Fluorine-Free Transparent Superhydrophobic Nanocomposite Coatings from Mesoporous Silica

In recent decades, there has been a growing interest in the development of functional, fluorine-free superhydrophobic surfaces with improved adhesion for better applicability into real-world problems. Here, we compare two different methods, spin coating and aerosol-assisted chemical vapor deposition (AACVD), for the synthesis of transparent fluorine-free superhydrophobic coatings. The material was made from a nanocomposite of (3-aminopropyl)triethoxysilane (APTES) functional mesoporous silica nanoparticles and titanium cross-linked polydimethylsiloxane with particle concentrations between 9 to 50 wt %. The silane that was used to lower the surface energy consisted of a long hydrocarbon chain without fluorine groups to reduce the environmental impact of the composite coating. Both spin coating and AACVD resulted in the formation of superhydrophobic surfaces with advancing contact angles up to 168°, a hysteresis of 3°, and a transparency of 90% at 550 nm. AACVD has proven to produce more uniform coatings with concentrations as low as 9 wt %, reaching superhydrophobicity. The metal oxide cross-linking improves the adhesion of the coating to the glass. Overall, AACVD was the more optimal method to prepare superhydrophobic coatings compared to spin coating due to higher contact angles, adhesion, and scalability of the fabrication process

Wetting properties of protective coatings based on structured surfaces

Extreme water repellency observed on the surface of lotus leaf and the mechanism behind it is well studied and used for the fabrication of self-cleaning materials. Recently, the slippery mechanism of the rim of Nepenthes pitcher plant was revealed and mimicked in the so called Slippery Liquid-Infused Porous Surfaces (SLIPS). Here, we have fabricated silica nano-porous coatings using hydrophobic fumed silica. The partial impregnations of the coatings with different amounts of non-volatile oil (squalane) allows us to gradually tune its wetting behaviour from a superhydrophobic state in the absence of oil, through various intermediate wetting states, to slippery surfaces at high loading of oil. Superhydrophobic and slippery surfaces produced were found to possess low retention of water drops and good anti-biofouling characteristics towards algae cells. The intermediate coatings between the superhydrophobic and to slippery surfaces were found to exhibit different degrees of water and algae cells adhesions in similar trends. A simplified one-step deposition for large scale production of the coatings has been developed from a single formulated dispersion. In order to understand the wetting mechanism of the squalane-impregnated surfaces, the coatings were duplicated into solid states by similar partial impregnation using curable polydimethylsiloxane, PDMS (Sylgard 184). In these forms, the SEM images have clearly revealed the morphology of these gradual oil-impregnated coatings to spans from hierarchical nano/micro-structured coatings through larger scales of roughness to smoother coatings. Studies with curable Sylgard have led to the development of a simple and environmentally friendly method to grow ultra-thin polymeric films on solid surfaces for the fabrication of a more stable coatings and the modification of structured materials such as silica/aluminium oxide coatings as well as different metals and paper materials. In-depth characterisation of the fabricated substrates including, the porosity and thickness of the coating as well as the distribution and mass of the impregnated oil have been determined using capillary-driven impregnation, Dektak profiling system, fluorescence microscopy, and gravimetric analysis, respectively. Wetting properties of the coatings such as contact angles, sliding angles, sliding forces, contact angle hysteresis and adhesion of algae cells have been investigated and are discussed in term of the resulted changes in the surface architectural structures and surface chemistry following the gradual oil impregnation

