## 3 Results and Discussion
### Direct Laser Interference Patterning of Dielectric Polymers
Although the fabrication of hierarchical structures was possible, it is worth mentioning that through the double-irradiation method is not possible to create well-balanced structures (DLIP pillars on large pillars) within two irradiation steps. Another strategy to fabricate balanced hierarchical structures consists on performing the structuring process with a pulse-based method. This consists in moving the sample using the translational stages to a certain position and then irradiating it with a certain number of laser pulses. After that, the sample is moved in X and Y to a new position and this procedure is repeated until the surface is totally treated. Since the structuring of polycarbonate presents a Gaussian unselective ablation for every pulse, for high number of pulses this method can produce an alternation of valleys and heights in both directions and hierarchical structures can be fabricated in a single-step process. Choosing correctly the pixel distance and the number of pulses,hierarchical structures consisting of line-like structures on the top of pillars can be fabricated in a single step process, similar to the ones presented in Figure 3d. Consequently, a second structuring process can be performed by irradiating orthogonally to the first DLIP lines with a low-overlap scan-based process, which can selectively ablate only this pattern, creating balanced pillars on the top of large pillars (Figure 3e). The peak-to-valley height of the small-scale pillars is around 0.7 \(\upmu\)m, sensibly lower than the one reached in the scan-based process (1.0 \(\upmu\)m), leading to a smoother hierarchical topography (Figure 3f). This behavior can be explained by taking into account that two processes concur in the creation of the pillars, while only a single process is sufficient to fabricate the upper line-like structures. The change in the topography, structure geometry and orientation resulted to have an effect on the wettability response of the treated polycarbonate samples. For this purpose, the water contact angle (WCA) has been measured in orthogonal directions, corresponding to the direction parallel (front) and perpendicular (lateral) to the DLIP lines of the structures. The results, correlated with the pulse overlap of the structures, are presented in Figure 4, reporting the results for the structures presented in Figure 1d,e, respectively. As can be seen, in the case of anisotropic structures (Figure 4a) the WCA in both directions increase with the number of pulses, since the structure depth increases as well. However a strong  
Figure 3: (**a**) Top-view SEM image of the hierarchical line-like on line-like structures parallel oriented, (**b**) line-like on line-like structures perpendicularly oriented, (**c**) line-like structures on pillars before cleaning, (**d**) line-like structures on pillars after cleaning, (**e**) pillar-like on pillar-like structures and (**f**) details of the hierarchy of pillar-like on pillar-like structures [30]. difference (-20\({}^{\circ}\)) can be appreciated between the WCA values measured in the front and lateral direction. This can be ascribed to the strong anisotropy of the structures realized with this method. On the other hand, when the structures are sufficiently balanced in both directions, the hydrophobicity increases with the structure depth and the WCA values are comparable in both directions (Figure 4b). As a result of the different high-rate irradiation processes, the structuring has made possible to change the polycarbonate wettability, increasing the static contact angle from 87.8\({}^{\circ}\)\(\pm\) 0.4\({}^{\circ}\) to 117.6\({}^{\circ}\)\(\pm\) 1.8\({}^{\circ}\), without significantly change the surface chemistry [30]. Moreover, due to the different structure geometries obtained, different wetting regimes and preferential wetting directions were possible.