Component Effects in Binary Droplet Impact Behaviors on the Heated Plate: Comparison Study of Ethanol/Propanol and Ethanol/Water Droplets and Observation of Novel Bubble Shrinkage Phenomenon

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This work compares the impact dynamics of ethanol/propanol and ethanol/water droplets on the heated plate and a novel unique bubble shrinkage phenomenon is observed and analyzed for ethanol/water droplets.

Abstract

This work aims to investigate the effect of liquid physical properties on the behavior of binary droplets impact on the heated smooth aluminum alloy plate with a high-speed imaging system. Two groups of mixed solutions with similar boiling point differences are selected as the working liquid, in which the low-boiling-point components are both ethanol and the high-boiling point components are propanol and water, respectively. Compared to the ethanol/propanol binary droplets, the experimental results show that the ethanol/water binary droplets have diverse impact phenomena and significantly broad transition boiling regimes, as well as the reduced droplet residence time and increased Leidenfrost temperature point. With the decreasing ethanol content in ethanol/water binary droplets, these effects become more prominent. For secondary atomization, the ethanol/water binary droplet undergoes parent droplet breakup into fragment droplets with larger diameters (D_s > 0.3 mm). Both binary droplets produce satellite droplets with small diameters (D_s < 0.3 mm) by puffing and ejection. In terms of the ethanol/propanol binary droplet impact, the probability of puffing is higher and the satellite droplet diameters are small. In the ethanol/water binary droplet impact, the probability of ejection is higher and the satellite droplet diameter distribution is wider. When an ethanol/water binary droplet of 25 vol.% ethanol content impacts the heated wall at T_s = 120 °C, a novel large bubble shrinkage phenomenon occurs at the late stage of droplet evaporation. This phenomenon is proposed to be relevant to the increasing surface tension and saturation temperature with decreasing ethanol content, as well as the decreasing ambient temperature above the top surface of the bubble.

1. Introduction

Droplet impact is widespread in nature and in industrial applications, involving complex heat, mass, and energy-transfer problems. The significance and complexity of droplet impact have stimulated extensive research over the past few decades, which is reviewed in references [1,2,3]. When the impacted plate is heated, an impacting droplet undergoes four stages with increasing surface temperature: film evaporation, nucleate boiling, transition boiling, and film boiling [4,5]. The difference between the surface temperature (T_s) and the liquid saturation temperature (T_b) is defined as the degree of superheat (ΔT = T_s − T_b). When ΔT is low, the droplets are attached to the wall after impact and evaporate slowly, i.e., in the film evaporation state. When ΔT increases and bubble nucleation is initiated, nucleate boiling takes place. In the first two stages, the evaporation time decreases with the increase in ΔT until reaching the critical heat flux (CHF) point, which is also the onset of transition boiling. In this stage, as ΔT rises, the number of bubbles multiplies and even starts to obstruct the heat transfer between solid and liquid, leading to a lower evaporation rate. The droplet initially undergoes nucleate boiling and emits numerous secondary droplets. Finally, the droplet may even detach from the wall and rebound. When T_s increases to the Leidenfrost point (LFP) temperature, the point of maximum evaporation time, film boiling occurs, which is also called the Leidenfrost phenomenon. In this stage, a stable vapor film forms at the bottom of the droplet and separates the direct contact between the solid and liquid. Consequently, the droplet evaporation rate is significantly reduced due to the larger thermal resistance of the vapor, posing potential safety concerns in specific heat-dissipation applications.

Aside from surface temperature, several factors can influence impact behavior [5], including the liquid properties [6,7,8,9] (such as surface tension, viscosity, and boiling point), droplet impact parameters [10,11,12] (such as droplet diameter and velocity), substrate attributes [13,14,15,16] (such as wetting properties, roughness, chemistry, and thermophysical properties), and environmental conditions [17,18,19] (such as ambient pressure and temperature). Until now, numerous studies have explored the behaviors of pure droplets impact on different hot surfaces with various impact parameters [5]. However, in practical industrial applications, droplets are typically complicated mixtures rather than pure liquids. In spray combustion applications, for example, the fuel droplets are typically a mixture of multiple organic liquids. Besides, water-emulsified fuels have been used to improve the atomization effect and control flame characteristics and pollution emissions [20,21,22]. Moreover, in spray cooling applications, previous studies have shown that alcohol additives can alter droplet evaporation and atomization characteristics as well as the dynamic LFP temperature [23,24,25]. Therefore, it is of paramount importance to conduct research into the impact dynamics of multi-component droplets on heated surfaces.