Development Of Polydimethylsiloxane Mixed Matrix Membrane

Mixed matrix membranes (MMMs) were prepared from Polydimethylsiloxane (PDMS) filled with surface-treated fumed silica (SiO2). Techniques (TGA, SEM, and FTIR) were employed to characterize the neat membrane (PDMS) and the mixed matrix SiO2-PDMS membrane (SiO2-PDMS). The results confirmed SiO2-PDMS has improved thermal property over neat PDMS. Uniform dispersion of SiO2 within the membrane was observed. Good material interaction between PDMS and fumed SiO2 was observed. The effects of SiO2 on the transport pattern and permeability of O2, N2, CO2, and CH4 were studied. Results showed that the presence of SiO2 in PDMS matrix had a significant impact on N2 transport pattern. The incorporation of SiO2 into polymer matrix gave rise to an increase in solubility dominant flux and a corresponding decrease in the diffusivity of the gases through the MMMs as evident in the permeability trend of: PCO2 \u3e PCH4 \u3e PO2 \u3e PN2. 10% SiO2-PDMS maintained stability under continuous and repeated exposure to oxygen. 10% SiO2-PDMS exhibited both improved O2 permeability of about 640 Barrer and O2/N2 selectivity of 3.42 against neat PDMS with O2 permeability of about 520 Barrer and O2/N2 selectivity for O2 over N2 of 2.59. However, neat PDMS had a fair performance for the separation of CO2/CH4 gas pair with CO2 permeability of about 3239 Barrer and selectivity of 4.16 against 10% SiO2-PDMS with 2967 Barrer and selectivity 4.29. This confirmed that SiO2 as nano filler in PDMS is not a suitable material for separation of CO2/CH4 gas pair but could be suitable for the separation of CO2/N2 gas pair

Preparation and characterization of mixed matrix membrane based on Polysulfone (PSF) and Lanthanum Orthoferrite (LaFeO3) for gas separation

The aim of this study is to investigate the effects of polysulfone (PSF) and lanthanum orthoferrite (LaFeO3) incorporated mixed matrix membrane (MMM) on gas permeation and selectivity properties. PSF/LaFeO3 MMMs were prepared with various weights loading of LaFeO3. The membranes obtained were characterized using scanning electron microscope (SEM), thermal gravimetric analysis (TGA) and Fourier-transform infra-red (FT-IR). The gas transport properties of MMM were measured using single gas permeation set up (CO2, CH4, O2 and N2) at ambient temperature, and feed pressure of 2, 4 and 6 bar. The permeation test showed that the mixed matrix membrane exhibited high permeability. With increasing LaFeO3 weight loading to 1.0%, the highest permeability values were 47.74 GPU for CO2, 29.85 GPU for CH4, 57.56 GPU for O2, and 40.66 GPU for N2. The results also showed that by incorporating 1.0wt% of LaFeO3 into PSF matrix, the highest CO2/CH4 and O2/N2 selectivity of 1.60 and 1.42 respectively were obtained. Overall, all the resultants MMM showed higher permeability and selectivity compared to pure PSF membrane

Composites of Poly(dimethylsiloxane) and Silica Nanoparticles as an Approach to Manipulating the Conductivity of Stretchable Gold Films

Stretchable conductors play a fundamental role in wearable electronics as device electrodes and interconnects. Polydimethylsiloxane (PDMS) is conventionally used as the platform for stretchable conductors which are fabricated by depositing a thin layer of gold metal onto the surface. These gold films fail electrically under minimal strains (15-20% linear elongation) due to uncontrolled crack propagation through the metal film. Adding a rough microstructured polymer layer onto the PDMS surface preserves electrical conductivity in metal films to higher elongations of 60% due to the numerous sites for strain localization. However, such heterogeneous layered systems are vulnerable to delamination, leading to device failure. In this thesis, we fabricate a composite layer by combining silane modified silica nanoparticles together with PDMS prepolymer on top of an elastomeric substrate to enable the fabrication of stretchable gold films. Chapter 2 describes the fabrication and deposition of this composite layer onto an elastomeric substrate for robust interlayer adhesion for the fabrication of stretchable gold films. Changing the loading and dispersity of the particles in the composite layer enables gold films to stretch to 50 - 70% elongations. There is a very linear change in resistance with elongation that is unique to each particle loading creating a tunable system with the opportunity for different strain sensor applications. Chapter 3 expands on the work in Chapter 2 by examining the effect of dramatically changing the particle size, from 12 nm (Chapter 2) to 300 nm on the organization and structure of the composite surface and the influence on the retention of conductivity with stretching of overlying gold films. By increasing the size of the particles, they associated less with one another and with the polymer resulting in films with less topography and higher resistances upon elongation

