Contact angles and wettability: towards common and accurate terminology

Abstract

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...

Meaningful Contact Angles in Flotation Systems: Critical Analysis and Recommendations

“…The contact angle is called 'apparent contact angle', and it is the angle measured between the intersection of the bubble surface and specimen projected plane. 19 The measurements should be done for the aqueous phase rather than the gas phase. If measurements are done for the 'just-attached' small gas bubble, the experimental value is sometimes called the 'contact angle' or 'static contact angle'.…”

“…It often refers to either the Young (for a smooth and homogeneous surface), Cassie (for a smooth but heterogeneous surface), Wenzel (for a rough and homogeneous surface) or Cassie-Baxter (for a rough surface and homogenous surface, where roughness is not completely penetrable by a liquid -extremely rare cases in practical flotation systems, which, if happen, are rather a result of the metastable state) angle and can be referred to as equilibrium contact angle if measured under equilibrium conditions. 19 This contact angle is poorly accessible experimentally, although acoustic and/or mechanical vibrations imposed on the solid or liquid or both could drive the liquid to the absolute minimum. 51 As a consequence, the most stable/equilibrium contact angle is mainly a theoretical concept, but it might play an important role in the interpretation of flotation systems, as discussed later.…”

Direct Measurements of Adhesion Forces for Water Droplets in Contact with Smooth and Patterned Polymers

“…The contact angle for PDMS matches exactly the value reported by Shahsavan et al 43 The contact angle measured for EVA is higher by about 7°than that reported by WaldoMendoza et al 44 The elevated roughness of the EVA sample (Table 1) is likely responsible for this difference in contact angle. Nylon and PET are under the category of hydrophilic polymers, 45,46 with advancing contact angles of about 63 and 77°, respectively. These values agree with contact angles reported in the literature.…”

Fabrication of superhydrophobic zirconium surface with a facile electrodeposition process

“…From related data, the ratio of carbon/oxygen/zirconium in the superhydrophobic surface was 62:9:1, and the theoretical atomic ratio of carbon/oxygen/zirconium in Zr(CH 3 (CH 2 ) 14 COO) 4 is 64:8:1. The two ratios are so close; therefore, it can be determined that zirconium palmitate (Zr(CH 3 (CH 2 ) 14 COO) 4 ) constitutes the superhydrophobic surface.…”

“…Such effect is named superhydrophobicity, wherein the surface water (advancing) contact angle (CA) is larger than 150°and the sliding angle (SA) is smaller than 10°. 3,4 A rough surface microstructure and a low surface energy composition, [3][4][5][6] on the whole, are indispensable for the creation of superhydrophobicity. Until now, general means for fabrication of superhydrophobic surface include sol-gel processing, 7,8 anodic oxidation, 9,10 spray deposition, 11 chemical etching, [12][13][14][15][16] electrospinning 17,18 and so on.…”