# Superhydrophobic Surfaces Enabled by Femtosecond Fiber Laser-Written Nanostructures
## 3 Results and Discussion
### Light-Trapping Effect  
Another important finding is the inverse correlation of hydrophobicity with the reflectance of structured metal surfaces. Hierarchical structures with presumably higher aspect ratios absorb light more efficiently, producing darker surfaces [31; 33]. In our study, we discovered that surfaces with less reflectance tended to be more hydrophobic than ones with a higher reflectance (Figure 9). Therefore, the level of reflectance would serve as a quick indicator of hierarchical structure and consequently of hydrophobicity.  
Figure 8: A series of frames of water droplets cleaning aluminum oxide powder on the hydrophobic metal surface over a time interval of 8 seconds. (**a**) First water droplet makes contact with the powder and hydrophobic region of the metal. (**b–e**) As droplets move down the surface, the powder on the metal is collected. (**f**) The water droplet with the powder rolls off the processed area.  
Figure 7: A series of frames of water droplets bouncing off a hydrophobic metal surface fabricated by fs fiber lasers over a time interval of 7 s. (**a**) Water droplet exits the needle. (**b**) First water droplet makes contact with metal. (**c**) First water droplet rolls off the hydrophobic region of the metal. (**d**) Second water droplet drops on the metal surface. (**e**) A third water droplet makes contact with the second water droplet still on the hydrophobic region of the metal. (**f**) The second and third water droplets roll off the processed area and merge with the first water droplet.  
Figure 9: A processed metal surface with <2% reflectance shows a larger contact angle (left) than the unprocessed one with about 12% reflectance (right).  
Figure 10 shows the spectral reflectance for various process conditions. A combination of proper pulse energy (i.e., 16 \(\upmu\)), scanning speed (50-100 mm/s), and pitch (5 \(\upmu\)m) creates surfaces with the lowest reflection, which is less than 2% over the whole spectral region. These optimal laser and process parameters for low reflectance are consistent with those for superhydrophobicity, which further confirms that both hydrophobicity and low reflectance require hierarchical structures with a relatively high aspect ratio of 1 (measured by ZYGO's 3D optical profiler).  
Reflectance is an important property of hydrophobic surfaces as it provides immense applications in several fields, such as solar energy. A low reflectance percentage means that a solar panel can absorb more light from the sun to convert into energy while maintaining superhydrophobic and self-cleaning properties.  
We also performed preliminary experiments to evaluate the reliability and the durability of these laser-processed metals. The stainless steel metal surface had a high resistance to extreme temperature cycles. No contact angle changes were observed before or after the metal was exposed to extremely high (71 \({}^{\circ}\)C) and low temperatures (-40 \({}^{\circ}\)C) or to temperature shocks (from 20 \({}^{\circ}\)C to -40 \({}^{\circ}\)C). This indicates that the hierarchical structures on these metal surfaces will not likely be degraded or displaced by temperature-induced stress.