# Fabrication of a Simultaneous Highly Transparent and Highly Hydrophobic Fibrous Films
## 4 Discussion
### High Hydrophobicity of the Aligned Fibers  
Figure 9 shows AFM images that reveal the surface morphology. The mean roughness values of the aligned fibers ranged from 360 \(\pm\) 35 nm to 560 \(\pm\) 130 nm; the mean roughness values of the randomly distributed fibers ranged from 175 \(\pm\) 15 nm to 262 \(\pm\) 90 nm. The average fiber diameter of the aligned fibers was 178 \(\pm\) 25 nm as shown in Figure S1 (Supplementary Materials); the average fiber diameter of the randomly distributed fibers was 250 \(\pm\) 50 nm as shown in Figure S2 (Supplementary Materials). The roughness values of the aligned fibers were higher than that of the randomly distributed fibers. Recently, [PERSON] et al., observed that the water contact angle depends on the surface roughness [25; 26; 27; 28; 29]. In addition, [PERSON] et al., reported that when air is trapped inside the rough surface in grooves underneath a liquid drop, heterogeneous wetting occurs. This is described by the Cassie-Baxter contact model. The volume of air pockets underneath a liquid drop increases as the surface roughness increases. Therefore, the contact angles (hydrophobicity) of aligned fibers increased because of the increase in the surface roughness. The contact angle of the aligned fiber was higher than that of the randomly distributed fibers due to the higher surface roughness [30; 31; 32].  
Figure 8: Cross-sections (dark area) of light scattering for both aligned fibers (**A**) and randomly distributed fibers (**B**).  
175 \(\pm\) 15 nm to 262 \(\pm\) 90 nm. In summary, we obtained simultaneously highly hydrophobic (\(\theta_{\rm W}\) > 120\({}^{\circ}\)) and highly transparent (T > 80%) films with the electrospinning time of 30 s using the aligned electrospun fiber scaffolds. The aligned electrospun fibers enabled the production of PS scaffolds on the top of a glass substrate that were more transparent and more hydrophobic than the conventional randomly distributed electrospun fibers. The increased transmittance of aligned electrospun fibers leads to increased light absorption, and their hydrophobicity is likely to provide self-cleaning surfaces. The hydrophobicity and transparency of aligned electrospun fibers can play an important role in improving the efficiency of solar cells, such as transparent building integrated photovoltaics, known as BIPV.  
The following are available online at [[https://www.mdpi.com/article/10.3390/app11125565/s1](https://www.mdpi.com/article/10.3390/app11125565/s1)]([https://www.mdpi.com/article/10.3390/app11125565/s1](https://www.mdpi.com/article/10.3390/app11125565/s1)), Table S1: Water contact angle obtained for randomly distributed electrospun fibers. Table S2: Water contact angle obtained for aligned electrospun fibers. Figure S1: Scanning electron microscope (SEM) image of randomly distributed electrospun fibers. The inset is the enlarged image of the fiber diameter. Figure S2: Scanning electron microscope (SEM) image of randomly distributed electrospun fibers. The inset is the enlarged image of the fiber diameter.  
Conceptualization, K.-S.L. and D.-H.Y.; methodology, S.-K.J.; validation, M.K.; writing--original draft preparation, D.-H.Y. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the 'New & Renewable Energy' of a Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Trade, Industry & Energy (grant number 20193010014830).  
Not applicable.  
The authors would like to thank [PERSON] for the consultation of the electrospinning process at Nano Science Technology Institute, Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea. Also thank [PERSON], [PERSON], [PERSON], and [PERSON] for their cooperation in alignment jig designing at Tera Leader Co.  
The authors declare no conflict of interest.