## 5 Conclusions  
In this contribution we showed the capabilities of the flame deposition method to synthesize hydrophobic C-layers. Stainless steel substrates (D = 19 mm disks) were used for the formation of C-layers in a co-flow diffusion flame. It was found that the hydrophobicity and physical properties of the flame-formed C-layers vary as function of parameters, including: (1) flame position (inner-cone vs. outer-cone); (2) fuel type (gaseous vs. vapor); and (3) percent of oxygen content in the oxidizer stream. The introduction of solid substrates within the CH\({}_{4}\)/air flame's inner-cone resulted in two very distinct deposition zones. Zone-1 is an outer ring of darker contrast surrounding a circular layer of light contrast (Zone-2). The variation of the contrast is the result of the deposition rate or the presence of the condensed-phase material occurring within the outer flame region. The hydrophobicity of deposited material from the inner-cone of the flame varies with radius along the disk. However, C-layers formed in the outer-cone, regardless of the fuel type, are uniform and hydrophobic. The flame structure (inner-cone vs. outer-cone) varied by the type of fuel and concentration of oxygen used to form a flame. The increase of oxygen in the oxidizer stream resulted in compressed flames (height and diameter). The C-layers formed in the oxygen-enhanced flames exhibit unique structural defects including a smaller region of deposition, peeling of deposits, and cracking at the edges of the films. The C-layers formed in oxygen-enriched air flames are hydrophobic at tested levels of oxygen enrichment (35% and 50% O\({}_{2}\)). Impacts of this research show that by varying the fuel type, oxygen content in the oxidizer stream, substrate insertion height, and substrate residence time in the flame hydrophobic films can be formed with desired mass, thickness, surface structure, and stability. Specifically, by altering the structure of the deposited soot it is possible to control the hydrophobicity and stability of the formed C-layer. Further research of these flame-formed hydrophobic C-layers should explore their durability; all hydrophobic coatings lose their effectiveness due to wear and the eventual destruction of the microstructures on the coating's surface. The durability of the C-layer as well as potential methods to improve C-layer durability may play a significant role in determining the commercial viability and adoption of the flame deposition method for synthesizing hydrophobic surfaces. The robustness of a hydrophobic surface is typically characterized by its mechanical strength, corrosion resistance, thermal resistance, and (if the layer is deposited on a material) the strength of the material's adhesion [30; 44; 45]. Research groups [PERSON] et al. and [PERSON] et al. have quantified mechanical strength of hydrophobic surfaces via a water impact test. In this test, water droplets impact the hydrophobic surface (angled at 45\({}^{\circ}\)) from a set height [30; 45]. After a set amount of water volume has impacted the surface, the surface is analyzed for damage. The surface analysis typically consists of SEM imaging (to compare the damage to an untested sample) and measuring new water contact angles. As the hydrophobic surface experiences more damage the degree of hydrophobicity typically decreases and the water droplet CA decreases. Corrosion resistance of hydrophobic materials has been studied by [PERSON] et al., [PERSON] et al., and [PERSON] et al. by immersing the material into an acidic (pH < 7), basic (pH > 7), and neutral (pH = 7) solution for a set time period [30; 44; 45]. Once the immersion is complete, the hydrophobic material is removed, and SEM imaging/analysis is conducted. The SEM imaging typically reveals how the surface structure was impacted by the acidic, basic, and neutral solutions. CA analysis is also conducted to quantify any reductions in hydrophobicity. The ability of a hydrophobic material to resist high temperature has been studied by [PERSON] et al. and [PERSON] et al.