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Electrodeposition is an effective and scalable method to grow desired structures on solid surfaces, for example, to impart superhydrophobicity. Specifically, copper microcrystals can be grown using electrodeposition by controlling deposition parameters such as the electrolyte and its acidity, the bath temperature, and the potential modulation. The aim of the present work is the fabrication of superhydrophobic copper-based surfaces by electrodeposition, investigating both surface properties and assessing durability under conditions relevant to real applications. Accordingly, copper-based layers were fabricated on Au/Si(100) from Cu(BF4)2 precursor by electrodeposition, using cyclic voltammetry and square-pulse voltage approaches. By increasing the bath temperature from 22 °C to 60 °C, the growth of various structures, including micrometric polyhedral crystals and hierarchical structures, ranging from small grains to pine-needle-like dendrite leaves, has been demonstrated. Without any further physical and/or chemical modification, samples fabricated with square-pulse voltage at 60 °C are superhydrophobic, with a contact angle of 160° and a sliding angle of 15°. In addition, samples fabricated from fluoroborate precursor are carefully compared to those fabricated from sulphate precursor to compare chemical composition, surface morphology, wetting properties, and durability under UV exposure and hard abrasion. Results show that although electrodeposition from fluoroborate precursor can provide dendritic microstructures with good superhydrophobicity properties, surfaces possess lower durability and stability compared to those fabricated from the sulphate precursor. Hence, from an application point of view, fabrication of copper superhydrophobic surfaces from sulphate precursor is more recommended.

Contact angle measurement is an effective way to investigate solid surface properties. The introduction of low-cost digital cameras, as well as software and libraries for image analysis, has made contact angle measurement potentially accessible to every laboratory. In this review, we provide a comparison of the main methods developed to evaluate contact angle from digital images, including the so-called Young-Laplace method, the circle and polynomial fittings, as well as the mask method. All methods have been implemented and compared analyzing virtual and real drop images in an open-source software, Dropen, developed as an app in MATLAB environment. The code enables single image analysis evaluation, for the robust automatic identification of the contact points and contact angle evaluation, with the goal of minimizing user inputs, automatizing the process and facilitating measurements for all users, from less experienced to advanced wetting experts. Dropen and its code are made available at BOA, the Bicocca Open Access public repository, for use and further development.

The present study explores the exploitation of ladder-like polysilsesquioxanes (PSQs) bearing reactive functional groups in conjunction with SiO2 nanoparticles (NPs) to produce UV-curable nanocomposite coatings with increased hydrophobicity and good thermal resistance. In detail, a medium degree regular ladder-like structured poly (methacryloxypropyl) silsesquioxane (LPMASQ) and silica NPs, either naked or functionalized with a methacrylsilane (SiO2@TMMS), were blended and then irradiated in the form of a film. Material characterization evidenced significant modifications of the structural organization of the LPMASQ backbone and, in particular, a rearrangement of the silsesquioxane chains at the interface upon introduction of the functionalized silica NPs. This leads to remarkable thermal resistance and enhanced hydrophobic features in the final nanocomposite. The results suggest that the adopted strategy, in comparison with mostly difficult and expensive surface modification and structuring protocols, may provide tailored functional properties without modifying the surface roughness or the functionalities of silsesquioxanes, but simply tuning their interactions at the hybrid interface with silica fillers.

Drop interaction with solid surfaces upon impact has been attracting a growing community of researchers who are focusing more and more on ‘complex’ surfaces and ‘complex’ drops. Recently, we are observing an emerging research trend related to the investigation of compound drop impact. Compound drops consist of two or more distinct continuous phases sharing common interfaces, surrounded by a third phase. Examples are core–shell and Janus drops. In this review, we address the fundamental aspects of compound drop impact and discuss the current challenges related to experimental testing and numerical simulation of multiphase fluid systems. Furthermore, we provide a perspective on the technological relevance of understanding and controlling compound drop impact, ranging from 3D printing to liquid separation for water cleaning and oil remediation.

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil-water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water-soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro-/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh-and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of w ≈ d is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter-without any additional surface engineering or functionalization-to separate oil droplets (d < 100 μm) from a surfactant-stabilized emulsion with a flux of ∼11,000 L m-2 h-1 bar-1. We believe these findings can inform the design of future oil separation materials.

