# Superhydrophobic Surfaces Enabled by Femtosecond Fiber Laser-Written Nanostructures
## 3 Results and Discussion
### Effect of Laser Irradiation on Metal Wettability  
As laid out in Section 2.2, a series of experiments were performed to optimize the laser and process parameters, which mainly included the PRR, pulse energy, scanning speed, and pitch. In the experiments, the PRR was first evaluated from 100 kHz to 1 MHz. Two groups of parameters resulted in the highest contact angles, demonstrating the most promising results. Therefore, our discussion will be mainly focused on these two groups:  
1. the PRR of 100 kHz (energy: 8, 16, 32, 55 \(\upmu\)J; scanning speed: 50, 100, 200, 500, 800 mm/s; pitch: 2, 5, 7 \(\upmu\)m), and
2. the PRR of 200 kHz (energy: 8, 16, 32, 55 \(\upmu\)J; scanning speed: 50, 100, 200, 500, 800 mm/s; pitch: 2, 5, 7 \(\upmu\)m).  
Figure 5 plots the resulting contact angles for various pulse fluences and scanning speeds, shown on the left and right, respectively. Only one variable was adjusted for each set of measurements. From Figure 5, it is evident that when the pulse fluence increased from 0.5 to 2 J/cm\({}^{2}\), the contact angle increased with the pulse fluence and reached the maximum value at the fluence of 2 J/cm\({}^{2}\). The further increase in the pulse fluence to 20 J/cm\({}^{2}\) caused a reduction in the contact angle. This was because the ablation-dominated process where excess fluence was present removed too much material from the surface before re-solidification [31]. A similar trend can be observed for the scanning speed. The maximum contact angle was reached at the scanning speed of 100 mm/s. The further increase in the scanning speed caused the ablation to be less effective. These results indicate that a balance between material ablation and re-solidification is critical in creating the desired hydrophobic surface.  
Later in the study, the pitch dependence on hydrophobic performance was also investigated. Interestingly, it was found that the pitch had a more pronounced impact on the hydrophobic properties than the other parameters, as shown in Figure 6. With the other parameters kept constant, a pitch of 5-6 \(\upmu\)m exhibited the best hydrophobic properties (166\({}^{\circ}\) contact angle and 3\({}^{\circ}\) rolling angle). Pitches that were too narrow (e.g., 2 \(\upmu\)m) caused too much beam overlap between the two adjacent scanning lines, which led to a diminishing of randomly oriented structures. In contrast, too wide of a pitch (e.g., 8 \(\upmu\)m) left some areas less processed or completely unprocessed.  
Figure 4: The formation of heterogeneous hierarchical structures (a few circled in red as examples) on a metal surface that resulted in hydrophilic traits, presented with 100 \(\upmu\)m (left) and 10 \(\upmu\)m (right) scale bars.  
The optimal combination of fs fiber laser parameters was found to be a PRR of around 200 kHz, an energy of 16 \(\upmu\)J (i.e., 2 J/cm\({}^{2}\)) per pulse, a scanning speed of around 100 mm/s, and a pitch of 5 \(\upmu\)m. Under the optimal parameters and conditions, we achieved a contact angle as high as 171\({}^{\circ}\) and a rolling angle of less than 3\({}^{\circ}\). To the best of our knowledge, this result exhibits the best superhydrophobic properties of a metal surface.