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Hypothesis: Characterizing the microstructure of an ice/surface interface and its effect on the icephobic behavior of surfaces remains a significant challenge. Introducing X-ray Computed Tomography (XCT) can provide unprecedented insights into the internal (porosity) and interfacial structures, i.e. wetting regime, between (super)hydrophobic surfaces and ice by visualizing these optically inaccessible regions. Experiments: Frozen droplets with controlled volume were deposited on top of metallic and polymeric substrates with different levels of wettability. Different modes of XCT (3D and 4D) were utilized to obtain information on the internal and interfacial structure of the ice/surface system. The results were supplemented by conventional surface analysis techniques, including optical profilometry and contact angle measurements. Findings: Using XCT on ice/surface systems, the 3D and 4D (imaging with temporal resolution) structural information can be visualized. From these datasets, qualitative and quantitative results were obtained, not only for characterizing the interface but also for analyzing the entire droplet/surface system, e.g., measurement of porosity size, shape, and location. These results highlight the potential of XCT in the characterization of both droplets and substrates and proves that the technique can aid to develop hydrophobic surfaces for use as icephobic materials.

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.

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.

In this paper we present a simple but effective procedure to impart superhydrophobicity to aluminum through a one-step wet chemistry synthesis. The synthesis is performed in one step, thanks to simultaneous etching with NaOH and grafting with fluoroalkyl silane (FAS) molecules, which allow aluminum surface functionalization. By optimizing the proper amount of reagents, controlling the reaction batch temperature and particularly by allowing pre-activation of the solution, we showed that the process can be used turn into water repellant superhydrophobic surfaces four aluminum samples in sequence, using the same reaction batch. Also, if FAS is further added to the solution the procedure can impart superhydrophobicity to further aluminum samples. The synthesis with the optimized parameters is green and cost effective, since it was developed to minimize the use of both base and fluorinated molecules. In addition, it is easy to scale up to large area treatments and to industrial application, because the procedure is one-step, is reproducible, and allows multiple use of the same reaction batch, for the treatment of multiple samples. Finally, we show that not only the surfaces show excellent non-wetting properties in quasi-static conditions, with contact angles ∼150̊ and with very low contact angle hysteresis, ∼4̊, but also in dynamic conditions, as proved by means of drop impact analysis up to Weber number of 690.

In the development of anti/de-icing systems for aeronautics, wind turbines or telecommunication antennas to date, less attention is paid to coating strategies. The majority of studies dealing with coatings have focused mainly on reducing ice adhesion forces, to easily remove ice, once it has formed. In this study we focused on an alternative strategy that consists of promoting the shedding of liquid water as a way to reduce the total amount of water present on the surface that can freeze. Shedding of liquid from the surface can be enhanced by modification of surface wettability, by means of the application of superhydrophobic coatings, i.e. water repellent coatings, characterized by low water adhesion forces. To study the effect of superhydrophobic coatings on surfaces exposed to icing conditions, tests were performed in an open loop icing wind tunnel (IWT) on a standard NACA0021 airfoil in two different icing conditions. Three samples were used during the tests, each one is characterized by different wettability properties. To simulate the presence on anti/de-icing system, the wing was also equipped with an electrical heater, mounted at the inner side of the wing leading edge. Results from IWT tests demonstrated that surface wettability is an important controlling factor not only for reducing ice accretion on the wing, but also for reducing by up to 80% the energy required to avoid ice accretion on the wing. The findings from IWT tests as described, together with results from a previous work on drop shedding, reinforced the hypothesis that shedding of drops is the key controlling mechanism for an efficient icing mitigation strategy.