# Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing
## 3 Superhydrophobic, Self-Cleaning, and Antifouling Surfaces

Table 2: Overview of superhydrophobic modification of different materials by ultrafast laser processing. 400 kHz was used for structure fabrication. The surface of stainless steel underwent superhydrophobic modification by laser-inscribed structuring (column) superimposed with LIPSS or nanoroughness and silanization. Rime testing showed that the superhydrophobic surfaces only slightly slowed down the formation of rime on the coating surface, but it was demonstrated that the adhesion of frozen droplets was dramatically reduced during a ice rain test. In [68] the structured steel was proposed for reduction of ice friction. The reduction of ice friction is relevant for many applications, starting with sports and outdoor activity equipment and ending with sleds and vehicles for transport over snow and ice. Ice friction experiments with a structured slider showed a continuous decrease in the friction coefficient with increasing sliding speed in comparison to the polished slider (demonstrating initial decrease and substantial increase after passing through a minimum value). However, the main drawback of that application of superhydrophobic modification was the poor durability of the structure under the load and frictional motion. In [29], ultrafast laser-processed hierarchical superhydrophobic structures on copper were applied for drag reduction. Tests of these surfaces showed considerably higher drag reduction than what was predicted from theoretical and experimental models on non-hierarchical structures. Even better drag reduction was achieved after silanization of structured surfaces. Another application was related to the antifouling properties of superhydrophobic surfaces for prevention of bacterial attachment. Nature itself evolved superhydrophobicity in plants, predominantly as a defense mechanism against the attachment of pathogens (e.g., fungal spores) [79]. In [62,80] superhydrophobic titanium surfaces (columnar structures superimposed with nanoroughness without coating) were tested for their ability to retain different bacterial strains. It was shown that all tested bacteria strains were retained by the structured surface to different degrees. It is noteworthy that each strain was found to preferentially attach to crevices located between the columnar surface features. The upper regions of the microscale columns remained essentially bacteria-free. It was hypothesized that air trapped by the topographical features initially inhibited contact between the bacteria and the titanium substratum. Attachment of bacteria (_S. aureus_) was shown to increase substantially over a period of 1 h, which corresponds to the period of maximum replacement of air trapped between microscale columns by the incubation medium (Figure 8). Figure 8: Dynamic wettability of the lotus-mimicked superhydrophobic titanium surfaces immersed in water. Air replacement by water (**top**); graph (**bottom**) showing the percentage of air trapped in superhydrophobic titanium surfaces after 1.5-h immersion in water. (Reprinted with permission from [62]. Copyright 2011 American Chemical Society.) Replacement of air increases the surface area available for cells to make contact with the Ti surface. Notably, the quantification of air trapped by in situ small-angle X-ray scattering showed that nano-sized air bubbles trapped within nanoroughness still covered 45% of the titanium surface, even after immersion for 50 min [80]. The replacement of the entrapped air by incubation medium described above is an example of wetting transition. Generally, transition from a superhydrophobic state, described by the Cassie-Baxter equation, to a Wenzel state or a mixed Cassie-Baxter/Wenzel state is accompanied by a loosening of antifouling properties. These transitions were observed under evaporation, pressing, vibration, and bouncing of droplets [81] and are the reason, for example, for failing the rime test in [78]. The same structured surface can demonstrate excellent superhydrophobic properties under certain conditions while failing in other environments.