Previous studies on the impact dynamics of multi-component droplets on heated walls have focused on two main types: emulsion droplets and water/alcohol droplets. Jang et al. [26] and Piskunov et al. [27] carried out experimental studies on the impact of water-in-oil emulsion droplets on the heated quartz surfaces. They found that emulsion droplets can generate the non-contact splashing phenomenon and induce the secondary atomization process at a wall temperature much lower than that of pure dodecane droplets. Cen et al. [28] investigated the sputtering and micro-explosion characteristics induced by water/diesel emulsion fuel droplets impacting heated surfaces under different water contents (from 5 vol.% to 20 vol.% with an interval of 2.5 vol.%) and surface temperatures. They found that when the surface temperature is below the LFP temperature, various types of bubbles will be generated based on specific temperature values, while no bubbles can be observed for pure diesel droplets. For multi-component droplets of mixed organic liquid fuels, Chausalkar et al. [29,30] experimentally explored the impact dynamics of gasoline and n-heptane/n-decane binary droplets on a heated wall. They discovered that the impact regime of the binary droplet is similar to that of a pure droplet, with the exception of some differences in nucleate boiling caused by the distillation characteristics of specific components. To investigate the impact dynamics of water/alcohol droplets on the heated plate, numerous studies have been conducted, exploring the effects of different types and contents of alcohol additives on the impact behavior of binary droplets, and Cai et al. [9,23] provide a detailed overview of such experiments. For low-content alcohol additives, Cai et al. [8,9,31] conducted experimental studies and discovered that trace amounts (<8 vol.%) of alcohol additives can effectively promote the atomization and fragmentation of water droplets, raising the dynamic LFP temperature compared to that of pure water. For a wider range of alcohol additives, Zhong et al. [32] and Sen et al. [33] explored the impact dynamics of water/ethanol binary droplets on a titanium dioxide nanotube surface and a smooth aluminum surface, respectively. Both studies revealed that the impact dynamics of binary droplets in the transition boiling state are more diverse and novel compared to those of pure droplets.

Summarizing the previous work, it was found that differences in fluid physical properties can have a significant effect on impact behavior. For multi-component droplets, previous studies have focused on the effects of different mixing contents and surface temperatures in the same type of mixture. But there are few comparative studies of different types of mixtures, leading to a limited understanding of the influence of varying liquid properties on impact behaviors. In order to further investigate the effect of the interaction of different liquid physical properties on the behavior of droplet impact, a comparative study for a specific system is required. Further research and analysis are imperative to explore the boiling behavior, atomization characteristics, and bubble dynamics of multi-component droplet impact during the transition boiling stage.

This study is designed to examine how the physical properties of liquids influence the behavior of binary droplet impacts on a smooth heated aluminum-alloy surface using a high-speed imaging system. Two sets of mixed solutions with comparable boiling point differences are used as working fluids. The mixtures consist of ethanol as the low-boiling-point constituent and propanol or water as the high-boiling-point counterpart, respectively. In the Results and Discussion section, first, the impact morphology as well as the impact regime map and residence times of the two binary droplets are discussed and compared. Second, the satellite droplet production and corresponding bubble dynamics for the two binary droplets in the case of nucleate boiling at the same wall temperature (T_s = 120 °C) are analyzed. Finally, a novel large bubble shrinkage phenomenon of ethanol/water binary droplets is observed at T_s = 120 °C and an ethanol content of 25%. A brief mechanistic explanation for this observation is provided.