An investigation into the effects of substrate properties on the mechanics of corneal epithelial cells

Cells respond to mechanical changes in their extracellular environment, as reflected by various cell behaviours and observed through changes in the tissue biomechanics. Types of cell behaviour that are regulated by mechanical cues in the microenvironment of the cell are cell spreading, migration, proliferation and differentiation. Cell migration is a key part of many biological processes including corneal wound repair. Changes in the biomechanical properties of the cornea can be induced by refractive and therapeutic treatments and also by diseases of the eye or other illnesses. A Rabbit Corneal Epithelial (RCE) cell line was used to study cell mechanics and cell migration. Polydimethylsiloxane (PDMS), a biocompatible silicone elastomer, was used as a substrate to culture RCE cells. In order to promote cell attachment and growth, the hydrophilicity of the PDMS surface was increased by treating it with oxygen-rich cold atmospheric pressure plasma, which was confirmed by surface characterisation techniques. Cell attachment and growth studies over time comparing plasma and non-plasma treated PDMS showed an increase in RCE cell growth and area coverage on plasma treated PDMS. [Continues.

In-situ icing and water condensation study on different topographical surfaces

Icephobicity is intrinsically affected by rough asperities and the surface voids provide anchoring points for the ice. The anchor of ice is likely to form on the surface under high humidity conditions. In-situ water condensation and icing observation were conducted to understand water condensation and ice retracting patterns in controlled humidity, pressure and temperature conditions. It was observed that water micro-condensation and icing occurred on rougher surfaces and the water droplets condensed along the surface cracks of the superhydrophobic polydimethylsiloxane (PDMS) based nanocomposite coatings. Further analysis revealed that ice anchoring was present on both aluminum and superhydrophobic coating surface, but it was more severe and intensified on the as-received aluminum substrates. No water condensation or subsequent icing was found on smooth PDMS hydrophobic surfaces due to the incapacity of the smooth surfaces to anchor water drops. It is the first time to validate ice anchoring over retracting ice on different wettability surfaces from in-situ icing observation. Ice adhesion strengths were also measured on the studied surfaces and the results indicated a strong linkage between centrifugal shearing of ice and anchoring mechanism due to surface rough voids, and there was no clear relevancy between ice adhesion strength and the surface wettability or hydrophobicity

Novel Synthesis of Silica-Supported Fischer-Tropsch Catalysts for Second Generation Biofuels

The objective of this study is to improve the catalytic performance of silica-supported Fischer-Tropsch cobalt based catalyst. Iron and nickel catalyst were also briefly studied. Initial work focused on synthesis of porous silica via oxidative thermal decomposition of polydimethylsiloxane (PDMS) and its characterisation. It was shown that PDMS undergoes at least two thermal degradation steps to form silica powder. It was also demonstrated that increase in isothermal time at constant temperature and increase in temperature at constant time could be used to tune the surface area and pore volume of the synthesized silica powder. Subsequently, a novel one pot technique called the swelling in method (SIM) was developed, and employed to synthesize silica-supported cobalt, iron and nickel based Fischer-Tropsch catalyst. The results of silica-supported cobalt based catalyst prepared by the swelling in method were compared with those synthesized by incipient wet impregnation method. The colloidal method was also combined with the swelling in method to prepare silica-supported cobalt nanoparticles catalyst. Characterisation of cobalt, iron and nickel based catalyst prepared by the swelling in method showed that PDMS as the initial catalyst support converted to silica powder after oxidative calcination. Physicochemical properties of silica-supported cobalt, iron and nickel catalyst prepared by the swelling in method suggest that the oxides of each metal were present inside the silica pores while cobalt based catalyst prepared by the same method had better surface area and pore volume compared to the catalyst synthesized by the incipient wetness impregnation technique. Catalytic performance of the catalyst synthesized by the swelling in and incipient wetness methods were studied in High Temperature Fischer-Tropsch synthesis reaction condition. The results showed that silica-supported cobalt based catalyst prepared by the swelling in method was overall more active, generated less methane and less susceptible to deactivation by sintering and carbon deposition when compared to the catalyst prepared by the impregnation technique. Silica-supported cobalt nanoparticles catalyst had the best catalytic activity in comparison to all the catalyst studied in this work. Silica-supported cobalt based catalyst prepared by the swelling in method using cobalt nitrate exhibited the best catalytic activity while the catalyst synthesized from cobalt acetate had the least activity. The addition of ruthenium to silica-supported cobalt catalyst contributed in minimising the formation of methane when compared to the catalyst without ruthenium. Silica-supported iron and nickel based catalyst showed reasonable catalytic activity, and as expected the amount of methane generated by nickel catalyst was relatively very high compared to all the catalyst studied in this thesis

