coatings Article Improving the Laser Texture Strategy to Get Superhydrophobic Aluminum Alloy Surfaces Annalisa Volpe 1,2, * , Sara Covella 1 , Caterina Gaudiuso 1,2 and Antonio Ancona 1,2   Citation: Volpe, A.; Covella, S.; Gaudiuso, C.; Ancona, A. Improving the Laser Texture Strategy to Get Superhydrophobic Aluminum Alloy Surfaces. Coatings 2021, 11, 369. https://doi.org/10.3390/coatings 11030369 Academic Editor: Chi Wai Chan Received: 3 March 2021 Accepted: 22 March 2021 Published: 23 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Dipartimento Interateneo di Fisica, Università degli Studi di Bari, 70126 Bari, Italy; s.covella1@studenti.uniba.it (S.C.); caterina.gaudiuso@uniba.it (C.G.); antonio.ancona@uniba.it (A.A.) 2 Institute for Photonics and Nanotechnologies, CNR IFN, Via Amendola 173, 70126 Bari, Italy * Correspondence: annalisa.volpe@uniba.it Abstract: Changing the wetting properties of surfaces is attracting great interest in many fields, in particular to achieve a surface with a superhydrophobic behavior. Laser machining is an emerg- ing technique to functionalize materials with high precision and flexibility without any chemical treatment. However, when it is necessary to treat large area surfaces laser-based methods are still too slow to be exploited in industrial productions. In this work, we show that by improving the laser texture strategy it is possible to reduce the laser processing time to produce superhydrophobic aluminum alloy surfaces. Three different surface texture geometries were micromachined; namely, square, circular and triangular lattice grooves. We found that if the spacing between the grooves is narrow, i.e., when the percentage of the textured surface is high, the volume of air trapped inside the micromachined structures plays an important role in the wetting behavior. Meanwhile, when the groove spacing approaches the droplet dimensions, the texture geometry has a preponderant influence. Based on these findings an appropriate choice of the laser texture strategy allowed the fabrication of superhydrophobic aluminum alloy surfaces with a 10% reduction of processing time. Keywords: femtosecond laser; laser texturing; wettability; superhydrophobicity; aluminum alloy; aerospace applications; optimization 1. Introduction Modifying the wettability of surfaces plays an important role in many application fields from aerospace [1,2] to civil engineering and [3] microfluidics [4]. In particular, recreating the superhydrophobicity (SH), namely surfaces with a high water contact angle (>150 ◦ ) and a low contact angle hysteresis or roll-off angle (<10 ◦ )[5], is attracting many re- searchers aiming to obtain surfaces with anti-bacterial [6], anti-icing [7,8] and self-cleaning properties [9], for example. Modifying the surface morphology by short/ultrashort laser micromachining repre- sents a highly flexible green process that is applicable to a wide range of materials from metals [10,11] to polymers [12,13]. In particular, direct laser writing (DLW) is a flexible and fast choice to reproduce the superhydrophobicity of the lotus leaf. However, upscaling this technology to an industrial level to functionalize large area surfaces at reasonable costs and times still requires dedicated research efforts starting with the choice of the most appropriate and cost-effective laser source and beam steering system and, finally, defining the quickest and most efficient texturing strategy [14,15]. In DLW, several laser sources have been employed from nanosecond (ns) [16] to ul- trashort laser pulses [17]. Though ns-lasers represent a lower cost solution, when thermal effects have to be avoided and it is requested to machine precise and reproducible micro- metric structures, ultrafast lasers are usually preferred [18]. The recent availability on the market of ultrashort laser sources delivering hundreds of watts [19] has opened the possi- bility to significantly improve the machining throughput. However, the heat accumulation phenomena observed as soon as the average power exceeds tens of watts compromise the Coatings 2021, 11, 369. https://doi.org/10.3390/coatings11030369 https://www.mdpi.com/journal/coatings