Introduction: In this study,a novel numerical implementation for the adhesion of liquid droplets impacting normally on solid dry surfaces is presented. The advantage of this new approach, compared to the majority of existing models, is that the dynamic contact angle forming during the surface wetting process is not inserted as a boundary condition, but is derived implicitly by the induced fluid flow characteristics (interface shape) and the adhesion physics of the gas-liquid-surface interface (triple line), starting only from the advancing and receding equilibrium contact angles. These angles are required in order to define the wetting properties of liquid phases when interacting with a solid surface. Methodology: The physical model is implemented as a source term in the momentum equation of a Navier-Stokes CFD flow solver as an "adhesion-like" force which acts at the triple-phase contact line as a result of capillary interactions between the liquid drop and the solid substrate. The numerical simulations capture the liquid-air interface movement by considering the volume of fluid (VOF) method and utilizing an automatic local grid refinement technique in order to increase the accuracy of the predictions at the area of interest, and simultaneously minimize numerical diffusion of the interface. Results: The proposed model is validated against previously reported experimental data of normal impingement of water droplets on dry surfaces at room temperature. A wide range of impact velocities, i.e. Weber numbers from as low as 0.2 up to 117, both for hydrophilic (theta(adv) = 10 degrees-70 degrees) and hydrophobic (theta(adv) = 105 degrees-120 degrees) surfaces, has been examined. Predictions include in addition to droplet spreading dynamics, the estimation of the dynamic contact angle; the latter is found in reasonable agreement against available experimental measurements. Conclusion: It is thus concluded that the implementation of this model is an effective approach for overcoming the need of a pre-defined dynamic contact angle law, frequently adopted as an approximate boundary condition for such simulations. Clearly, this model is mostly influential during the spreading phase for the cases of low We number impacts (We < (similar to)80) since for high impact velocities, inertia dominates significantly over capillary forces in the initial phase of spreading. (C) 2014 Elsevier B.V. All rights reserved.

Understanding the interaction of supercooled metastable water with superhydrophobic surface textures is of fundamental significance for unraveling the mechanisms of icing as well as of practical importance for the rational development of surface treatment strategies to prevent icing. We investigate the problem of supercooled water drops impacting superhydrophobic textures for drop supercooling down to -17 degrees C and find that increased viscous effects significantly influence all stages of impact dynamics, in particular, the impact and meniscus impalement behavior, with severe implications to water retention by the textures (sticky versus rebounding drop) and possible icing. Viscous effects in water supercooling conditions cause a reduction of drop maximum spreading (similar to 25% at an impact speed of 3 m/s for a millimetric drop) and can significantly decrease the drop recoil speed when the meniscus partially penetrates into the texture, leading to an increase of the contact time up to a factor of 2 in supercooling conditions compared to room temperature. We also show that meniscus penetration upon drop impact occurs with full penetration at the center, instead of ring shape, common to room temperature drop impact. To this end, we describe an unobserved mechanism for superhydrophobicity breakdown: unlike for room temperature drops, where transition from bouncing to sticky (impaled) behavior occurs sharply at the condition of full texture penetration, with a bubble captured at the point of impact, under supercooled conditions, the full penetration velocity threshold is increased markedly (increasing by similar to 25%, from 2.8 to 3.5 m/s) and no bubble is entrapped. However, even though only partial texture penetration takes place, failure to completely dewet because of viscous effects can still prohibit complete supercooled drop rebound.