Fabrication of durable self-healing superhydrophobic coating to improve the performance of high voltage insulators during winter conditions

With the emerging advancements in different fields of new materials, polymers, and technologies which have been revolutionary in industries, the demand for new generation of multi-functional materials with specific properties is highly growing. Superhydrophobic and self-healing materials are among these developments and have arisen as an unstoppable demand in the recent decades. In real world applications, coatings and surfaces are subjected to mechanical damages that are severe threat to the integrity of the structures. Once polymeric structures are damaged, there might be few limited methods available to sustain their functional lifetime. By the inspirations from mother nature and biological systems, the self-healing composite materials are designed to trigger a self-repair response without any or slight external human intervention. Herein, we aimed at designing a multifunctional superhydrophobic coating in order to increase the effective life-span of high-voltage insulators by preventing and/or delaying the possible arcing and flashover driven damages that originated from wettability issues and mechanical damages. Firstly, a telechelic silanol terminated polydimethylsiloxane (DMS-S12) and catalyst (Dibutyl tin dilaurate, DBTL) were encapsulated inside poly (melamine-urea-formaldehyde) shells separately via emulsion polymerization technique. The encapsulation of core materials, surface morphology and size distribution of microcapsules, and thermal stability of microcapsules were investigated. The synthesized microcapsules were obtained within a size range of 10-110 μm showing a spherical and uniform morphology, and thermal stability up to elevated temperatures. The microcapsules were incorporated inside a polydimethylsiloxane (PDMS) elastomer matrix, namely SILGARD 184, and the healing performance of the silicone composite was evaluated by monitoring a crack repair via scanning electron microscopy (SEM) and measuring the extent of recovery in mechanical properties via tensile and tear tests. The composites containing microcapsules depicted self-healing efficiencies of 67% and 55% calculated based on the recovered toughness and tearing energy of the healed samples. Secondly, a silicone-based superhydrophobic (SHP) coating was developed using spray coating method which was applicable to a variety of substrates including glass, porcelain, aluminum, and steel. The developed coating exhibited contact angle of 163° and contact angle hysteresis of 2.3° with excellent self-cleaning (in both dry and wet pollution scenarios) and icephobic (low ice adhesion and high delay in freezing time) properties. Robustness and durability which are the Achilles heel of superhydrophobic materials were assessed via a set of mechanical and chemical testing techniques in which the great non-wetting properties of the as-prepared coating was shown to be maintained even after various extreme treatments, i.e., waterjet impacting, immersing in pollutants and acid/base solutions for 24 h, tape peeling test, and sandpaper abrasion. Thirdly, the performance of superhydrophobic coating under electrical stress was evaluated employing a variety of methods including dielectric spectroscopy analyses, flashover voltage measurements tests, condensation, and inclined plane tests. The SHP coating represented lower dielectric permittivity and loss factor compared to a pristine sample within the frequency range of 10-4 – 103 Hz. Also, the leakage current results showed that the coating successfully reduced the leakage current on its surface in environments with high humidity. Moreover, the developed coating was able to increase flashover voltage in different conditions including dry, wet, and polluted states. Lastly, the obtained self-healing microcapsules were adapted for incorporation in thin layer surface coatings by some modifications in the process parameters. The SEM observations illustrated mean diameters of 18 and 16 μm for microcapsules. For the evaluation of self-healing ability, the scratched coatings were visually inspected using microscopy imaging. Electrochemical Impedance Spectroscopy (EIS) was employed to quantitatively investigate the self-healing function of the as-prepared coating. The self-healing efficiency and delamination index of the coating were calculated using the obtained data from EIS measurements (charge transfer resistance (Rct) and impedance (Z0.01 Hz)). The data showed self-healing efficiencies up to 96% compared to the blank superhydrophobic coating. The delamination index of the samples also confirmed the healing of the microcrack after 48 hours. Avec l’émergence dans plusieurs domaines de nouveaux types de matériaux, polymères et technologies qui ont révolutionné les infrastructures sociales et industriels, la demande pour des matériaux