2. Materials and Methods

Figure 1 presents a detailed schematic of the experimental apparatus, which comprises a droplet generation device, a heating device, and an imaging system. The droplet is extruded at a controlled flow rate of 30 μL/min by a syringe pump from a needle that has an external diameter of 0.8 mm and an internal diameter of 0.51 mm. A droplet forms at the tip of the needle and detaches when gravity overcomes the surface tension, of which the volume is around 6 μL. The primary focus of this study is to investigate the effects of surface temperature and droplet component while maintaining a constant needle height, thereby ensuring a uniform droplet impact speed of 0.7 m/s. The research fluids employed in this study are the ethanol/propanol mixture and the ethanol/water mixture, both of which contain three mixing contents: 25 vol.%, 50 vol.%, and 75 vol.%.

Table 1. Pure liquid properties at room temperature.

Properties	Ethanol	Propanol	Water
ρl [kg/m3]	785.92	799.8	997.07
cp,l [kJ/(kg°C)]	2.439	2.678	4.184
hfg [kJ/kg]	922.79	765.68	2435.12
Tb [°C]	78.31	97.15	100.02
σ [mN]	21.7	26.3	72.1
μ [mPa s]	1.077	1.89	0.9125

displays the physical parameters of the pure liquids, which are obtained from NIST’s REFPROP V8.0 software [34].

Table 2. Experimental conditions and binary liquid properties at room temperature.

Term	*1 25E/75P	50E/50P	75E/25P	25E/75W	50E/50W	75E/25W
ΔTbp [°C]		21.71			18.84
Φ	25%/75%	50%/50%	75%/25%	25%/75%	50%/50%	75%/25%
ρl [kg/m3]	796.33	792.86	789.39	944.28	891.50	838.71
cp,l [kJ/(kg°C)]	2.619	2.560	2.500	3.821	3.415	2.958
hfg [kJ/kg]	804.44	843.55	882.99	2120.44	1768.50	1372.26
*2 Tsat [°C]	90.9	85.58	81.53	86.6	82.3	79.9
σ [mN]	24.9	23.7	22.6	39.5	30.2	25.5
D0 [mm]	2.20	2.21	2.20	2.34	2.27	2.24
V0 [m/s]	0.7	0.7	0.7	0.7	0.7	0.7
We	34.4	36.2	37.6	27.4	32.8	36.1
Bo	0.379	0.400	0.413	0.321	0.373	0.404

*¹ E, P, W denote ethanol, propanol, and water, respectively. Numbers in front of the letter indicates the percentage share. For example, 25E/75P means the volume fractions of ethanol and propanol in the binary mixture are 25 vol.% and 75 vol.%, respectively. *² For binary droplets, T_sat denotes the bubble point temperature and is calculated by the Aspen Plus Simulator V11.0.

displays the specific mixture ratio of binary droplets and the corresponding physical parameters. The density, specific heat, and latent heat of the mixture are calculated using the recommended mixing rules [9,35]: ${\rho }_{\mathrm{l}}={(y_1/{\rho }_{\mathrm{l}1}+y_2/{\rho }_{\mathrm{l}2})}^{-1}$, $c_{\mathrm{p},\mathrm{l}}=y_1{c}_{\mathrm{p},\mathrm{l}1}+y_2{c}_{\mathrm{p},\mathrm{l}2}$, $h_{\mathrm{f}\mathrm{g}}=y_1{h}_{\mathrm{f}\mathrm{g}1}+y_2{h}_{\mathrm{f}\mathrm{g}2}$. The saturation temperature of the binary mixture liquid is calculated by the Aspen Plus Simulator V11.0. The surface tension of the ethanol/water mixture is obtained from the database [36]. The difference in surface tension between ethanol and propanol is modest, and the surface tension of the mixture is calculated by mole fraction weighted averaging: $\sigma =x_1{\sigma }_1+x_2{\sigma }_2$.

The target substrate used for this study is a finely polished 6061 aluminum alloy plate of circular shape, 80 mm diameter, and 2 mm thickness. The surface roughness (R_a) is about 7 nm, which is measured by a 3D profiler (KLA-Tencor P7, KLA, Milpitas, CA, USA). The plate is heated by a CNC heating platform (400 W) that has a temperature control range of 30 °C to 400 °C with a precision of less than 1%. The surface temperature is maintained between 100 °C and 400 °C. A K-type thermocouple, located at the center of the plate, is used to measure the surface temperature before the droplet impact and is subsequently removed during the impact process. To mitigate any potential cooling effect from previous droplet impacts on the surface temperature, a designated minimum time gap of 3 min is imposed between sequential trials.