Understanding the interaction between liquids and deformable solid surfaces is a fascinating fundamental problem, in which interaction and coupling of capillary and viscoelastic effects, due to solid substrate deformation, give rise to complex wetting mechanisms. Here we investigated as a model case the behavior of water drops on two smooth bitumen substrates with different rheological properties, defined as hard and soft (with complex shear moduli in the order of 10(7) and 10(5) Pa, respectively, at 1 Hz), focusing both on wetting and on dewetting behavior. By means of classical quasi-static contact angle measurements and drop impact tests, we show that the water drop behavior can significantly change from the quasi-static to the dynamic regime on soft viscoelastic surfaces, with the transition being defined by the substrate rheological properties. As a result, we also show that on the hard substrate, where the elastic response is dominant under all investigated conditions, classical quasi-static contact angle measurements provide consistent results that can be used to predict the drop dynamic wetting behavior, such as drop deposition or rebound after impact, as typically observed for nondeformable substrates. Differently, on soft surfaces, the formation of wetting ridges did not allow to define uniquely the substrate intrinsic advancing and receding contact angles. In addition, despite showing a high adhesion to the soft surface in quasi-static measurements, the drop was surprisingly able to rebound and escape from the surface after impact, as it is typically observed for hydrophobic surfaces. These results highlight that measurements of wetting properties for viscoelastic substrates need to be critically used and that wetting behavior of a liquid on viscoelastic surfaces is a function of the characteristic time scales.

Icing of surfaces is commonplace in nature and technology, affecting everyday life and sometimes causing catastrophic events. Understanding (and counteracting) surface icing brings with it significant scientific challenges that requires interdisciplinary knowledge from diverse scientific fields such as nucleation thermodynamics and heat transfer, fluid dynamics, surface chemistry, and surface nanoengineering. Here we discuss key aspects and findings related to the physics of ice formation on surfaces and show how such knowledge could be employed to rationally develop surfaces with extreme resistance to icing (extraordinary icephobicity). Although superhydrophobic surfaces with micro-, nano-, or (often biomimetic) hierarchical roughnesses have shown in laboratory settings (under certain conditions) excellent repellency and low adhesion to water down to temperatures near or below the freezing point, extreme icephobicity necessitates additional important functionalities. Other approaches, such as lubricant-impregnated surfaces, exhibit both advantages and serious limitations with respect to icing. In all, a clear path toward passive surfaces with extreme resistance to ice formation remains a challenge, but it is one well worth undertaking. Equally important to potential applications is scalable surface manufacturing and the ability of icephobic surfaces to perform reliably and sustainably outside the laboratory under adverse conditions. Surfaces should possess mechanical and chemical stability, and they should be thermally resilient. Such issues and related research directions are also addressed in this article.

Drop impact onto surfaces has long been a popular and important subject of experimental, numerical and theoretical studies to explain phenomena observed both in nature and in many engineering applications. Progress in understanding and describing the hydrodynamics involved in drop impacts has been rapid in recent years, due partly to the availability of high-speed cameras, but also because of accompanying advances in theoretical and numerical approaches. Thus, for simple surfaces, i.e. smooth surfaces of uniform chemistry, the outcome of a drop impact can be well predicted over a large range of impact parameters, including quantitative values of spread dynamics and splash characteristics. This article comprehensively reviews the present level of understanding for such impact situations. However many practical applications involve impacts onto surfaces of higher complexity, either morphologically or chemically, involving textured or porous surfaces or surfaces with non-uniform wettability characteristics. This expands greatly the parameter space for which descriptions of the impact must be found and the present understanding is significantly more rudimentary compared to drop impacts onto simple surfaces. In this review such impacts are discussed by considering effects introduced by morphological changes to the surface and by changes of the wettability. Comparisons to corresponding impacts onto simple surfaces are drawn to underline the additional physical mechanisms that must be considered.

The shortcomings of the current formulation for calculating the adhesion force for drops and bubbles with noncircular contact lines are discussed. A general formulation to evaluate the adhesion force due to surface forces is presented. Also, a novel methodology, that is, IBAFA, image based adhesion force analysis, was developed to allow implementation of the general formulation. IBAFA is based on the use of multiple profile images of a drop. The images are analyzed (1) to accurately reconstruct the contact line shape, which is analytically represented by a Fourier cosine series, and (2) to measure contact angles at multiple locations along the contact line and determine the contact angle distribution based on a linear piecewise interpolation routine. The contact line shape reconstruction procedure was validated with both actual experiments and simulated experiments. The procedure for the evaluation of the adhesion force was tested using simulated experiments with synthetic drops of known shapes. A comparison with current methods showed that simplifying assumptions (e.g., elliptical contact line or linear contact angle distribution) used in these methods result in errors up to 76% in the estimated adhesion force. However, the drop adhesion force evaluated using IBAFA results in small errors on the order of 1%.