de nouvelle génération multifonctionnels à propriétés spécifiques enregistre une forte croissance. Notamment, les matériaux superhydrophobes et autoréparant ont connu une ascension fulgurante, au cours des dernières décennies. Dans des applications pour la vie quotidienne, les revêtements et les surfaces subissent de sévères contraintes mécaniques qui affectent l’intégrité des structures à long terme. Pourtant, il existe très peu de méthodes disponibles pour maintenir l’efficacité et leur durabilité lorsque les surfaces polymériques sont abimées. Ainsi, s’inspirant de mère nature et du potentiel auto-régénérassions des systèmes biologiques, des matériaux composites autoréparant ont été développé pour apporter une solution auto-réparatrice avec très peu voire aucune intervention humaine. Ici, la conception d’un revêtement superhydrophobe multifonctionnel capable d’augmenter la durée de vie effective des isolateurs à haute tension est au coeur de notre préoccupation. Ce dernier vise à prévenir voire retarder les éventuels dommages mécaniques causés par les arcs électriques et les contournements résultant de problèmes de mouillabilité et de ces dommages. Dans un premier temps, un polydiméthylsiloxane doté d’une terminaison en silanol téléchélique (DMS-S12) et un catalyseur (dilaurate de Dibutylétain, DBTL) ont été encapsulés séparément dans membrane de poly (mélamine-urée-formaldéhyde) via la technique de polymérisation en émulsion. L'encapsulation des matériaux du noyau ainsi que la morphologie de surface, la distribution de leur taille et la stabilité thermique des microcapsules ont été étudiées. Les microcapsules synthétisées présentaient un diamètre compris entre 10-110 μm avec une morphologie sphérique et uniforme, et une stabilité thermique jusqu'à des températures élevées. Celles ont été incorporées par la suite à l'intérieur d'une matrice élastomérique de polydiméthylsiloxane (PDMS), dénommée SILGARD 184 pour former le revêtement composite. Le potentiel de cicatrisation du composite de silicone ainsi obtenu a été évalué en surveillant une réparation de fissure par microscopie électronique à balayage (MEB) et en mesurant l'ampleur de la récupération des propriétés mécaniques par des essais de traction. Dans un second temps, le revêtement développé a été appliqué par pulvérisation sur une variété de substrats notamment le verre, la porcelaine, l'aluminum et l'acier. Le revêtement présent un angle de contact de 163° et une hystérésis d'angle de contact de 2.3° avec d'excellentes propriétés autonettoyantes (évalué en pollution sèche et humide) et glaciophobes (faible adhérence à la glace et retard élevé du temps de congélation). La robustesse et la durabilité représentent généralement le talon d'Achille des matériaux superhydrophobes. C’est pourquoi un ensemble des essais mécaniques et chimiques ont été effectués pour évaluer la robustesse du revêtement final vis à vis des conditions réels. Les résultats recueillis ont confirmé la stabilité des propriétés du revêtement développé soumis à conditions extrêmes. Troisièmement, la performance du revêtement superhydrophobe final (SHP) sous contrainte électrique a été évaluée à l'aide de diverses méthodes tels que la spectroscopie diélectrique, des tests de mesure de tension d'amorçage, de condensation et des tests de plan incliné. Le revêtement SHP offrait une permittivité diélectrique et un facteur de perte inférieurs à ceux d'un échantillon vierge dans la plage de fréquences est comprise entre 10-4 et 103 Hz. De plus, les résultats portant sur le courant de fuite ont montré que le revêtement réduisait avec succès le courant de fuite à sa surface dans des environnements à forte humidité. En plus, une augmentation de la tension de contournement dans différentes conditions qu’il s’agisse d’un état sec, humide ou pollué a été observée. Enfin, les microcapsules autoréparants obtenues ont été adaptées/ redimensionnées pour être incorporées dans des revêtements de surface de manière à obtenir une couche mince grâce à certaines modifications des paramètres du procédé. Les observations MEB ont révélé des diamètres moyens de 18 et 16 μm pour les microcapsules. Concernant l'évaluation de la capacité d'auto-guérissons, les revêtements ont été rayés et inspectés visuellement en utilisant l'imagerie par microscopie. La spectroscopie d'impédance électrochimique (EIS) a été également utilisée pour étudier quantitativement la fonction d'auto-guérissons du revêtement tel que préparé. L'efficacité d'auto-guérissons et l'indice de délaminage du revêtement ont été calculés à l'aide des données obtenues à partir des mesures EIS (résistance de transfert de charge (Rct) et impédance (Z0.01 Hz)). Les données ont montré des efficacités d'auto-guérissons allant jusqu'à 96% par rapport au revêtement superhydrophobe vierge. L'indice de délaminage des échantillons a également confirmé que la cicatrisation de la microfissure a lieu après 48 heures

