## 3 Results and Discussion
### Direct Laser Interference Patterning of Metallic Substrates
Similarly, cone-like or spike-like geometries (corresponding to LIPSS morphologies) with high aspect ratio also required of high cumulated fluence vales (e.g., 300 pulses with \(\sim\)1 J/cm\({}^{2}\) = 300 J/cm\({}^{2}\), for steel) [18]. Like in the line-like patterns, also LSFL and HSFL were observed, but because of their orientation it can be assumed that they were fabricated during the second irradiation step. Finally, water contact angle (WCA) measurements of the produced pillar-like patterns were performed. In the case of the ferritic steel substrate, a WCA of 5\({}^{\circ}\) was measured directly after the laser process denoting a hydrophilic character of the surface. For the Ti6 Al4V material, a similar behavior was observed, with a WCA of 30\({}^{\circ}\). However, after 30 days, the WCA significantly increased for both surfaces up to 120\({}^{\circ}\) and 76\({}^{\circ}\) for the ferritic steel and Ti6 Al4V, respectively. This increase has been observed not only for DLIP but also other laser based processing methods and has been explained in relation with an increasing amount of carbon in the processed surface when exposed in normal environment conditions [39]. In other materials such stainless steel, contact angles over 160\({}^{\circ}\) could be obtained denoting also the importance of the material and thus the surface chemistry [37]. The relatively long depth of interference field provides the ability to structure not only rough surfaces but also the tilted surface that result in the structure with asymmetrical topography. An example of such structure formed by DLIP on ferritic steel is showed in Figure 7b. The structure was formed by scanning the 2 beam interference spot of 83 \(\upmu\)m in diameter across the tilted by 45  
Figure 5: Line-like periodic structures fabricated by Direct Laser Interference Patterning using a laser fluence of 1.42 J/cm\({}^{2}\) and 99% of pulse-to-pulse overlap on (**a**) ferritic steel (with an spatial period of 5.2 \(\upmu\)m) and (**b**) on Ti6 Al4V (with and spatial period of \(\Lambda\) = 2.9 \(\upmu\)m). The insets show the appearance of HSFL. LSFL can be also observed, mainly at the positions corresponding to the interference maxima. surface and keeping the overlap of 97% between the spots. The picosecond laser emitting pulses of 70 ps at 1 kHz repetition rate with the fluence of 0.87 J/cm\({}^{2}\) was applied in this case. The topography cross-section of the fabricated structure measured with confocal microscope is plotted in Figure 7a. It can be seen, that the resultant structure contains periodic line-like grooves formed of inclined planes. The plane inclination angle of 7\({}^{\circ}\) was measured (note that the scales in Figure 7a are different). As can be seen from the magnified SEM image of the single inclined plane (Figure 7c) the formation of the previously mentioned LIPSS oriented perpendicularly to the direction of the structure is observed on top of those planes. The measured period of these LIPSS was in this case \(\sim\)886 \(\pm\) 50 nm, which is close to the used laser wavelength of 1064 nm and therefore corresponds to the low spatial frequency LIPSS (LSFL). The high spatial frequency LIPSS (HSFL) with the period of 180 \(\pm\) 30 nm was also found in between the grooves of the LSFL (see inset of Figure 7c). Taking into account all different scale structures formed in this case, the overall morphology of such multiscale-structure can be attributed to the hierarchical structure with asymmetrical topography. The resultant asymmetry of the topography alters the isotropy of the induced functional properties, conferring them directional properties, which, for example, can be used to transport water droplets and soft materials, control liquid spreading, provide directional adhesion and exhibit directional friction [40]. Figure 6: Pillar-like structures fabricated by Direct Laser Interference Patterning on (**a,b**) ferritic steel and (**c,d**) Ti6 Al4V substrates.