended matrix, infused lubricant, and hydroxyl-terminated lubricant. The slippery coating was developed by impregnating hydroxyl-terminated-lubricant-loaded carriers into a blend of polydimethylsiloxane (PDMS) and alkoxy-siloxane resin. The prepared coating showed desirable anti-icing and de-icing properties and long-lasting stability against UV exposure and humidity. The icephobic characteristics of the coating can be attributed from its slipperiness and formation of unfrozen molecules. The electrical properties of the coating were evaluated using a comprehensive set of methods, including dielectric spectroscopy, flashover, condensation, and inclined plane tests. In comparison with the reference sample, the coating containing lubricant-loaded carriers exhibited lower dielectric permittivity and loss factor. Moreover, the coating reduced the leakage current under high humidity. The lubricant-loaded-carrier-containing coating also heightened the flashover voltage in different conditions. The results confirmed that the coating containing lubricant-loaded carriers could be considered as a potential candidate for applying on high-voltage insulators. La surface poreuse glissante infusée de liquide (SPGIL), inspiré de la plante sarracénie, suscite un intérêt croissant dans de nombreuses applications, notamment pour les dangers associés à la formation de la glace. Outre la démonstration de leur potentiel hydrophobe, ces surfaces possèdent une faible hystérésis d'angle de contact inférieure à 2.5° et un faible angle d'inclinaison inférieur à 5°. Toutefois, les (SPGIL), sont confrontées à des défis liés au lessivage de l'huile qui réduit leur durée de vie. Ici, deux approches distinctes, notamment la micro-texturation et l'utilisation d'absorbants d'huile, sont présentées dans l’optique de produire efficacement des surfaces glissantes pour améliorer la durabilité de ces matériaux imprégnés de lubrifiant. Dans la première approche, la méthode de réplication a été employée pour produire des surfaces texturées imprégnées de liquide (STIL) par le biais de la technique de gravure chimique. Des analyses, comme la déplétion/récupération du lubrifiant, ont confirmé que la présence de micro-textures couplée à la viscosité du lubrifiant peuvent jouer un rôle significatif dans la régulation du taux de migration de l'huile. De plus, la combinaison de la micro-texturation et de la glissance dans les STIL peut améliorer de manière considérable les performances glaciophobes. En effet, une telle surface a démontré une contrainte d'adhérence de la glace inférieure à 20 kPa, ce qui représente quatre ordres de grandeur de moins que celle de la surface d'origine non traitée dépourvue de micro-textures. En plus, leur glaciophobicité a été montrée plus stable au fil du temps comparée à la surface d'origine dépourvue de micro-textures. Pour la deuxième stratégie, des porteurs chargés de lubrifiant ont été utilisés pour empêcher la consommation rapide d'huile, prolongeant ainsi la durée de vie des revêtements préparés. En conséquence, une pression négative a été appliquée pour favoriser l'incorporation du lubrifiant dans les pores des porteurs et augmenter ainsi sa capacité de charge. Afin, d’évaluer quantitativement l'infusion de lubrifiant, une analyse thermogravimétrique (TGA) et la méthode BET (Brunauer, Emmett et Teller) ont été utilisées. Habituellement, un taux élevé de lubrifiant (avoisinant 50 % m/m) est couramment utilisé pour améliorer les performances glaciophobes. Cependant, la présence d'une quantité excessive de lubrifiant dans la matrice peut entraîner une diminution des caractéristiques mécaniques des surfaces préparées. La combinaison de différents mécanismes anti-givres, comme la localisation de contrainte, la glissance et la formation de molécules non gelées, peut être bénéfique pour l’obtention des propriétés glaciophobes désirables à partir de faibles concentrations de lubrifiant. Ces mécanismes ont été obtenus en utilisant une matrice faite d’un mélange de résines, un lubrifiant infusé et un lubrifiant hydroxyl-terminé. Le revêtement glissant résultant a été développé en imprégnant des supports chargés de lubrifiant hydroxyle-terminé dans un mélange de résines polydiméthylsiloxane (PDMS) et d'alkoxy-siloxane. Le revêtement préparé a montré des capacités d’anti-givrage et de dégivrage intéressantes, ainsi qu'une bonne résistance à l'exposition aux UVs et à l'humidité. Les caractéristiques glaciophobes du revêtement peuvent être attribuées à sa glissance et à la formation de molécules non gelées. Les propriétés électriques du revêtement ont été évaluées à l'aide d'un ensemble complet de méthodes, notamment la spectroscopie diélectrique, les tests de contournement, de condensation et de plan incliné. En comparaison avec l'échantillon de référence, le revêtement contenant des porteurs chargés de lubrifiant a montré une permittivité diélectrique et un facteur de perte plus faibles. De plus, une réduction du courant de fuite, dans le cas d'humidité élevée, a été observée avec le revêtement développé. Ce dernier a également montré une augmentation de la tension d'éclatement dans différentes conditions. En somme, les résultats ont confirmé que la solution de revêtement développée contenant des supports chargés de lubrifiant pourrait être considérée comme un candidat potentiel pour une application glaciophobe sur des isolateurs à haute tension

