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The analysis of wetting properties of superhydrophobic surfaces may be a difficult task due to the restless behavior of drops on this type of surfaces and the limitations of goniometry for high contact angles. A method to validate the performance of superhydrophobic surfaces, rather than standard goniometry, is required. In this work, we used bouncing drop dynamics as a useful tool to predict the water repellency of different superhydrophobic surfaces. From bouncing drop experiments conducted over a wide range of superhydrophobic surfaces, we found that those surfaces with a proper roughness degree and homogeneous chemical composition showed higher water-repellency. We also conducted a drop condensation study at saturating conditions aimed to determine whether there is direct correlation between water repellency and condensation delay. We found that the drop condensation process is strongly related to the surface topography, as well as the intrinsic wettability. The condensation is promoted on rough surfaces but it is delayed on intrinsically hydrophobic surfaces. However, the differences found in condensation delay between the superhydrophobic surfaces explored in this study cannot be justified by their chemical homogeneity nor their efficiency as water repellent surfaces, separately.

One of the main challenges in cookware and baking industry is the fabrication of enhanced nonstick coatings including two main features: highly hydrophobic, resistant and durable. In general, life-long release coatings are prepared with a polymer-filler composite of several microns thickness deposited on food-compatible metals such as aluminum, stainless steel or aluminized steel, as the dimensions and functionality of the product. In this work we present several routes to fabricate water-repellent surfaces on aluminized steel as alternative to current nonstick coatings but with enhanced release properties and high durability. For this purpose, we explored two different strategies to directly create hierarchical texture on aluminized steel surfaces: sandblasting in combination to acid etching and single acid etching. The rough metal surfaces were further hydrophobized by using two different organic coatings: a Teflon film and a silane monolayer. The release properties were quantified through the study of the adhesion of water drops by means of sliding angle and contact angle measurements. We identified the most appropriate texturing process by assessing a balance between the water repellency properties and the lowest damage caused on the metal surface. This damage was quantified through thickness loss measurements of the aluminum layer. The durability of the samples was analyzed with Scotch-peeling test and next, with sliding angle measurements. We analyzed the roughness and morphology of the surface textures by contact profilometry and FE-SEM. The surface chemical composition was also analyzed by EDX. Our results pointed out to that the most durable repellent surfaces were those ones silanized. The combination of sandblasting and acid etching and the hydrophobization process with silane molecules enable to fabricate super-nonstick coatings on aluminized steel of high durability.

Colloidal interactions have been extensively studied due to the wide number of applications where colloids are present. In general, the electric double layer force and the van der Waals interaction dominate the net force acting between two colloids at large separation distances. However, it is well accepted that some other phenomena, especially those acting at short separation distances, might be relevant and induce substantial changes in the force profiles. Within these phenomena, those related to the surface contact angle, the hydration degree of the ions, or the pH, may dominate the force profiles features, not only at short distances. In this paper, we analyzed the effect of the pH and counterion type on the long-range as well as short-range forces between polystyrene colloidal particles by using the colloidal probe technique based on AFM. Our results confirm that the features of the force profiles between polystyrene surfaces are strongly affected by the pH and hydration degree of the counterions in solution. Additionally, we performed a study of the role of the pH on the wettability properties of hydrated and nonhydrated polystyrene sheets to scan the wettability properties of this material with pH. Contact angle measurements confirmed that the polystyrene surface is hydrophobic in aqueous solutions over the entire range of pHs investigated. These results are in good agreement with the features observed in the force profiles at low pH. At high pH, a short-range repulsion similar to the one observed for hydrophilic materials is observed. This repulsion scales with the pH, and it also depends on the hydration degree of the ions in solution. This way, the short-range forces between polystyrene surfaces may be tunable with the pH, and its origin does not seem to be related to the hydrophobicity of the material.

