Citation: Myronyuk, O.; Baklan, D.; Rodin, A.M.; Vanagas, E.; Yong, Z. Owens–Wendt Characterization of Femtosecond-Laser-Textured Hydrophobic Aluminum Surfaces. Coatings 2023, 13, 1104. https:// doi.org/10.3390/coatings13061104 Academic Editor: Chi Wai Chan Received: 28 April 2023 Revised: 6 June 2023 Accepted: 12 June 2023 Published: 15 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). coatings Article Owens–Wendt Characterization of Femtosecond-Laser-Textured Hydrophobic Aluminum Surfaces Oleksiy Myronyuk 1 , Denys Baklan 1 , Aleksej M. Rodin 2, * , Egidijus Vanagas 2 and Zuo Yong 1 1 Department of Chemical Technology of Composite Materials, Chemical Technology Faculty, Igor Sikorsky Kyiv Polytechnic Institute, Beresteiskyi Ave. 37, 03056 Kyiv, Ukraine; o.myronyuk@kpi.ua (O.M.); d.baklan@kpi.ua (D.B.); zuo.yong@lll.kpi.ua (Z.Y.) 2 Solid State Laser Laboratory, Department of Laser Technologies, Center for Physical Sciences and Technology, Savanoriu Ave. 231, 02300 Vilnius, Lithuania * Correspondence: aleksej.rodin@ftmc.lt; Tel.: +370-60140057 Abstract: The eligibility of applying the Owens–Wendt approach to determining the free surface energy of liquid-repellent aluminum surfaces, with micro- and nanotextures formed by a femtosecond laser, was considered. This approach has been shown to be applicable using two essential parameters that can be derived from the graphs. The first is related to the fraction of the contact area between the liquid and the solid surface in the Cassie state. The second is related to the degree of intrinsic polarity of the surface material or the applied organic modifier. The presented interpretation was used to compare the liquid repellency of the obtained textures. A microtexture with a period of 60 μm and a groove width of 45 μm has been shown to be the most liquid repellent. Among the modifiers, 1H,1H,2H,2H-perfluoroctyltriethoxysilane was the most effective, and stearic acid was only slightly inferior, but promising in terms of cost and environmental friendliness. It was shown that spontaneous hydrophobization provided a contact angle with water up to 159 ◦ , but the stability of such textures was inferior to the considered modifiers. Keywords: functional materials; superhydrophobic surface; laser micro-texturing; nanostructures; liquid-repellent coatings; alkoxysilanes; surface energy; contact angle; Owens–Wendt approach 1. Introduction Texturing is an effective way to achieve liquid-repellent surfaces bioinspired by lotus leaves, reeds, and rose petals [1]. A high throughput of metal surface micro-texturing is provided by femtosecond laser ablation [2,3]. It also allows the formation of nanotextures, including highly ordered laser-induced periodic surface structures (LIPSS). The hierar- chical surface structure makes it possible to realize a stable Cassie wetting state, when a liquid drop contacts with the protrusions of the texture, and trapped air is located in its depressions [4,5]. This provides a surface contact angle above 150 ◦ , which is defined as a superhydrophobic state [6]. An important factor in this case is the intrinsic hydropho- bicity (non-polarity) of the surface, given that metals are polar surfaces, and their oxides are predominantly hydrophilic materials [7]. Numerous studies have been devoted to methods of hydrophobization of textured surfaces, such as treatment in solution, chemical vapor deposition, as well as the phenomenon of spontaneous hydrophobization, leading to an increase in the surface contact angle above 160 ◦ after texturing [8]. Hypotheses explain this phenomenon by adsorption from the atmosphere and binding on the surface of hydrocarbons, as well as carbon dioxide [9]. The effectiveness of both induced and spontaneous methods is often assessed by the contact angle with water, as well as by indicators such as contact angle hysteresis and rolling angles [10,11]. In some publications, the surface energy of hydrophobic textured surfaces is also characterized by the Zisman and Owens–Wendt methods, which are more comprehensive tools for describing the stability of wetting states [12–14]. The first method Coatings 2023, 13, 1104. https://doi.org/10.3390/coatings13061104 https://www.mdpi.com/journal/coatings