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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Polystyrene Petri dishes, aminated by a plasma deposition process, were surface modified by the covalent linking of two different enzymatically modified hairy regions (HRs) from pectin containing, for example, rhamnogalacturonan-I and xylogalacturonan structural elements. The two polysaccharide preparations share the same structural elements of apple pectin, but the relative amounts and lengths of the neutral side chains present differ. Surface analysis by X-ray photoelectron spectroscopy, contact angle measurement, and atomic force microscope (AFM) force-separation curves was used to characterize the effects on surface chemistry and interfacial forces of the surface modification process. Cell adhesion experiments using continuous L-929 fibroblasts and primary aortic smooth muscle cells were performed to evaluate the effect of the polysaccharide nature on cell adhesion. Results show that immobilization of the HR affects the interfacial field of forces and the cell behavior: "equilibrium" contact angles, obtained by a recently introduced vibrational approach, decrease after HR immobilization reaching a value close to 20 degrees . AFM force-separation curves show a more extended (or softer) interface in the case of the HR bearing longer side chains. Accordingly, depending on the HR preparation, cells shifted from spread morphology and adhesion behavior quantitatively comparable to that observed on conventional tissue culture polystyrene to rounded morphology and significantly lower adhesion. These data show that engineering of plant pectins can be a valuable tool to prepare novel and finely tuned polysaccharides having different chemico-physical and biological properties, to be used in the surface modification of medical devices and materials.

The contact angle is one of the most sensitive experimental values describing a junction between three phases, being influenced by the composition and properties of contacting media as well as the structure and composition of interfaces involved. The origins and importance of the contact angle in analysis of three-phase systems date back to the famous works on cohesion and adhesion of fluids published by Thomas Young in 1805 and later by Athanase Dupré in 1869. Since then, the contact angle has remained one of the most important values measured experimentally during characterization of solids and their wetting characteristics. Such measurements, however, involve solid surfaces that deviate from the idealized ones used in thermodynamic and mechanical modeling of three-phase junctions by Young, Dupré and others, and there is typically more than one value of contact angle measured on such surfaces. As a result, the attention of scientists and researchers in the past two centuries has been on development of methods for accurate contact angle measurements, interpretation of experimental values and understanding of the causes of contact angle value variation and contact angle hysteresis. This paper reviews advancements made in interpretation of experimental contact angles and their use in characterization of solid surfaces.

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