## 4 Conclusions  
In this work, the impact behaviors of ethanol/propanol and ethanol/water binary droplets, which possess similar boiling point differences, on a smooth heated aluminum plate are comparatively investigated. Different liquid components (25 vol.%, 50 vol.%, and 75 vol.%) and surface temperatures (ranging from 100 \({}^{\circ}\)C to 400 \({}^{\circ}\)C) are considered. The impact morphology, secondary atomization, and bubble dynamics are analyzed and discussed. The main conclusions, which are anticipated to enhance the understanding of how different liquid properties caused by variation in liquid compositions affect spray cooling and spray combustion applications, are outlined as follows:  
1. Compared to ethanol/propanol binary droplets, the impact behavior of ethanol/water binary droplets exhibits greater diversity. The transition boiling regime for ethanol/water binary droplets is significantly extended and thus elevates the LFP temperature. However, the LFP temperature of the ethanol/propanol binary droplet exhibits minimal variation with increasing ethanol content. Ethanol/water binary droplets can significantly reduce the droplet residence time within a certain temperature range, but this reduction effect decreases with increasing ethanol content. 2. During the secondary atomization process, ethanol/water binary droplets undergo a unique breakup of the parent droplet, leading to the formation of several fragment droplets with large diameters (\(D_{\text{s}}>0.3\) mm). Both types of binary droplets generate satellite droplets with smaller diameters (\(D_{\text{s}}<0.3\) mm) through mechanisms of puffing and ejection. Puffing tends to produce smaller satellite droplets and is more likely to occur in ethanol/propanol binary droplets. Conversely, ejection produces satellite droplets with larger diameters; this is more likely to occur in ethanol/water binary droplets. The distribution of the satellite droplet diameter for ethanol/water binary droplets is relatively wider, and the satellite droplet diameter decreases as the ethanol content increases. The satellite droplet diameter of the ethanol/propanol binary droplet is relatively small and undergoes minimal changes with an increase in ethanol content. This discrepancy is mainly attributed to the higher surface tension of the ethanol/water binary droplets. Figure 10: (**a**) Schematic diagram of a large bubble shrinkage phenomenon at two different moments. (**b**) Binary boiling point diagram of ethanol/water binary mixture calculated by the Aspen Plus Simulator V11.0. (**c**) The variation in surface tension for ethanol/water binary mixture as a function of the ethanol mole fraction, with data plotted from the Dortmund Data Bank (DDB)—Thermophysical Properties [36]. The subscripts ’1’ and ’2’ in the variables denote moment 1 and moment 2, respectively. The subscripts ‘e’ and ’w’ in the variables denote ethanol and water, respectively. 3. A novel large bubble shrinkage phenomenon is observed during the later stages of droplet evaporation when a 25E/75W binary droplet impacts on a heated wall at \(T_{\rm s}\) = 120 \({}^{\circ}\)C. Different from the previously reported bubble shrinkage phenomenon in n-butanol/PODE4 binary droplet combustion [41; 42], this work identifies a combination of three factors leading to bubble shrinkage. As the ethanol content decreases, there is an increase in the surface tension and saturation temperature of the binary mixture. Moreover, the ambient gas temperature above the bubble surface decreases with bubble growth. This causes an imbalance in the pressure inside and outside the bubble, which causes the bubble to shrink. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14114459/s1](https://www.mdpi.com/article/10.3390/app14114459/s1)]([https://www.mdpi.com/article/10.3390/app14114459/s1](https://www.mdpi.com/article/10.3390/app14114459/s1)), Video SI: 25E/75W\(-T_{\rm s}\) = 120; Video S2: 25E/75P\(-T_{\rm s}\) = 120. Conceptualization, Y.L. and X.Y. ; methodology, X.Y.