## 3 Results and Discussion  
### Flow Field Structures of the Investigated Droplets  
Figure 3 illustrates the cross-section internal flow field in the pure PODE\({}_{2}\) droplet and the five investigated binary droplets. As shown in Figure 3a, two opposite radial flows with two symmetrically distributed vortex cores near the droplet surface can be observed in the cross section of the pure PODE\({}_{2}\) droplet. This flow field structure has been observed in previous studies of Leidenfrost droplets where the upward vapor flow around the droplet is formed [10; 16]. In this work, the high-temperature fuel vapor can be evaporated from the droplet surface by the heating of the flame. The fuel vapor can flow upward along the droplet surface because of the buoyancy effect. This upward vapor flow around the droplet can drive the liquid on the droplet surface to flow upward, which in turn pushes the liquid in the center of the droplet to flow downward. Therefore, the upward vapor flow can induce two opposite radial flows in the cross section of the droplet. As shown in Figure 3a, the flow field structure exhibits two perfect opposite radial flows, which indicates that the convective flow in the pure PODE\({}_{2}\) droplet is mainly induced by the surrounding vapor flow. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Composition** & **Content Ratio** & \(\Delta T_{\text{bp}}\) **(K)** & **Nomenclature** \\ \hline \multirow{5}{*}{PODE\({}_{2}\)/PODE\({}_{4}\)} & 10\%/90\% & & 10P2/90P4 \\  & 30\%/70\% & & 30P2/70P4 \\  & 50\%/50\% & 97 & 50P2/50P4 \\  & 70\%/30\% & & 70P2/30P4 \\  & 90\%/10\% & & 90P2/10P4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A list of the investigated binary solution droplets in this work, along with their content ratios, \(\Delta T_{\text{bp}}\), and nomenclatures. Figure 3: Flow fields in (**a**) the pure PODE\({}_{2}\) droplet and (**b–f**) the five investigated binary droplets at the middle of the stable evaporation stage during combustion. For the \(\mathrm{PODE_{2}/PDE_{4}}\) droplets, as shown in Figure 3b-f, their main internal flow field structures are generally like the pure \(\mathrm{PODE_{2}}\) droplet. However, the radial flows and vortex cores are less symmetric and stable, while sometimes one of the vortices even disappears. For example, as shown in Figure 3b,c,e, the radial flow on the right side disappears and only one vortex core exists and is located near the left side of the droplet surface, while for the 90P2/10P4 droplet in Figure 3f, the situation is contrary. It should be noted that the disappearance of the vortex is quite random; that is, for each binary droplet both left and right vortices may disappear in repeated experiments. Nevertheless, these results reveal that the two opposite radial flows induced by the upward vapor flow around the droplet surface can be disturbed in the binary droplets. Detailed analysis of these phenomena is given in the following two paragraphs. Unlike the pure droplets, the components in binary droplets are not uniformly distributed because of the preferential evaporation of the highly volatile component on the droplet surface, which can induce the content gradient from the droplet surface to the droplet interior [8]. Due to the different densities and surface tensions of \(\mathrm{PODE_{2}}\) and \(\mathrm{PODE_{4}}\), the content gradient can lead to the density gradient and surface tension gradient in the binary droplets. Therefore, the convective flow in the binary droplets is not only driven by the surrounding vapor flow but also influenced by the gravitational effect induced by the density gradient [15] and the Marangoni effect induced by the surface tension gradient [12]. For the gravitational effect, the liquid on the droplet surface with higher content of \(\mathrm{PODE_{4}}\) (1.06 g/mL in density [20]) tends to flow downward, while the liquid inside the droplet with higher content of \(\mathrm{PODE_{2}}\) (0.96 g/mL in density [20]) tends to flow upward.