## 3 Results
### C-Layers Formed in CME/Oxygen-Entriched Air Flames
The introduction of increased oxygen in the oxidizer stream prompted the formation of surface cracks on the C-layer (35% and 50% O\({}_{2}\); Figure 5(b2,d2)) as well as layer peeling (50% O\({}_{2}\); Figure 5(c1)). These surface instabilities were attributed to the increase in temperature that accompanies increased oxygen content in the oxidizer stream during combustion. The CME/air flame at an HAB of 15 mm produced a flame temperature of 788 \({}^{\circ}\)C. Flame temperature increased to a peak of 938 \({}^{\circ}\)C at an HAB of 8 mm when oxygen content increased to 50% O\({}_{2}\). Furthermore, these instabilities occurred at the edge of the C-layer, which corresponds to the edge of the flame (location of flame/oxidizer interaction) where the flame temperature would be highest [43]. The introduction of an intermediate amount of oxygen in the oxidizer stream (35% O\({}_{2}\)) resulted in black sooty film deposits with visible evidence of structural instability and non-uniformity. This is evidenced by distinct deposition zones: (Region 1) an area of weblike structures bordered by (Region 2) a region of compacted soot (Figure 5(a1,a2,b1-b3)). These varying deposition areas/zones are created by significant temper  
Figure 5: Comparative diagrams, light microscopy images, and SEM images of C-layers formed in CME/35% O\({}_{2}\) and CME/50% O\({}_{2}\) flames. At 35% O\({}_{2}\), a light and dark contrast was observed (**a1–a4**), with the lighter-contrast area producing weblike structures (**a2**) and the area of darker contrast consisting of compacted soot with microscale cracks (**b1–b4**). CME flames with 50% O\({}_{2}\) produced three regions of deposits (**c1**). the first formed by typical weblike structures (**c2**); the second made up of early soot deposits that have been curled (**c3,c4**); and the third comprising compacted soot that has been significantly fractured (**d1–d4**). flame as well as the thermal stress experienced by the cold substrate interacting with the hot CME/oxygen-enriched air flame. The deposited film formed at 35% O\({}_{2}\) shows significant modification of its surface morphology compared to the air-counterpart. The deposited carbonaceous film exhibits two different contrasts under the electron beam (Figure 5(a1)), a larger inner circular deposition of a \"light-contrast\" enclosed by a thin outer \"darker-contrast\" area (Region 1 and Region 2 in Figure 5(a1)). HR-SEM imaging of Region 1 reveals that this part of the deposited film is composed of the typical structure of mature soot: deposits are formed of shorter, chain-like structures made up of tens of nearly-spherical carbon beads and branched aggregates (Figure 5(a2)); this type of morphology is very similar to that found in the CME/air flames. However, the entirety of the deposited C-layer does not share this morphology. Region 1 and Region 2 are clearly separated by a boundary (Figure 5(a1,a3,a4)); higher resolution SEM imaging shows large morphological variations between soot deposits of Region 1 and Region 2. The carbonaceous deposits on the \"darker-contrast\" portion of the layer (Region 2) do not display the typical weblike structures, instead it appears as if the particles are joined together, and the agglomerates have been compacted to fill the voids (Figure 5(b3,b4)). Further SEM imaging of Region 2 reveals unique structural cracking (Figure 5(b1,b2)). These cracks are initial signs of thermal stress/fatigue, and while very small (widths \(<\) 1 \(\upmu\)m) they can be seen at multiple locations across Region 2. Such unique defects are neither observed in Region 1 nor the CME/air flame. We hypothesize that these effects are due to thermal stresses caused by the increase in flame temperature inherent to oxygen-enriched combustion. Furthermore, flame temperature increases from the center of the flame to its outer radius, providing an explanation as to why these cracks are only found on the perimeter of the C-layer.