## 3 Results and Discussion
### Observation and Preliminary Analysis of Novel Bubble Shrinkage Phenomenon  
In the case of 25E/75W binary droplets' impact on a heated wall at \(T_{\rm s}\) = 120 \({}^{\circ}\)C, a novel bubble shrinkage phenomenon can be observed in the late stage of the impact, as depicted in Figure 9. After \(t\) = 2058.5 ms, several bubbles gradually form, enlarge, and then coalesce  
Figure 8: Typical satellite droplet generation modes and bubble dynamic behavior of a 25E/75W binary droplet at \(T_{\rm s}\) = 120 \({}^{\circ}\)C: (**a**) puffing, (**b**) ejection, and (**c**) big bubble growth and breakup. The dynamic impact process is shown in Supplementary Video S2. In subfigure (**a,b**), the area in the red dashed box is further zoomed in and placed on the right side of the original figure at an interval of 0.33 ms.  
Figure 7: Typical satellite droplet generation modes and bubble dynamic behavior of a 25E/75P binary droplet at \(T_{\rm s}\) = 120 \({}^{\circ}\)C: (**a**) puffing, (**b**) ejection, and (**c**) big bubble growth and breakup. The dynamic impact process is shown in Supplementary Video S1. In subfigure (**a,b**), the area in the red dashed box is further zoomed in and placed on the right side of the original figure at an interval of 0.33 ms.  
with each other to form a large bubble. Unlike what is shown in Figure 8, after reaching a maximum size, the large bubble starts to shrink rather than breaking up. Finally, the bubble vanishes and merges with the attached liquids. [PERSON] et al. (2014); [PERSON] et al. (2014) also report a novel bubble shrinkage phenomenon in the combustion of n-butanol/PODE4 binary droplets. Additionally, they proposed a simple evaporation and content evolution model to elucidate the phenomenon. They believe that bubble shrinkage is induced by vapor condensation at the interface between the bubble and the liquid film, a process driven by the vapor-liquid nonequilibrium. Moreover, a lower bubble growth velocity is observed to be associated with the nonequilibrium state.  
The schematic diagram of the droplet shrinkage phenomenon is depicted in Figure 10a, where the two graphs represent two different moments. Unlike in the work of [PERSON] et al. (2014, 2014), the external environment of the bubble in this work comprises a mixture of air and vapor, exhibiting temperature gradients rather than a high-temperature flame. While the plate has a heating effect on the surrounding gas, this heating effect weakens with increased distance from the plate. Therefore, the further away the plate, the lower the ambient gas temperature (\(T_{\text{g}}\)) is. During the late stage of evaporation, the ethanol content has decreased tremendously, accompanied by a rise in water content. This results in elevated saturation and dew point temperatures as well as an increase in the surface tension of the binary mixture, as illustrated in Figure 10b,c. With the growth of bubbles, the gas temperature above the bubble surface lowers, leading to a decrease in the liquid film temperature as well as the interfacial temperature (\(T_{\text{i}}\)). Consequently, condensation occurs inside the droplet, causing a reduction in the internal pressure of the bubble. In conclusion, the combined effect of the increasing surface tension and saturation temperature due to decreasing ethanol content, as well as the decreasing ambient gas temperature above the top surface of the bubble, causes the bubbles to shrink after reaching a critical size. This bubble shrinkage phenomenon is not observed in ethanol/propanol binary droplets. The minor difference in surface tension between the two types of alcohol is insufficient to allow significant bubble growth, preventing the bubbles from encountering the cooler ambient gases above the bubble.  
Figure 9: Bubble growth, coalescence, and shrinkage phenomena in a 25E/75W binary droplet impact on the heated wall at \(T_{\text{s}}=120\)\({}^{\circ}\)C.