# A Review on Ultrafast Laser Enabled Excellent Superhydrophobic Anti-Icing Performances
## 3 Anti-Icing Mechanisms of SHSs
### Relationship between Superhydrophobicity and Icephobicity  
In terms of the interfacial states between droplets and micro-nanostructures, droplets on superhydrophobic surfaces mainly exhibit two states: Cassie-Baxter (CB) and Wenzel. Although droplets in both states have CAs exceeding 150\({}^{\circ}\), their icephobicity performs differently [5]. Icephobicity works only when droplets are supported by air pockets to retain the CB state. However, once air pockets are broken, droplets will rapidly penetrate into the valleys and be stuck by surrounding micro-nanostructures. In this case, the SHSs not only lose their icephobicity but also bring about more icing danger such as easy-icing and ice difficult-removal [109]. [PERSON] et al. reported that the Wenzel ice on SHSs even showed over four times higher ice adhesion strengths than on hydrophilic surfaces [25]. Hence, superhydrophobicity is not equal to icephobicity but is one necessary but not insufficient condition.  
To ensure the icephobicity of superhydrophobic surfaces, droplets should retain the CB state but not the Wenzel state. Nonetheless, much research indicates droplets tend to passively transit from the CB state to the Wenzel state under external disturbances [79; 110; 111], such as freezing, vibration, external pressure, and so on. Affected by the energy barrier between the Wenzel state and the CB state, the spontaneously reversible transition was once regarded as impossible. Many experts have systematically investigated the icing and melting processes on different micro-nanostructured superhydrophobic surfaces [79]. Although some found ultralow ice adhesion strengths for mL-scale ice columns, the transition from the CB state to the Wenzel state still inevitably occurred when the droplet size diminished to the \(\mu\)L-scale. Figure 6 shows the icing and melting processes of droplets on laser-fabricated superhydrophobic surfaces. Compared to other SHSs, it was discovered that droplets on laser-fabricated SHSs possessed a significantly delayed icing ability, which contributed to more bubbles dissolving in supercooled droplets and further dissolving out in ice droplets. During melting processes, these bubbles could spontaneously and rapidly impact downwards under the Marangoni effect, leading to the recovery of the CB state. The dewetting transitions were closely related to surface structures, which further affected the superhydrophobicity, the delayed icing time, and the liquid retraction resistance forces. Although it was confirmed that laser-induced micro-nanostructures could well ensure the robustness of dewetting transitions, the room-temperature superhydrophobicity was still overwhelmingly different from the low-temperature icephobicity owing to complex interfacial interactions at low temperatures [112]. To ensure the anti-icing properties of SHSs, extraordinary superhydrophobicity is undoubtedly one of the most significant conditions, but there are still many other factors that need to be guaranteed. As follows, three main failure factors of superhydrophobic anti-icing surfaces are discussed.