# 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  
Five stages are divided into the whole icing processes: supercooling, nucleating, recalescing, freezing, and solid cooling [79; 126]. The delayed icing time generally denotes the duration time of the first three stages, which start in the original cooling from room temperatures and end in the occurrence of recalescence. Among these stages, avoiding ice embryo nucleation is the key to delaying icing [127]. Nucleation processes are divided into two types: homogeneous nucleation and heterogeneous nucleation [128]. The former generally takes place in environments without any disturbance and is completed by structure fluctuation and energy fluctuation. However, in practical environments, heterogeneous nucleation generally takes the dominant [129]. The nucleation rate of liquids can be expressed by:  
\[R(T)=R_{\text{bulk}}(T)V+R_{\text{lv}}(T)S_{\text{lv}}+R_{\text{ls}}(T)S_{ \text{ls}} \tag{5}\]  
where \(R(T),R_{\text{bulk}}(T),R_{\text{lv}}(T)\) and \(R_{\text{ls}}(T)\) are the nucleation rates of the total, the bulk, the liquid-vapor interface, and the liquid-solid interface, respectively; \(S_{\text{lv}}\) and \(S_{\text{ls}}\) are the contact areas of the liquid-vapor interface and the liquid-solid interface, respectively; \(V\) is the liquid volume, and \(T\) is the surface temperature. Compared to \(R_{\text{ls}}(T)\), the value of \(R_{\text{lv}}(T)\) is very small. Hence the total nucleation rate \(R(T)\) is mainly dependent of \(R_{\text{ls}}(T)\) and \(S_{\text{ls}}\)[5]. Among them, \(R_{\text{ls}}(T)\) is determined by the heterogeneous nucleation energy barrier, which is expressed by:  
\[\Delta G_{\text{het}}=\frac{16\pi\gamma_{\text{lv}}^{3}}{3\Delta G_{\text{V}}^ {2}}.\frac{(2+cos\theta)(1-cos\theta)^{2}}{4} \tag{6}\]  
where \(\Delta G_{\text{het}}\) is the energy barrier of heterogeneous nucleation; \(\gamma_{\text{lv}}\) is the liquid-vapor interface energy; \(\Delta G_{\text{V}}\) is the Gibbs free energy per unit volume; and \(\theta\) is the contact angle of the ice embryo on a surface. The value of \(\Delta G_{\text{het}}\) is proportional to the contact angles, where higher contact angles could induce a higher heterogeneous nucleation energy barrier, further slowing down the nucleation rate. However, as discussed above, superhydrophobic delayed icing properties work only when the CB state is retained. Once droplets transit to the Wenzel state, the significantly increased solid-liquid contact area will greatly accelerate the icing proceedings. Hence, maintaining the CB stability at low temperatures is of great importance. Many researches reveal that it is mainly affected by surface wettability and micro-nanostructure morphologies [11; 30]. Undoubtedly, wettability is the key factor that influences delayed icing properties. However, the above analysis confirmsthat the micro-nanostructure topologies play a more important role in low-temperature air pocket stability [118]. To explore the optimal ULSASs, numerous state-of-the-art micro-nanostructures have been developed. [PERSON] et al. combined femtosecond laser ablation and chemical etching to fabricate the triple-scale micro-nanostructures on copper and aluminum alloy materials [130], where the open microcone arrays were covered by densely distributed submicroflowers and nanograss (Figure 11a). It was found that the triple-scale micro-nanostructures showed a significantly longer delayed icing time of 52 min 39 s, longer than on smooth surfaces (8 min 15 s) and double-scale structured SHSs (34 min 38 s). [PERSON] et al. fabricated a hybrid micro-nanostructure with square pillars integrated Siberian-Cocklebur-like microstructures on PTFE by controlling femtosecond laser parameters [131], as shown in Figure 11b. They found that the surfaces not only had excellent superhydrophobicity but also exhibited significantly enhanced delayed icing time. [PERSON] et al.