Self-regulating surfaces for efficient liquid collection

To achieve efficient liquid collection, a surface must regulate incoming liquid accumulation with outgoing liquid transport. Often, this can be proposed to be achieved by functionalizing surfaces with non-wetting characteristics. Yet, there remain fundamental, practical limits to which non-wetting surfaces can effectively be employed. We instead utilize filmwise wetting to achieve liquid regulation via a Laplace pressure gradient induced by solid surface curvature. The key parameters affecting this capillary flow are then introduced, namely solid properties like scale and curvature and liquid properties like surface tension and density. The liquid regulation mechanism can then be employed in condensation and aerosol processes to generate enhanced flow, while the solid geometry needed to create this capillary flow itself is capable of affecting and enhancing liquid generation. Ultimately, the surface design framework can be customized to each unique application to optimize processes in HVAC, industrial steam generation, chemical depositions, and atmospheric water harvesting.Comment: 16 pages, 4 figure

Tuning Surface Wettability Through Volumetric Engineering

abstract: Many defense, healthcare, and energy applications can benefit from the development of surfaces that easily shed droplets of liquids of interest. Desired wetting properties are typically achieved via altering the surface chemistry or topography or both through surface engineering. Despite many recent advancements, materials modified only on their exterior are still prone to physical degradation and lack durability. In contrast to surface engineering, this thesis focuses on altering the bulk composition and the interior of a material to tune how an exterior surface would interact with liquids. Fundamental and applied aspects of engineering of two material systems with low contact angle hysteresis (i.e. ability to easily shed droplets) are explained. First, water-shedding metal matrix hydrophobic nanoparticle composites with high thermal conductivity for steam condensation rate enhancement are discussed. Despite having static contact angle <90° (not hydrophobic), sustained dropwise steam condensation can be achieved at the exterior surface of the composite due to low contact angle hysteresis (CAH). In order to explain this observation, the effect of varying the length scale of surface wetting heterogeneity over three orders of magnitude on the value of CAH was experimentally investigated. This study revealed that the CAH value is primarily governed by the pinning length which in turn depends on the length scale of wetting heterogeneity. Modifying the heterogeneity size ultimately leads to near isotropic wettability for surfaces with highly anisotropic nanoscale chemical heterogeneities. Next, development of lubricant-swollen polymeric omniphobic protective gear for defense and healthcare applications is described. Specifically, it is shown that the robust and durable protective gear can be made from polymeric material fully saturated with lubricant that can shed all liquids irrespective of their surface tensions even after multiple contact incidences with the foreign objects. Further, a couple of schemes are proposed to improve the rate of lubrication and replenishment of lubricant as well as reduce the total amount of lubricant required in making the polymeric protective gear omniphobic. Overall, this research aims to understand the underlying physics of dynamic surface-liquid interaction and provides simple scalable route to fabricate better materials for condensers and omniphobic protective gear.Dissertation/ThesisDoctoral Dissertation Mechanical Engineering 201

How Does Chemistry Influence Liquid Wettability on Liquid-Infused Porous Surface?

