# A Review on Ultrafast Laser Enabled Excellent Superhydrophobic Anti-Icing Performances
## 4 Advances and Challenges of ULSASs
### Passive Superhydrophobic Anti-Icing Surfaces
#### 4.1.1 Delayed Icing/Frosting
adopted a picosecond laser to fabricate the cauliflower-like protrusions with nanostructures on the open micro-gratings [132], as shown in Figure 11c. They revealed that the laser-processed multi-tier micro-nanostructures moderated the heat loss to delay the icing process. Summarizing recently reported data on ULSASs [133; 134; 135; 136; 137; 138], Figure 12 shows the delayed icing times versus different droplet volumes and materials. Compared to organic and inorganic ULSASs, most metallic surfaces have better delayed icing properties. This is attributed to the differences of the formed micro-nanostructures on different materials owing to different light-matter interaction mechanisms (discussed in Section 2.2). Some chemical component changes during ablating also account for the differences. Figure 11: Delayed icing properties of ULSASs. (**a**) Image sequence of icing processes on the pristine copper surface, the double-scale structured ULSAS, and triple-scale structured ULSASs (reproduced  
with permission from [130]. Copyright (2021) Chinese Laser Press). (**b**) SEM images of the hybrid micro-nanostructure with square pillars integrated Siberian-Cocklebur-like microstructures (reproduced with permission from [131]. Copyright (2020) Elsevier). (**c**) Schematics showing the cauliflower-like protrusions with nanostructures on the micro-gratings (reproduced with permission from [132]. Copyright (2020) Elsevier). In contrast to droplet freezing, frosting is a more complicated process, involving more phase transformations [137; 138; 139]. Generally, two types of frosting modes are included: desublimation frosting and condensation frosting [140]. Desublimation frosting involves the gas-solid phase change and occurs mostly when the surface temperature is lower than the triple point of water. In condensation frosting, vapor first condenses onto surfaces, and then condensates freeze to form frosts when surface temperatures decrease below the icing point [4]. On hydrophobic surfaces, the required supersaturation degree for desublimation frosting is an order of magnitude larger than on hydrophilic surfaces [137]. Therefore, in practical applications, the delayed frosting on SHSs generally represents the delayed condensation frosting. Numerous studies have been made on understanding the mechanisms of condensation frosting on SHSs and developing related anti-frosting surfaces [24; 68; 141; 142]. The most remarkable is the discovery of droplet jumping on SHSs induced by the decreased Laplace pressure and droplet coalescence [21; 24]. Taking advantage of the spontaneous droplet jumping, condensates could be swept off the surfaces in time before freezing. However, some remaining droplets can still freeze to inevitably form frosts. To diminish the frosting ratio, two strategies have been mainly developed: (i) designing and optimizing micro-nanostructures to inhibit the growth of frost layers; (ii) designing the chemical components of surfaces to enhance droplet jumping. For ULSASs, the first strategy is adopted more frequently. By adjusting the microstructure sizes and morphologies, the frost growing extent and spreading directions could be well con  
Figure 12: Delayed icing times on ULSASs versus different droplet volumes and materials. The test temperature of all data was \(-10\)\({}^{\circ}\)C. Data were from the references [61; 131; 132; 133; 134; 135; 136]. trolled. Many experts have investigated frosting behaviors on ULSASs. [PERSON] et al. adjusted laser fluence and pulse repetition frequency to fabricate different micro-nanostructures and investigated the relationship between surface morphologies and anti-frosting properties [143]. They found the smaller tip area and the larger nanostructures were preferred for delaying frosting. [PERSON] et al. fabricated 3D conical microtextures covered with nanowires on copper materials [144]. In condensation environments, the fine structures could contribute to the departure of condensate droplets, thus realizing the dropwise condensation and delaying frosting.