## 3 Results and Discussion
On exposed surfaces, the droplets were placed 180 days after the exposition (15\(\times\)(Ti/Zr)) and 2 days after the exposition (15\(\times\)(Zr/Ti) and 8\(\times\)(Zr/Cr/Ti)). Figure 2: Representative 2D-FT of the image shown in Figure 1a. Inset: detailed view of the central part. Intensity scale (bar on the right) is in arbitrary units. As a consequence of ultra-short laser beam interaction with metal surfaces in open air, the surface plasmon polariton (SPP) occurred at the interface between the metal and the dielectric (air) surfaces, inducing periodic distribution and accumulation of the energy on the surface. Because fs pulse duration is shorter than the characteristic relaxation time of the material, energy deposition is shock-like and leads to the ablation of the material, leaving parallel trenches--LIPSS. The spatial period of LIPSS is less than the incoming wavelength. The heat load to the surrounding material was minimal. The ablated material was dispersed away in the form of nanoparticles with the dimensions of up to 100 nm. In this  
Figure 3: The image of the droplet of distilled water on the surface of: (**a**) 15\(\times\)(Ti/Zr) on Si (Ti is the topmost layer), as-deposited, the CA = 72.11\({}^{\circ}\); (**b**) 15\(\times\)(Ti/Zr) on Si (Ti is the topmost layer), laser-modified, the CA = 137.15\({}^{\circ}\); (**c**) 15\(\times\)(Zr/Ti) on Si (Zr is the topmost layer), as-deposited, the CA = 77.29\({}^{\circ}\); (**d**) 15\(\times\)(Zr/Ti) on Si (Zr is the topmost layer), laser-modified, the CA = 144.49\({}^{\circ}\); (**e**) 8\(\times\)(Zr/Cr/Ti) on Si (Zr is the topmost layer), as-deposited, the CA = 68.10\({}^{\circ}\); (**f**) 8\(\times\)(Zr/Cr/Ti) on Si (Zr is the topmost layer), laser-modified, the CA = 123.45\({}^{\circ}\). White bar denotes 5 \(\upmu\)m. Golden lines are added for the visualization. way, the deposited energy was mostly invested in the ablation and not in heating, which implies that, for fs interaction, the creation of a thick oxide layer did not take place. The values of the LIPSS periods estimated from the micrographs, as well as measured values of contact angles of both surfaces, are summarized in Table 1. The period matches between 72 and 77% of the value of the implemented wavelength. The uniformly distributed LIPSS are oriented in the normal to the polarization direction of the incoming beam and their period is not close to the used laser wavelength for all observed samples. These types of LIPSS originate from the interference of the incident laser beam with surface electromagnetic wave excited during the laser treatment [8]. The CA values indicate that non-corrugated surfaces are hydrophilic. Similar CA values for surfaces unmodified by the laser suggest that they are not much influenced by the difference in the composition. The difference in CA between the untreated samples of Zr/Ti and Zr/Cr/Ti could be explained by long-range wetting transparency [30]. For surfaces modified by laser beam, the values also do not differ much. However, a strong increase after the interaction by the fs laser beam suggests it is dominantly caused by the structural change--the occurrence of LIPSS--and not by the composition change. Different to the case where a 40\({}^{\circ}\) rise in hydrophobicity of Ti surface was obtained by \(\upmu\)m corrugations [17], better results in this work were obtained by nm corrugations. In open air ambience, all transition metals (except a few) are covered with a thin native oxide layer [31]. Different values of CA in the literature are the consequence of different preparation methods, which lead to differences in the chemical composition on the interface. Thin zirconium oxide layer (80-90 nm) on the SiO\({}_{2}\) substrate shows small values of CA (hydrophilicity), but thicker layers show higher values of CA [32; 33]. Oxides of titanium have various values of water contact angles [34].