Design of Nepenthes pitcher-inspired slippery liquid-infused porous surface (SLIPS) appeared as an important avenue for various potential and practically relevant applications. In general, hydrophobic base layers were infused with selected liquid lubricants for developing chemically inert SLIPS. Here, in this current study, an inherently hydrophilic (soaked beaded water droplet with ∼20° within a couple of minutes), porous and thick (above 200 μm) polymeric coating, loaded with readily chemically reactive acrylate moieties yielded a chemically reactive SLIPS, where residual acrylate groups in the synthesized hydrophilic and porous interface rendered stability to the infused lubricants. The chemically reactive SLIPS is capable of reacting with the solution of primary amine-containing nucleophiles in organic solvent through 1,4-conjugate addition reaction, both in the presence (referred as “in situ” modification) and absence (denoted as pre-modification) of lubricated phase in the porous polymeric coating. Such amine reactive SLIPS was further extended to (1) examining the impact of different chemical modifications on the performance of SLIPS and (2) developing a spatially selective and “in situ” postmodification with primary amine-containing nucleophiles through 1,4-conjugate addition reaction. Moreover, the chemically reactive SLIPS was capable of sustaining various physical abrasions and prolonged (minimum 10 days) exposure to complex and harsh aqueous phases, where infused lubricants protect the residual acrylate groups from harsh aqueous exposures. Such, principle will be certainly useful for spatially selective covalent immobilization of water-insoluble functional molecules/polymers directly from organic solvents, which would be of potential interest for various applied and fundamental contexts

Advancing nanofabrication processes for the generation of multifunctional surfaces

Ubiquitous in the natural world, micro- and/or nano-structured surfaces can afford simultaneous control over a range of interfacial properties; providing an attractive solution for where the accumulation of fluids (fog/rain/oil) and bacteria, and the mismanaged interaction of photons, can impede the safety or efficiency of the surface. Although surfaces found in nature provide a wealth of inspiration, replicating the structures synthetically persists to be a challenge, particularly so when striving for scalability and simplicity to encourage industrial/commercial uptake. Furthermore, the fabrication challenges become amplified when aiming for sub-wavelength structures; often necessary to unlock or enhance additional functionality. In this thesis, I present novel fabrication routes based on lithography and reactive ion etching (RIE) to achieve a range of ordered structures at the nano-scale in glass and silicon, and further replicate the resultant structures into polymers. I explore scalable masking techniques including block copolymer (BCP) lithography, laser interference lithography (LIL) and nanoimprint lithography (NIL), to achieve a series of pitches from 50 – 600 nm. By coupling the masking with novel combinations of etching chemistries, and taking advantage of the etch resistivity of different materials, I fabricate high aspect ratio nanostructures through simplified processes and demonstrate their ability to target applications in wettability, photonics and anti-bacterial action. Specifically, for silicon and glass nanocones, I focus on their anti-fogging, superhydrophobic, anti-reflective and anti-bacterial properties. I also investigate the impact of the nanostructure morphology on a sub-class of water-repellent surfaces, namely, slippery liquid infused porous surfaces, and their ability to retain lubricant under dynamic conditions; continuing on the theme of smart nanostructure design and simplified fabrication to pave a route to multifunctional surfaces. It is anticipated that the surfaces and their properties will find use as car windscreens, coatings for solar panels, high-rise glass facades, and high-touch surfaces to name a few

Design of anti-icing surfaces: smooth, textured or slippery?

Passive anti-icing surfaces, or icephobic surfaces, are an area of great interest because of their significant economic, energy and safety implications in the prevention and easy removal of ice in many facets of society. The complex nature of icephobicity, which requires performance in a broad range of icing scenarios, creates many challenges when designing ice-repellent surfaces. Although superhydrophobic surfaces incorporating micro- or nanoscale roughness have been shown to prevent ice accumulation under certain conditions, the same roughness can be detrimental in other environments. Surfaces that present a smooth liquid interface can eliminate some of the drawbacks of textured superhydrophobic surfaces, but additional study is needed to fully realise their potential. As more attention begins to shift towards alternative anti-icing strategies, it is important to consider and understand the nature of ice repellency in all environments to identify the limitations of current solutions and design new materials with robust icephobicity.Chemistry and Chemical Biolog