## 3 Results and Discussion
### Flow Velocity Evolution and Distribution of the Investigated Droplets
#### 3.2.2 Velocity Distributions of the Investigated Binary Droplets  
Figure 6 shows the velocity fields in the five investigated binary droplets at four moments of the stable evaporation stage. It can be seen that the maximum flow velocities are generally located in the droplet interior, while the minimum flow velocities are generally located near the droplet surface. This is mainly because the vortex cores in the binary droplets are formed near the droplet surface, as can be seen in Figure 3. The flow field structures in the studied different binary droplets are generally unstable, so there exists fluctuation of the flow velocities. Figure 7 shows the corresponding vorticity fields in the five investigated binary droplets at the same four moments of the stable evaporation stage. It can be seen that the maximum vorticities are generally located near the droplet surface, while the minimum vorticities are generally located in the droplet interior, which is opposite to the trend of velocity fields. For the 50P2/50P4 binary droplet, the atomization delay time is the smallest. Therefore, the velocity field and vorticity field of the 50P2/50P4 binary droplet in Figures 6 and 7 at \(t=500\) ms are missing because the binary droplet has already entered into the atomization stage. Figure 5: Comparison of the spatially averaged flow velocities at the stable evaporation stage of the investigated pure and binary droplets. Figure 6: Velocity fields in the five investigated binary droplets at four moments (125, 250, 375, and 500 ms) of the stable evaporation stage. Figure 8 shows the corresponding velocity profiles of the five investigated binary droplets along the droplet horizontal radial direction at four moments of the stable evaporation stage. The velocities at \(r/R_{\rm d}\) close to \(\pm 1\) are not presented as this region is very close to the droplet surface and out of the effective data region. The velocity profiles mainly peak in the droplet interior, while the local minima represent the vortex cores formed by the radial flows. The main body of a velocity profile generally has a parabolic shape, except for those at \(t=125\) ms (the shortest moment after ignition in Figure 8). The weak parabolic shapes or even plain shapes at \(t=125\) ms are probably due to the vortex structures in the droplets still not being formed and the flow velocity not showing superiority in the droplet interior. It can be observed from Figure 8 that for each binary droplet, the peak velocity  
Figure 7: Vorticity fields in the five investigated binary droplets at four moments (125, 250, 375, and 500 ms) of the stable evaporation stage. first increases from \(t=125\) ms to \(t=250\) ms to a value of 5-9 mm/s as the radial flows and vortex structures are formed. Then, the peak velocity gradually decreases to a value of around 3 mm/s from \(t=250\) ms to \(t=500\) ms as the droplet approaches the atomization stage, which is in accordance with the findings in previous work [16]. From 125 ms to 500 ms, the evolution of the velocity is fluctuant for each of the binary droplets because of the disturbance in the flow field. The velocities of these binary droplets are all at the same magnitude, and their differences are small. In this work, it should be mentioned that the investigated binary droplets can all undergo the atomization process. This observation implies that although the internal flow in the binary droplets can enhance the mass transfer process and affect the content gradient, the influence is not strong enough to break the content gradient in the droplet. Actually, the averaged flow velocities (\(V_{\text{aver}}\)) in the five investigated binary droplets are only around 3 mm/s, as shown in Figure 5, which is a rather small value compared with the flow velocities in the Leidenfrost droplets, which are generally at the magnitude of 10 cm/s [10; 16].