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This is the closing issue of 2017, in which we offer eight distinctively unique scientific contributions. The collection comprises one communication and seven research articles. The first three papers are on contact angles and wetting phenomena, followed by two papers on coatings of biomaterials, a paper on manipulation of oxide layer of a titanium alloy, a paper on thin inorganic films and a final paper on nanocomposite polymer electrolytes.Asymmetry of liquid droplets on rough and heterogeneous solid surfaces with fabricated microscopic patterns of grooves and strips has been observed and reported in the literature in the past. However, for the first time, a team from Israel led by Abraham Marmur demonstrates that a large liquid droplet prefers an axisymmetric shape on rough surfaces made of parallel microscopic grooves if forced to overcome mechanical barriers and reach the most stable thermodynamic state.1 This observation -supported by experimentation -is an important development in the science of wetting on rough surfaces and we strongly recommend everyone who studies contact angles and wetting phenomena to read the paper by Cwickel et al. 1Zhang and Kornev analyzed the wetting of ribbon-shaped fibers in comparison to cylindrical fibers. 2In this interesting contribution, the authors show that the sharp edges of ribbons prevent formation of barrel-like drops characteristic of cylindrical fibers. They also demonstrate a transition from the liquid column to the clam-shell drops that are not controlled by liquid volume but by the contact angle. The authors explain this intriguing and surprising phenomenon through analysis of surface tension and capillary forces acting on the droplet.Hundreds of publications on superhydrophobic surfaces and coatings were published in a variety of journals since the middle of the 1990s. In spite of this enormous progress, there are still market-driven demands for coatings that are durable, environmentally friendly and weather resistant. The research team led by Alidad Amirfazli designed a new coating made of siloxane resin reinforced with nanosilica.3 Chemical bonding between silica and resin was controlled using a tin catalyst. The authors demonstrate fabrication of this durable superhydrophobic coating on aluminum surface using a spraying technique.Surface manipulation, modification or coating is commonly explored in engineering of medical implants. Shomali et al. examined the effect of poly(L-lactic acid) (PLLA) polymer on biodegradation and biocompatibility of zinc. 4 Zinc and its alloys have been introduced in 2013 as a new class of biodegradable medical metal, whereas PLLA is biodegradable polymer commonly used in the medical field as a coating in drug-eluting vascular stents. Shomali et al. demonstrate a successful coating of zinc wire with a microscopic uniform film of PLLA that delays degradation of zinc implant in the vascular environment of rats. This polymer, however, reduces biocompatibility of the metal, a surprising outcome that was never reported in the literatu...

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Experiments to understand the effect of surface wettability on impact characteristics of water drops onto solid dry surfaces were conducted. Various surfaces were used to cover a wide range of contact angles (advancing contact angle from 48° to 166°, and contact angle hysteresis from 5° to 56°). Several different impact conditions were analyzed (12 impact velocities on 9 different surfaces, among which 2 were superhydrophobic). Results from impact tests with millimetric drops show that two different regimes can be identified: a moderate Weber number regime (30 < We < 200), in which wettability affects both drop maximum spreading and spreading characteristic time; and a high Weber number regime (We > 200), in which wettability effect is secondary, because capillary forces are overcome by inertial effects. In particular, results show the role of advancing contact angle and contact angle hysteresis as fundamental wetting parameters to allow understanding of different phases of drop spreading and beginning of recoiling. It is also shown that drop spreading on hydrophilic and superhydrophobic surfaces occurs with different time scales. Finally, if the surface is superhydrophobic, eventual impalement, i.e., transition from Cassie to Wenzel wetting state, which might occur in the vicinity of the drop impact area, does not influence drop maximum spreading.

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A balance of surface science and aerodynamic knowledge is brought to bear to elucidate the fundamental parameters determining the incipient motion (runback) for a drop exposed to shearing airflow. It was found that wetting parameters such as contact angle are very influential in determining the minimum required air velocity for drop shedding. On the basis of experimental results for drops of water and hexadecane (0.5-100 microL) on PMMA, Teflon, and a superhydrophobic aluminum surface, an exponential function is proposed that relates the critical air velocity for shedding to the ratio of drop base length to projected area. The results for all of the water systems can be collapsed to self-similar curves by normalization. Results from other researchers also conform to the exponential self-similar functional form proposed. It was shown that the data for hexadecane drops can be matched relatively well to those for water drops by means of a corrective factor based on fluid properties and contact angles. Also, the critical air velocity for shedding from the superhydrophobic surface is seen to be more constant over a range of volumes than for the other surfaces. Finally, contact angle measurements from airflow shedding experiments are compared to measurements made by tilted plate and quasi-static advancing and receding tests. The observed differences between contact angles from different measurement methods show that the transfer of contact angle data among various applications must be done with care.

The effect of surfactants on wetting behavior of super-hydrophobic surfaces was investigated. Super-hydrophobic surfaces were prepared of alkylketene dimer (AKD) by casting the AKD melt in a specially designed mold. Time-dependent studies were carried out, using the axisymmetric drop shape analysis method for contact angle measurement of pure water on AKD surfaces. The results show that both advancing and receding contact angles of water on the AKD surfaces increase over time ( approximately 3 days) and reach the values of about 164 and 147 degrees , respectively. The increase of contact angles is due to the development of a prickly structure on the surface (verified by scanning electron microscopy), which is responsible for its super-hydrophobicity. Aqueous solutions of sodium acetate, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, and n-decanoyl-n-methylglucamine were used to investigate the wetting of AKD surfaces. Advancing and receding contact angles for various concentrations of different surfactant solutions were measured. The contact angle results were compared to those of a number of pure liquids with surface tensions similar to those of surfactant solutions. It was found that although the surface tensions of pure liquids and surfactant solutions at high concentrations are similar, the contact angles are very different. Furthermore, the usual behavior of super-hydrophobic surfaces that turn super-hydrophilic when the intrinsic contact angle of liquid on a smooth surface (of identical material) is below 90 degrees was not observed in the presence of surfactants. The difference in the results for pure liquids and surfactant solutions is explained using an adsorption hypothesis.