Aluminum-magnesium (Al-Mg) alloy and aluminum-coated steel (aluminized steel) are typically used for the manufacturing of baking trays and molds. For these applications, these materials must be modified to develop release and hydrophobic properties. With this aim, the bare substrates are typically coated with low-surface energy materials such as fluoropolymers, elastomers, or sol-gel layers. In this work, some alternative strategies to prepare these functional surfaces are presented. We used three-step processes involving (i) micro-texturing, (ii) nano layer deposition through immersion and electrodeposition, and (iii) hydrophobization. The raw substrates were sanded or sandblasted at the micro scale, accordingly. Texturization at the nano scale was achieved with a cerium layer formed by electrodeposition or solution immersion. The cerium layers were hydrophobized with fatty acids. The wetting properties of the samples were studied with tilting-plate and bouncing drop methods. We measured the surface roughness of the samples by contact profiling and analyzed their surface morphology using a field emission scanning electron microscope (FESEM). The elemental chemical composition of the samples was analyzed by energy-dispersive X-ray spectroscopy (EDX). The wettability results indicated that the best performance for the Al-Mg substrates was reached by sandblasting and later immersion in a cerium nitrate solution. For aluminized steel substrates, the best results were obtained with both electrodeposition and immersion methods using a cerium chloride solution., The authors were supported by the University of Cordoba (Spain), which financed this work through the Own Research Plan 2019. This research was partially financed by the State Research Agency (SRA) and European Regional Development Fund (ERDF) through the project MAT2017-82182-R.

In this article, a compilation of results on direct force measurements between colloidal particles in monovalent salts carried out with the colloidal probe technique based on Atomic Force Microscopy was presented. The interaction forces between similar and dissimilar particles was studied and it was concluded that, in general, these force profiles may be satisfactorily quantified by the DLVO theory down to distances of few nanometers. However, in the specific case where the charge of one of the involved particle is close to neutral, it was found that the surface potential of this particle may change its sign depending on the sign of charge of the opposite particle. In this respect, the assumption that the surface potential of a particle is a property only related to the particle surface features and the bulk properties is called into question. Microsc. Res. Tech. 80:144-152, 2017. © 2016 Wiley Periodicals, Inc.

The design of durable superhydrophobic coatings for metal surfaces is a subject of interest and research. Galvanized steel is one of the most used metallic materials for components of automobiles, building structures and roofing. In spite of its wide number of applications, galvanized steel has been scarcely modified to reach superhydrophobicity. The main reason for this is that galvanized steel is essentially a zinc-coated steel surface and most of the strategies to prepare superhydrophobic coatings on metal substrates require partial removal of the surface material. For this reason, providing a non-aggressive strategy to create superhydrophobic galvanized steel (or other metal coated materials) is an important challenge. With this aim, we propose in this paper a methodology based on a two-step texturing process (sandblasting and “soft” acid-etching) followed by a fluoropolymer deposition as a non-invasive strategy to produce water repellent surfaces on this material. The roughness of the samples was analyzed by confocal microscopy and FE-SEM imaging, the surface chemical composition by EDX and the wettability properties by contact angle measurements. Our results show that both texturing methods are necessary to create hierarchically micro/nano-structured surfaces on galvanized steel. This structure induces superhydrophobic properties once the metal is subsequently Teflon-coated. Sandblasting introduces a micro-scale texture, while soft acid-etching incorporates nano-asperities.

Hypothesis Wettability of solid surfaces is mostly probed with sessile drops rather than bubbles because this method is readily followed out. This recurrent use may lead to a misleading connection of certain phenomena to the hydrophobicity/hydrophilicity of materials. For instance, the Cassie-Baxter regime and the wicking effect are generally associated only to hydrophobic and hydrophilic surfaces, respectively. However, the same phenomenology should be observed when air bubbles (underwater conditions) in contact with solid surfaces are used instead. In particular, one might expect that rough-hydrophilic surfaces become superaerophobic due to the appearance of a hybrid dewetting regime, like the Cassie-Baxter regime described for rough-hydrophobic surfaces. Otherwise, rough-hydrophobic surfaces might become superaerophilic due to air-wicking. Experiments To elucidate this issue, in this work, we analyzed the wettability of surfaces with very different intrinsic contact angle and roughness degree. The analysis was performed with both Sessile Drop and Captive Bubble methods. Findings Our results with captive bubbles for rough-hydrophilic surfaces revealed phenomena only explained by the occurrence of a transition from the Wenzel regime to an “inverse” Cassie-Baxter regime. In addition, our results with captive bubbles for rough-hydrophobic surfaces showed evidences of air percolation through the interconnected asperities. This effect reminds the wicking effect reproduced on rough-hydrophilic surfaces, responsible for superhydrophilicity.