The droplet impact process is captured using a high-speed camera (Phantom V2012, Vision Research, Inc., Wayne, NJ, USA) operating at 3000 frames per second with an exposure time of 3 μs. A macro lens (Nikon AF 200 mm f/4D, Nikon, Tokyo, Japan) is attached to the camera via an extension ring, achieving an image resolution ranging from 11.4 to 11.8 μm/pixel. Illumination for imaging is provided by a 640 nm laser (CAVILUX Smart, Cavitar Ltd., Tampere, Finland). Image processing is performed using Davis V8.0 software and a customized Python V3.8.5 program. The diameters of the binary droplets vary between 2.20 mm and 2.34 mm, yielding Weber numbers (We = ρ_lV₀²D₀/σ) between 27.4 and 37.6 and Bond numbers (Bo = ρ_lgR₀²/σ) between 0.321 and 0.413. It is observed that the Weber number remains under the splashing threshold at normal surface temperatures [37]. The effects of gravity are negligible in this study due to small Bond numbers. The error in determining the droplet boundaries is no more than ±1 pixel during image processing. Therefore, the measurement error uncertainty of droplet size and centroid position is ±0.012 mm. The impact velocity of the droplet is calculated using the change in droplet centroid position within 5 frames divided by the interval time. The temporal uncertainty for velocity computation is twice the exposure time. Utilizing the error propagation equation, the maximum relative uncertainty for velocity, Weber number, and Bond number are ascertained to be 1.39%, 3.32%, and 2.18%, respectively. The errors for other variables are calculated based on the standard deviation from the statistical results of multiple experimental outcomes.

3. Results and Discussion

3.1. Morphology and Regime Map of Different Impact Behaviors

Figure 2 and Figure 3 demonstrate the typical impact behavior of ethanol/propanol binary droplets and ethanol/water binary droplets on the heated wall for different liquid components and surface temperatures, respectively. For an ethanol/propanol binary droplet, when T_s = 100 °C, the droplet is in the film evaporation state and continues to evaporate throughout the entire process. No bubble rupture or generation of secondary droplets occurs, although occasional bubbles are observed inside the droplet. When T_s = 120 °C, the droplet is in the nuclear boiling state, and more bubbles and satellite droplets (D_s < 0.3 mm) with smaller diameters are generated. When T_s = 150 °C, the droplet is in the transition boiling state. At the initial stage, the droplet is in the nuclear boiling state and produces a large number of satellite droplets. With the increase in vapor production, the droplet gradually retracts, detaches from the wall, and eventually rebounds. When T_s = 180 °C, the droplet transitions to the Leidenfrost state.

Compared to the ethanol/propanol binary droplets, the impact behavior of the ethanol/water binary droplets is more diverse, as shown in Figure 3. When T_s = 120 °C, the ethanol/water binary droplet is in the nucleation boiling state, and a large number of bubbles as well as secondary droplets are generated. Due to the high surface tension of water, the generation of bubbles with large diameters can be observed during the boiling process of ethanol/water binary droplets, which has not been observed in ethanol/propanol binary droplets. This will be discussed in further detail later. With the gradual increase in surface temperature, droplets reach the transition boiling state, and the impact behaviors become more diverse. Besides producing satellite droplets, the ethanol/water binary droplet also generates fragment droplets (D_s > 0.3 mm) through the breakup of the parent droplet. This unique parent droplet breakup phenomenon has not been observed in the impact of the ethanol/propanol binary droplets or pure droplets on the smooth heated surface. The transitional boiling behaviors of ethanol/water binary droplets are investigated in more detail in another work [38] involving a rougher surface (R_a = 15 nm), which is not the focus of the current study. Finally, as the surface temperature increases further, the ethanol/water binary droplet gradually reaches the Leidenfrost state.

