# A Review on Ultrafast Laser Enabled Excellent Superhydrophobic Anti-Icing Performances
## 4 Advances and Challenges of ULSASs
### Passive/Active Combined Superhydrophobic Anti-Icing Surfaces
#### 4.2.1 Electrothermal/Superhydrophobic Anti-Icing Surfaces  
In terms of different heating methods, electrothermal/superhydrophobic anti-icing surfaces are divided into two main types [166; 167]: one is achieved by directly installing heating elements under SHSs; the other is enabled by electricity power input to transform from electricity to heat. The former has no requirement for the electrical conductivity of SHSs while the latter requires surfaces with good conductivity and electro-to-heat conversion performances. [PERSON] et al. used an ultrafast laser to fabricate the hierarchical micro-nanostructures on the NACA 0012 airfoil [151]. After chemical treatment, the surfaces showed strong water repellence with a contact angle of \(163^{\circ}\pm 6^{\circ}\) and a roll-off angle of \(8^{\circ}\pm 6^{\circ}\). Combining the SHSs and their bottom-heating elements (Figure 16a), the electrothermal/superhydrophobic deicing tests were conducted in the icing wind tunnel environment (TAS (true air speed) = 65 m s\({}^{-1}\); SAT (static air temperature) = \(-10^{\circ}\)C/\(-20^{\circ}\)C; LWC (liquid water content) = 0.2 g m\({}^{-3}\) and AoA (angle of attack) = \(3^{\circ}\)). Figure 16b shows the plot of the required deicing time as a function of heating power density. The power density and time of deicing on the reference surfaces were significantly higher than on the laser-fabricated SHSs. For the SATs of \(-10^{\circ}\)C and \(-20^{\circ}\)C, the deicing power densities on the reference surfaces reached up to \(\sim\)2.1 and \(\sim\)2.5 W cm\({}^{-2}\), respectively, while those on the SHSs decreased to \(\sim\)0.5 and \(\sim\)1.1 W cm\({}^{-2}\), respectively. [PERSON] et al. also compared the electrothermal/superhydrophobic deicing properties of pristine surfaces and laser-fabricated SHSs in the icing wind tunnel conditions (TAS = 0-200 m/s; SAT = \(-35^{\circ}\)C; LWC = 0.2\(\sim\)3.9 g m\({}^{-3}\)) [168]. They found that although external heating could avoid icing on the heating zone of the pristine surface, the runback ice would be accreted more severely at the tailing edge of airfoils. In contrast to the icing phenomena on pristine surfaces, the liquid water could quickly roll away from the laser-fabricated SHSs to prevent the runback iceing (Figure 16c).  
For the second electrothermal deicing technology, some new phases with electrothermal properties are usually added to substrates to reinforce the thermal conductivity and electro-to-heat conversion ability. The common electrothermal materials include carbon-based materials (e.g., graphene, carbon nanotube, etc.), Ag-based materials, and some conducting polymers [169]. [PERSON] et al. mixed multi-walled carbon nanotubes (MWCNT) and N-methylpyrrolidone solutions to prepare the MWCNT/PEI nanocomposite film [170]. An ultrafast laser was adopted to fabricate the micro-nanostructures, realizing the superhydrophobicity with a contact angle of 156.6\({}^{\circ}\). Under 12 V, the three-dimensional conductive network constructed by MWCNT could rapidly rise the temperature to 109 \({}^{\circ}\)C within 370 s. With excellent electrothermal and superhydrophobic properties, ice on the surfaces could completely melt in 170 s (Figure 16d).  
Figure 16: Electrothermal/superhydrophobic combined anti-icing surfaces. (**a**) Schematics showing the NACA0012 support and the heating element. (**b**) Comparison of the deicing time and deicing power on pristine surfaces and ULSASs (reproduced with permission from [151]. Copyright (2020) John Wiley and Sons). (**c**) Image sequences of electrothermal deicing on different surfaces in the icing wind tunnel conditions (reproduced with permission from [168]. Copyright (2021) Elsevier). (**d**) Schematic showing the electrothermal mechanisms when the current is directly applied on materials, and the thermographic image sequences of electrothermal deicing processes on ULSASs (reproduced with permission from [170]. Copyright (2023) Elsevier).