The chemistry and CVD of hydrophobic surfaces

This thesis details the use of chemical vapour deposition (CVD) to deposit hydrophobic surfaces, in addition to this, the functional properties are investigated and further characterisation of the surfaces extreme water repulsion (superhydrophobicity) is made. The design and manufacture of surfaces that repel water (hydrophobic) draws much inspiration from the natural world, including examples of superhydrophobic leaves. The way water can interact with a surface is characterised, with many examples of superhydrophobic surface generation provided from the literature, along with general routes toward their formation. The main aspects of CVD depositions are addressed and examples of hydrophobic surfaces using this technique are cited. The novel deposition of thermosetting and thermosoftening polymers has been investigated, with the role of the CVD deposition mechanism emphasised. The deposition of the polymer occurs via the preformation of polymer particles, which is not typical in CVD, these were then deposited onto the substrate. The result was an easy-to-produce and robust superhydrophobic thin film, constructed from an inherently hydrophobic material. The same principle is then expanded to silica microparticles, films of the particles were deposited on to a substrate with hydrophilic surfaces originally deposited. The silica films were subsequently rendered exceptionally superhydrophobic by a simple post-treatment. The formation of copper films is then reported, using copper nitrate precursors a relatively flat metallic copper film was formed. The films were then roughened by reaction to form copper hydroxide nano-crystals, this hydrophilic surface is again functionalised to render it superhydrophobic. All films deposited were characterised using energy dispersive X-ray analysis, glancing angle X-ray diffraction, UV/Vis spectroscopy, infra-red/Raman spectroscopy and scanning electron and atomic force microscopy were used to study surface morphology, with the hydrophobicities of each surface quantified. The superhydrophobic elastomer films underwent microbiological testing in order to examine the adhesion of bacteria. A substantial reduction in the ability of bacteria to attach to the superhydrophobic surfaces was observed and rationalised through a reduction in available contact between the media of the bacteria (water) and the surface material. The dynamic interaction between water and surfaces was examined through water bouncing. The dependence of water bouncing on surface hydrophobicity and microstructure was studied, in addition to the effect of water droplet volume and impact velocity. A new definition and scale for superhydrophobicity is proposed, through the ability of water droplets to bounce on a surface. Finally the insight gained from previous work carried out is used in developing a device for separating mixtures of oil and water, through the use of superhydrophobic meshes