# A Review on Ultrafast Laser Enabled Excellent Superhydrophobic Anti-Icing Performances
## 3 Anti-Icing Mechanisms of SHSs
### Failure Factors of Superhydrophobic Anti-Icing Surfaces  
#### 3.2.1 Cassie-Baxter Stability  
The CB stability represents the ability of SHSs to withstand the impalement of air pockets by droplets [113; 114]. In room-temperature environments, the CB stability is evaluated by the Laplace pressure of droplets, namely, \(P=2\gamma/R_{d}\), where \(\gamma\) is the liquid surface tension and \(R_{d}\) is the radius of droplets. Higher CB stability means that SHSs can withstand higher Laplace pressure to retain the CB state of droplets. The critical Laplace pressure \(P_{C}\) is generally determined by two criteria [80; 115]: (i) the critical Laplace pressure for the CA at the moment of droplet transiting from the superhydrophobic state to the hydrophobic state (\(P_{CCA}\)); and (ii) the critical Laplace pressure for the contact diameter at the moment of the triple-phase line stops retracting (\(P_{CCD}\)). In terms of the two criteria, \(P_{C}\) is expressed by:  
\[\begin{cases}P_{C}=P_{CCA},if\ P_{CCA}<P_{CCD}\\ P_{C}=P_{CCA},if\ P_{CCA}>P_{CCD}\end{cases} \tag{4}\]  
Figure 6: icing and melting processes on the superhydrophobic surfaces fabricated by ultrafast laser (reproduced with permission from [79], copyright (2022) Nature Publishing Group).  
Many researches have confirmed that some finely designed ultrafast laser-fabricated SHSs show superior CB stability at room temperature owing to highly controllable and robust micro-nanostructures [115; 116; 117]. It is widely accepted that SHSs with higher \(P_{C}\) could show better anti-icing properties. [PERSON] et al. combined ultrafast laser ablation and chemical oxidation to fabricate two types of three-tier micro-nanostructures: the microcone-microflower-nanosheet (MNSF) and the microcone-microflower-nanograss (MNGF) [80]. The two surfaces showed ultrahigh room-temperature CB stability with the highest critical Laplace pressure of \(\sim\)1450 Pa among all the published reports (Figure 7a,b). However, although the two surfaces showed similar CB stability at room temperature, there still existed significant differences in their anti-icing properties. For example, the ice adhesion strength on MNGF was only 1.7 kPa while that on the MNSF reached 16.9 kPa (Figure 7c). The results indicated that room-temperature \(P_{C}\) was not appropriate for evaluating the low-temperature Cassie-Baxter stability. Focusing on this problem, related calculation models based on icing phenomena were recently established to evaluate the low-temperature CB stability of SHSs with air pocket pressure (Figure 8a,b) [118]. It was found that both microstructure morphologies and temperatures had a great influence on the air pocket pressure. Among these, the open-cell microstructures had higher air pocket pressure at low temperatures than the semi-open or closed microstructures, thus showing higher CB stability. As demonstrated in Figure 8c,d, the anti-icing properties corresponded well to the air pocket stability induced by microstructures, where the SHSs with more open-cell structures showed better and robust anti-icing properties. Hence, to avoid the failure of superhydrophobic anti-icing surfaces, open-cell structures should be adopted to maintain the low-temperature CB stability.  
Figure 7: Morphologies (**a**), CB stability (**b**) and ice adhesion strengths (**c**) of laser-fabricated superhydrophobic surfaces. MNGF denotes the surface with microcone-microflower-nanosheet structures while MNSF denotes the surface with microcone-microflower-nanograss structures. The SEM images in (**a**) are from the MNGF surfaces. (Reproduced with permission from [80]. Copyright (2020) American Chemical Society.)