## 3 Results and Discussion
### Direct Laser Interference Patterning of Metallic Substrates  
DLIP has shown to be an outstanding technology for the processing of metallic surfaces. In the past, this technology has been applied a wide number of materials including aluminum, stainless steel, copper and titanium between others using nanosecond and picosecond pulsed laser systems [31; 32; 33; 34; 35]. Depending on the parameters used to irradiate the materials, very different surface topologies can be obtained. For instance, the pulse-to-pulse overlap has shown to have significant effect on the height of the produced patterns. This effect is shown exemplary in Figure 5, for the case of line-like periodic arrays produced in ferritic steel and Ti6 Al4V using high pulse-to-pulse overlaps in Figure 5a,b, respectively. For the irradiation process, a picosecond pulsed laser system (10 ps) was used and the overlapping angle was varied to obtain 5.2 \(\upmu\)m and 2.9 \(\upmu\)m periods for Figure 5a,b, respectively. For both materials, a pulse-to-pulse overlap value of 99% was used as well as a laser fluence of 1.42 J/cm\({}^{2}\). In addition to the very well defined periodic line-like patterns, additional repetitive features can be observed on both materials. The nanostructures mostly visible at the interference maxima positions of the DLIP structures consist of periodical features which can be classified as to different types of laser-induced periodic surface structures (LIPSS) [36]. The features observed parallel to the DLIP structures correspond to low spatial frequency LIPSS (LSFL) with a spatial period of approximately 800 nm, which is similar to the used laser wavelength (1064 nm). In addition, also high spatial frequency LIPSS (HSFL) can be observed (see inset in Figure 5a), but in this case they are perpendicular to the DLIP structures with a spatial period of approximately 200 nm. The same features could be observed in the irradiated Ti6 Al4V (see inset in Figure 5b) with similar feature sizes. The orientation of both LSFL and HSFL are related to the direction of the polarization of the laser radiation. While the LSFL are in general oriented perpendicular to the beam polarization, the HSFL are parallel oriented which is in good agreement with the literature [37]. Figure 4: Water contact angle response measured in orthogonal directions for the different pulse-overlap conditions, for anisotropic line-like on pillars structures (**a**) and isotropic pillars on pillars structures (**b**) [30]. In addition to line-like patterns, using two-beam DLIP, also pillar like patterns can be produced, using the procedure previously described for the polymer materials. Examples of these structures are shown in Figure 6 on ferritic steel and Ti6 Al4V, using picosecond (10 ps) IR (1064 nm) radiation. Additional information about the methodology employed can be found in [37]. Also in this case, the pulse-to-pulse overlap was set to 99%. The spatial period for both materials was 5.2 \(\upmu\)m and a laser fluence of 1.42 J/cm\({}^{2}\) was used. The used spatial period is similar to the feature sizes observed in the Colocasia esculent surface, with super-hydrophobic properties [38]. The DLIP treatment at the mentioned conditions allowed achieving depths of 5.0 \(\pm\) 0.5 \(\upmu\)m and 4.2 \(\pm\) 0.3 \(\upmu\)m on ferritic steel and Ti6 AlV4, respectively. Consequently, aspect ratios (ratio between structure depth and spatial period) of 0.96 and 0.80 were achieved, for a total cumulative fluence of 284 J/cm\({}^{2}\). The last value clearly shows the high amount of energy per unit of area that is required for fabricating high-aspect ratio patterns using ps-laser sources. However, they are comparable with other experiments in the literature for instance involving direct laser writing, where cumulated fluences up to \(\sim\)400/cm\({}^{2}\) have been reported for producing patterns with relative high aspect ratios (\(\sim\)1-2) [15].