## 3 Results
### C-Layers Formed in CME/Oxygen-Entriched Air Flames  
The role of oxygen in the formation of soot has been studied by numerous authors [36; 38; 39; 40; 41; 42], and it has been found that introducing oxygen into a flame increases the rate of soot inception, formation, agglomeration, and oxidation. These fundamental effects on the flame-formed soot may have a profound impact on the structure and hydrophobicity of the flame-formed C-layers. In Flame III the effects of oxygen-enhanced combustion on deposited carbon films were studied using a CME/oxygen-enriched air flame. The SS substrate was inserted into the flame volume at three-quarters the flame height. This relevant insertion height was used due to the overall height of the flame shrinking as oxygen content in the oxidizer stream increased (Figure 2c). Oxygen contents of 21%, 35%, and 50% O\({}_{2}\) were used, with respective substrate insertion heights of 15 mm, 9 mm, and 8 mm (Table 2). At all three O\({}_{2}\) contents the substrate was introduced to the flame's luminous zone and the produced C-layers were hydrophobic (Figure 4(a1-c1)). At 35% O\({}_{2}\) the C-layer produced was superhydrophobic (CA = 154\({}^{\circ}\)) with a mass of 6 mg, a 1 mg increase compared to the air flame. This mass increase coincides with the increase in soot volume fraction between CME combustion with air and 35% O\({}_{2}\)[35], indicating a greater abundance of soot in the flame. Furthermore, at 35% O\({}_{2}\) the deposited material consists of similar weblike structures that were seen in the CME/air and CH\({}_{4}\)/air flames (Figure 4(a1-a3,b1-b3)). Once the oxygen content was increased to 50% O\({}_{2}\), the flame deposits were no longer superhydrophobic, producing a hydrophobic CA of 146\({}^{\circ}\). The decrease in the performance of the layers as a hydrophobic material was accompanied with a drop in mass to a low of 1 mg (compared to 5 mg at 21% O\({}_{2}\) and 6 mg at 35% O\({}_{2}\)). This correlates well with our previous work on soot formation, where it was reported that the soot peak volume fraction of a CME/50% O\({}_{2}\) flame was significantly larger than the CME/21% O\({}_{2}\) flame; however, when mass of the total soot was measured the mass in the 21% O\({}_{2}\) flame was larger than the 50% O\({}_{2}\) flame, indicating that soot is undergoing an oxidation process [35]. Oxidation of deposited soot accounts for the reduction in mass as well as the difference in soot development/structure seen in the TEM images (Figure 4(a5-c5)). The soot formed in the CME/air and CME/35% O\({}_{2}\) appears to be \"irregular-shaped\" [34], indicated by a lack of distinct shapes or boundaries of the soot, providing evidence that the soot has not yet been fully carbonized (Figure 4(a4,b4)). It is evident from the TEM images that there is a gradual decrease in the amount of \"irregular-shaped\" structures, as oxygen is increased from 21% to 35% and finally to 50%. It is important to note that at all three oxygen contents the hydrophobic portions of the flame-formed C-layers share similar branched, weblike, porous structures (Figure 4(a1-a3),b1-b3,c1-c3)). This not only demonstrates the flame method's capability to consistently form hydrophobic soot depositions under vastly different conditions, but also emphasizes the importance of these branching hierarchical structures. The hy  
Figure 4: Comparative SEM images of C-layers formed in a CME/air flame (**a1–a3**), CME/35% O\({}_{2}\) flame (**b1–b3**), and CME/50% O\({}_{2}\) flame (**c1–c3**). TEM images of soot collected at the respective flame insertion heights of the substrate for the CME/air flame (**a4–a6**), CME/35% O\({}_{2}\) flame (**b4–b6**), and CME/50% O\({}_{2}\) flame (**c4–c6**). drophobicity of the current deposited carbon films is due to their surface structure; this is a physical characteristic, not a chemical one. Therefore, the unique topography created via soot deposition is the mechanism that facilitates the hydrophobicity exhibited by the deposited C-films.