**1. Introduction**

Liquid-repellent surfaces have drawn huge interest in the last years due to their inherent self-cleaning properties [1–3] and other potential related advantages e.g., antior de-icing properties [4–6], drag and friction reduction [7–9], and anti-fouling behavior [10,11]. All these properties might have positive fallout on a wide range of industrial fields and applications like aircraft, building materials, ships, machinery, and countless others. Therefore, the fabrication of liquid-repellent surfaces has become one of the hottest topics in the material science community. The first and most well-known approach to the fabrication of liquid-repellent surfaces starts from the mimicry of the dual-scale (i.e., hierarchical) surface structure observed on the lotus leaf [12]. By coupling the creation of micro/nanoscale surface features with the tailoring of surface chemical composition to achieve low surface energy, superhydrophobic surfaces can be obtained [13,14]. These materials are characterized by high water contact angles, i.e., larger than 150◦, and high mobility of water drops that are placed on their surface. Such extreme water repellence is due to the trapping of air pockets between surface morphological features, leading to minimal adhesion between water drops and the solid surface. However, the lotus leaf-like surfaces show some limitations: for instance, they are not able to repel liquids with a surface tension lower than that of water (72 mN·m−<sup>1</sup> at 25 ◦C) like oils and alkanes. For that reason, the scientific community started to design surfaces with simultaneous repellence to

**Citation:** Veronesi, F.; Guarini, G.; Corozzi, A.; Raimondo, M. Evaluation of the Durability of Slippery, Liquid-Infused Porous Surfaces in Different Aggressive Environments: Influence of the Chemical-Physical Properties of Lubricants. *Coatings* **2021**, *11*, 1170. https://doi.org/ 10.3390/coatings11101170

Academic Editor: Rubén González

Received: 6 September 2021 Accepted: 23 September 2021 Published: 27 September 2021

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**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/).

water (i.e., hydrophobicity) and other liquids, mainly non-polar oils (i.e., oleophobicity). Such combination is commonly termed as amphiphobicity [15,16] or, in case the repellence is extended also to complex fluids like blood or milk, omniphobicity [17,18].

Another limitation of the lotus-mimicking approach is that the trapped air pockets can be displaced by fast impinging drops or when the liquid pressure is too high. Perhaps the most promising fabrication approach to overcome this limit has been proposed by Wong et al. [19] in 2011 and takes inspiration from the surface of Nepenthes plant [20], which is covered by a layer of liquid lubricant that makes it extremely slippery for the insects that land on it. The proposed Slippery, Liquid-Infused Porous Surfaces (SLIPS) display much lower contact angles compared to superhydrophobic surfaces but drops of many liquids can still move very easily on them. Moreover, these materials possess much improved stability when submerged, along with other enhanced properties [21,22].

Due to such exciting potential, SLIPS have drawn large interest and many papers have focused on the design criteria to achieve the best omniphobicity [23]. In principle, SLIPS can be fabricated by using any liquid as infused lubricant, but three fundamental criteria for SLIPS design always hold: the fluid to be repelled must be immiscible with the infused lubricant; the lubricant must wet the solid structures completely, even when no outer liquid is present; the outer liquid must form discrete droplets on the SLIPS. Notwithstanding these design guidelines for SLIPS, the literature still lacks an assessment of their durability in different environments that simulate their performance in real operational conditions. Moreover, a systematic investigation of the relationship between infused lubricant and durability is missing.

In this paper, we report the fabrication of SLIPS infused with different lubricants and study the evolution of their wetting properties after testing in several conditions, namely UV irradiation, immersion in chemically aggressive solutions, and abrasion. Both fluorinated and fluorine-free lubricants were tested, as the former are known for inducing excellent oleophobicity but are also deemed to be environmentally harmful [24], while the latter can only generate hydrophobicity but are much more eco-friendly. The results provide relevant insight on the behavior of SLIPS and draw guidelines for the choice of the lubricants to be infused for long-lasting repellence.
