# Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing
## 3 Superhydrophobic, Self-Cleaning, and Antifouling Surfaces  
According to the impressive estimation from [8] superhydrophobic plant leaves (e.g., grasses) comprise in total an area of around 250 million km\({}^{2}\), which is about 50% of the total surface of our planet. The surfaces of many plant leaves are known to be superhydrophobic and water-repellent for a long time. However, only since the 1970s, with the introduction of scanning electron microscope studies, has it been revealed that their unique properties are related to the specific surface structure of the leaf surfaces. Since then, the surface structures and wetting properties of almost 20,000 biological superhydrophobic species have been discovered and investigated [8, 49, 50, 51, 52]. One of the most famous biological superhydrophobic species is _Nelumbo nucifera_ (Lotus). It was found that the specific surface structure of lotus leaves is built up of microstructures (distinctively convex to papillose micrometer-sized epidermal cells), which are covered by nanoroughness (a very dense layer of epicticular waves) (Figure 7). Biological research initiated technical implementation of self-cleaning functional surfaces and, therefore, in this case, the biomimetic development process can be clearly referred to as biology push or bottom-up. Since the beginning of the 1990s, a large number of scientific papers have been dedicated to the analysis and abstraction of superhydrophobic surfaces. Below, the basics of wettability phenomena, which are required for the abstraction of superhydrophobicity, are briefly summarized [Box 1]. For deeper insight, the following works are recommended: [1, 53, 54]. \begin{table}
\begin{tabular}{c l c c} \hline \hline
**Process Parameters** & **Influence on Morphology of Column Structures** & **Material** & **References** \\ \hline \multirow{4}{*}{Collimated beam diameter} & Average column height and distance between neighboring & Silicon (Si) & [48] \\  & columns decrease by decreasing the collimated beam diameter & & \\  & at a fixed fluence. & & \\  & Threshold fluence for the formation of columnar structures & Steel & [18] \\  & decreases with increasing beam diameter. & & \\ \hline \multirow{2}{*}{Angle of incidence} & By variation of angle of incidence inclined columns can & Nickel (Ni) & [39] \\  & be generated. & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 7: A flowering plant of the Lotus family (_Nelumbo nucifera_) (**a**); a spherical water droplet on a superhydrophobic leaf (**b**); SEM image of the lotus leaf surface (**c**). The wettability of the solid surface by the liquid depends on interfacial tensions \(\gamma_{\rm sa}\), \(\gamma_{\rm sl}\), and \(\gamma_{\rm la}\)--solid-air (\(\gamma_{\rm sa}\)), liquid-solid (\(\gamma_{\rm sl}\)), and solid-air (\(\gamma_{\rm la}\))--and is characterized by the measurement of the contact angle. The correlation between the contact angle and the interfacial tensions is the famous Young equation:  
\[\cos\theta_{\rm Y}=\frac{\gamma_{\rm sa}-\gamma_{\rm sl}}{\gamma_{\rm la}}.\]  
The Young equation describes the case of an ideal solid surface, which is assumed to be smooth, rigid, chemically homogeneous, insoluble, and non-reactive. Surfaces that are characterized by a contact angle <91\({}^{\circ}\) are referred to as wetting (if the liquid is water, they are called hydrophilic); those with a contact angle \(\geq\)91\({}^{\circ}\) as non-wetting (or hydrophobic in the case of water). On smooth, low-energy surfaces, a maximum water contact angle of approx. 120\({}^{\circ}\) can be achieved [55]. By the interaction of liquids with a structured surface, different wetting states can be realized. In the first state, the liquid wets the structured surface completely, forming a homogeneous solid/liquid interface.