## 3 Results and Discussion
### Structural Characterization
For the above-mentioned example, this results in a roughness ratio r of 1.34. As mentioned above, the decrease in the apparent contact angle with laser fluence and pulse overlap can be assigned to the increasing extension of the surface, as generated by the laser structuring. This is qualitatively confirmed and highlighted in Figure 7, showing the correlation of the apparent contact angle and the roughness ratio r versus the pulse overlap. With increasing pulse overlap, i.e., increasing track energy, the roughness ratio increases, while the apparent contact angle decreases accordingly. However, comparing the roughness ratios as determined by the surface topography to the roughness ratios calculated by the Wenzel equation reveals a discrepancy. For example, for the above given example (a fluence of 1.96 J/cm\({}^{2}\) and a pulse overlap of 98%) that yields a roughness ratio of r = 1.34, the apparent contact angle is measured as 8.7\({}^{\circ}\). Using [PERSON]'s contact angle of 64\({}^{\circ}\) (see above) and the Wenzel equation, the roughness factor is determined to be 2.15. Figure 8 shows the roughness ratios as determined by the surface topography and the roughness ratios calculated by the Wenzel equation using the measured apparent contact angles and [PERSON]'s contact angle. Apparently, both methods result qualitatively in similar behavior, namely, an increase in r with increasing laser fluence and pulse overlap. However, the roughness ratio as calculated by the Wenzel equation is consistently higher than the roughness ratio determined by the surface topography. Figure 6: AFM measurement of the LSFL structure (**a**) for a laser fluence of 0.87 J/cm\({}^{2}\) and 99% overlap. (**b**) The height profile of the LSFL can be described using a \(|\sin(x)|\) function. Figure 7: First day apparent contact angle and calculated roughness factor r for a fluence of 1.96 J/cm\({}^{2}\). As a consequence, we conclude that the changes in the surface topography are not exclusively responsible for the altered wetting behavior, as it has been assumed by other studies on copper, titanium, stainless steel, and brass [29; 34; 35]. Thus, a yet unknown surface chemistry effect must be taken into account to further understand the wetting behavior of laser-structured areas. We therefore performed XPS measurements of unstructured reference samples and structured samples on different days after laser treatment. All XPS measurements have been done on previously unmeasured specimens in order to exclude any modifications of the surface chemistry during XPS measurements itself or temporarily storage under high vacuum in the XPS spectrometer. For the element surface concentration, we find immediately after laser surface structuring that the present carbon concentration strongly decreases from 46.9 to about 22%, while the oxygen concentration increases from about 30 to 52%, respectively, with oxide layers typically being associated with a hydrophilic behavior [29]. These concentrations remain almost unchanged over all days. High resolution detailed XPS carbon spectra (see Figure 9a) additionally reveal a significant increase in oxidic bonds particularly in the form of O-C=O as compared to the reference. In the oxygen spectra (Figure 9b), C-O, Si-O, C=O, and metallic oxides can be found for both the reference and laser-treated specimen, but the content of metallic oxides are increased after laser treatment. The small amount of Si occurring in the measurements is a result of locally introduced SiC particles by grinding and polishing of the reference surface. Again, these findings remain almost unchanged over days elapsing from the day of laser treatment. We therefore conclude that the changes in element concentration and the binding conditions of carbon and oxygen account for the strong decrease in the apparent contact angle \(\Theta\) upon laser structuring. However, as the XPS spectra do not reveal pronounced changes over the following days, the corresponding change in \(\Theta\) during the following days remains unclear.