# A Review on Ultrafast Laser Enabled Excellent Superhydrophobic Anti-Icing Performances
## 1 Introduction
Meanwhile, compared to other conventional lasers with continuous wave and short pulses, ultrafast lasers have an ultrashort pulse width from several tens of femtoseconds to tens of picoseconds, which is equivalent to or shorter than the heat conduction time of an electron lattice. Hence, a smaller  
Figure 1: Number of published articles per year indexed by Web of Science. The keywords for searching are “anti-icing” and “superhydrophobic anti-icing”. heat-affected zone and higher processing quality could be realized [40, 41]. Generally, the surface structures formed by ultrafast laser irradiation can be divided into two types: laser-ablated structures (arrayed microstructures) and laser-induced structures (random and/or periodic nanostructures) [42, 43]. The former's geometry is mainly determined by the ablation process with intense laser energy interaction with materials and can be closely controlled by user-defined structure shapes. The most typical representative of the latter, the laser-induced periodic surface structure (LIPPS), is formed by linearly polarized laser radiation with fluences slightly above the ablation threshold [44]. By varying laser processing conditions such as wavelength, pulse duration, power density, incidence angle, scanning speed, and processing environments, the morphologies of LIPPS could also be closely controlled. Adopting the above two structures, microscale and nanoscale structures on surfaces could be easily and tunably fabricated with ultrafast lasers. In the meantime, during laser ablation, the plasma plume contains abundant energetic species, which reunite and rapidly deposit on microstructures to form nanostructures, thus constituting the hierarchical micro-nanostructures [45, 46, 47]. The fabrication of various micro/nanostructures enabled by ultrafast lasers has paved a solid pathway to preparing SHSs. Although most materials such as metals and ceramics are intrinsically hydrophilic so that the superhydrophilic interfaces are produced after laser processing, the superhydrophobicity can be easily achieved by surface chemical modification, including vapor deposition, solution immersing, spinning coating, and so on [48, 49, 50]. Many reports and advances have been produced in fabricating SHSs and controlling their superhydrophobic states by ultrafast lasers [51, 52]. Compared to the SHSs fabricated by other methods, laser-fabricated superhydrophobic surfaces have shown tremendous advantages in durability, stability, and repeatability as well as high tunability [53, 54, 55, 56]. Many reviews have summarized laser-fabricated superhydrophobic surfaces in various application scenarios [40, 42, 57]. Specifically, research articles about ultrafast laser-fabricated superhydrophobic anti-icing surfaces (ULSASs) have sprung up in recent years [58, 59, 60, 61], as shown in Figure 2. However, to our best knowledge, related reviews and perspectives are rarely reported. In response to the special issue \"Superhydrophobic and Icephobic Coatings\", it is of significance and value to summarize recent progress in ULSASs and to propose current challenges and future prospects. In this review, we focus on ultrafast laser enabled superhydrophobic anti-icing performances and draw examples from surface fabrication, anti-icing mechanisms, and advances and applications, as shown in Figure 3. The fundamental wettability and ultrafast laser micro-nano fabricating examples are discussed first, elucidating the structural diversity and tunability of ultrafast laser fabricating. Adopting ultrafast laser-fabricated SHSs, some novel and interesting phenomena involving icing and melting processes are discussed with a deepened understanding of the relationship between superhydrophobicity and ice-phobicity. We clarify that superhydrophobicity is one necessary but insufficient condition of icephobicity and present three common failure factors of superhydrophobic anti-icing surfaces, including Cassie-Baxter stability, surface durability, and environmental adaptivity.