## 3 Results and Discussion
### Wetting Behavior  
As has been reported, laser-structured surfaces alter the wetting behavior with a temporal evolution of the apparent contact angle over days. Here, we focus on a detailed analysis of the apparent contact angle \(\Theta\) as a function of the applied laser fluence and pulse overlap with specific attention on the temporal change in \(\Theta\). Figure 4 depicts the apparent contact angle versus time for different fluences between 0.87 and 1.96 J/cm\({}^{2}\). The individual error bars show the influence of the adapted pulse overlap and takes the standard deviation of a threefold measurement into account. The apparent contact angle \(\Theta\) was measured directly after the laser irradiation (referred to as Day 1) and on Days 3, 5, 9, 10, and 12. Starting from the initial contact angle of an unstructured surface of 64\({}^{\circ}\), i.e., the hydrophilic range (this contact angle is calculated by [PERSON]'s equation for a flat surface), for all fluences, the apparent contact angle after laser irradiation and surface structuring initially drops further into the hydrophilic range, a behavior that has been reported by several other groups [29, 34, 35]. During the following days, \(\Theta\) continuously increases, indicating a saturation-type behavior reaching values between 100\({}^{\circ}\) and 140\({}^{\circ}\), in accordance with previous observations on several metals [29, 34, 35]. An explanation for this is given in [29] and [21]. A CO\({}_{2}\) decomposition reaction is triggered by the laser process and generates new non-polar carbon groups on the surface which have a hydrophobic character. It is worth mentioning that, depending on the specific laser structuring parameters, such as  
Figure 3: SEM images of a laser-structured brass surface (**a**,**c**) and corresponding transformed 2D Fourier spectra for the analysis of LSFL and u-ripples (**b**,**d**). fluence, pulse overlap, and wavelength, we find that brass saturation is achieved after a period of up to 12 days. With increasing fluence, the apparent contact angle, after the initial drop, increases and leads to a higher apparent contact angle at the end of the observation period of 12 days, shown in Figure 4. Showing the influence of both laser fluence and pulse overlap on the apparent contact angle, Figure 5 details the dependence of these laser structuring parameters on the first and 12 th days after irradiation, exemplary for an intermediate fluence of 1.96 J/cm\({}^{2}\). The apparent contact angle on Day 1 significantly decreases with increasing pulse overlap, i.e., increasing track energy, whereas, on Day 12, \(\Theta\) increases only slightly with pulse overlap. This behavior can, by trend, also be found for the investigated fluences of 0.87 and 1. 63 J/cm\({}^{2}\), respectively. In particular, we find that the combination of the highest fluence (1.96 J/cm\({}^{2}\)) and the highest specific pulse overlap (98%) leads to the smallest contact angle of 8.7\({}^{\circ}\) (Figure 5). In contrast, the combination of the smallest fluence (0.87 J/cm\({}^{2}\)) and the smallest specific pulse overlap (92%) leads to the highest apparent contact angle of 45.3\({}^{\circ}\) on the first day. A similar effect was also reported from [PERSON] et al. (2016) for stainless steel and aluminium. At the end of the observation period, i.e., Day 12, all surfaces reached a hydrophobic behavior (Figure 4), and those surfaces with the highest drop of the apparent contact angle after irradiation (Day 1) reached the highest apparent contact angle on the 12 th day. In the following section, we will show that this behavior can be attributed to the laser-induced topography, its associated increase in the surface, and changes in the surface chemical composition. Figure 4: Apparent contact angle in the dependence of time and the applied laser fluence (error bars indicate the standard deviation of 12 daily measured samples). The reference contact angle on flat brass is 64\({}^{\circ}\).