Tortuosity can indicate curvature a

Tortuosity can indicate curvature and/or undulations in carbon layers, and it is only associated with non-planar lamella [16]. Fig. 11b shows the tortuosity distribution of soot particles. All the distributions for fringe tortuosity at different HAB are close to each other, meaning there is no significant difference in tortuosity of these particles. According to the mean tortuosity data presented in Table 2, the fringe tortuosity of soot particles has a tendency of decreasing first and then increasing. The particles at 45mm HAB, just below the flame tip，have the minimum tortuosity value, whereas particles in the flame tip have the maximum value. The decrease of FT is due to that particle graphitization becomes obvious and the curving degree of the fringes reduces. The reason why FT increases from 45 mm HAB to the flame tip is that, most crystallites in the outer shell are oxidized, causing a great decrease in the graphitic order, and the residual layers close to the core have relatively large tortuosity.Fig. 11c shows the inter-fringe spacing distribution of soot particles. It can be seen that, the 35 mm HAB curve has the minimum peak value at the rightmost position, indicating that the samples at this point have the maximum inter-fringe spacing, and the data distribution is relatively dispersed. The 40 mm HAB curve has the maximum peak value at the leftmost position, indicating the samples at this point have the minimum inter-fringe spacing, and the data distribution is concentrative. Changes in peak values and their positions suggest that from 30 mm to 35 mm HAB, the inter-fringe spacing increases significantly, then downstream from 35mm to 40mm HAB, it decreases significantly, and finally from 40 mm to 52 mm HAB, it increases again. This is consistent with the spacing data in Table 2. From 30 mm HAB to 35 mm, the soot particles are in growth process, and the repulsion between adjacent carbon layers, can increase the inter-fringe spacing [16]. Further downstream, the role of soot oxidation begins to appear, which results in the spacing decreasing. As approaching the flame tip, more and more crystallites are oxidized. The spacing of residual carbon layers near the soot core is relatively large, therefore, there is a certain degree of increase in inter-fringe spacing near the flame tip.