Figure 4 illustrates the impact behavior regime map for the two kinds of binary droplets. The ethanol/water binary droplets significantly broaden the transition boiling region and have higher Leidenfrost temperatures compared to the ethanol/propanol binary droplets. Moreover, the Leidenfrost temperature of ethanol/propanol binary droplets remained essentially constant with increasing ethanol content, but the Leidenfrost temperature of ethanol/water binary droplets decreased significantly with increasing ethanol content. According to the previous work of van Limbeek et al. [17] and Pacheco-Vázquez et al. [39], Leidenfrost temperature depends on the boiling point of the liquid, the latent heat, and the specific heat of gas. The latter two physical parameters need to take into account the effect of temperature, and the values at saturation temperature are used; these are obtained from NIST’s REFPROP V8.0 software [34] as shown in

Table 3. Pure liquid properties at boiling points.

Properties	Ethanol	Propanol	Water
Tb [°C]	78.31	97.15	100.02
cp,g [kJ/(kg°C)]	1.7174	1.7225	2.08
hfg [kJ/kg]	849.91	670.211	2256.40
*TL [°C]	162.8	179.5	217.7

*T_L is calculated using the relation given by Pacheco-Vázquez et al. [39].

. In this work, the Leidenfrost temperature for pure liquids is calculated using the relation given by Pacheco-Vázquez et al. [39]: ${\theta }_{\mathrm{L}}=1.17{\theta }_{\mathrm{b}}+0.05$, where ${\theta }_{\mathrm{L}}=T_{\mathrm{L}}{c}_{\mathrm{p},\mathrm{g}}/h_{\mathrm{f}\mathrm{g}}$, ${\theta }_{\mathrm{b}}=T_b{c}_{\mathrm{p},\mathrm{g}}/h_{\mathrm{f}\mathrm{g}}$. The ratio $c_{\mathrm{p},\mathrm{g}}/h_{\mathrm{f}\mathrm{g}}$ can be interpreted as the relative amount of energy available for evaporation compared to that being lost from the droplet to the surrounding gas [17]. The Leidenfrost temperatures calculated in this work are slightly larger than the results given by Pacheco-Vázquez et al. [39]. This may be due to a slight difference in the saturation temperatures used for the calculations employed. As shown in Table 3, whereas propanol and water have comparable boiling points and specific heat of gas, the latent heat differs significantly. Thus, the Leidenfrost temperature of propanol is about 38 °C lower than that of water. Moreover, ethanol has a higher latent heat than propanol, and the specific heat of gas is similar. The difference in Leidenfrost temperature between the two alcohols is only 16 °C, while the difference in Leidenfrost temperature between ethanol and water is about 55 °C. Therefore, for ethanol/water binary droplets, changes in the content of ethanol have a significant effect on the Leidenfrost temperature.

When the droplet reaches the transition boiling stage, nuclear boiling occurs in the early stages of evaporation. Then, the droplet retracts and rebounds to reach the Leidenfrost state. Upon reaching the Leidenfrost state, due to the persistence of the vapor film, droplets impact and retract after spreading to a maximum. The time interval between the moment when the droplet makes contact with the plate and the moment when the droplet first rebounds away from the surface is defined as the residence time (t_r). At a normal surface temperature, droplets stick to the hydrophilic aluminum plate after impact. Therefore, in the case of droplet impact on a heated wall, the residence time can be used to reflect the potential of vapor bubbles to accelerate the retraction and bouncing process of the droplet. Figure 5 illustrates that, for both types of binary droplets, the residence times are comparatively prolonged during the transition boiling phase, followed by a decrease as the surface temperature rises. Since the ethanol/propanol binary droplet reaches the Leidenfrost state very quickly, the residence time is essentially constant with various surface temperatures and liquid components. This is consistent with the findings of Cai et al. [31], who reported that the presence of iso-propanol and variations in surface temperature scarcely influence the residence time in the Leidenfrost state. For the ethanol/water droplet, the residence time is significantly reduced within a certain range of temperature in the transition boiling state, which is caused by the rapid breakup of the parent droplet. As the ethanol content increases, the temperature range that leads to a reduction in residence time gradually narrows, and the extent of the reduction in residence also diminishes due to the weakened breakup of parent droplets.

3.2. Characteristics of Satellite Droplet Formation and Bubble Dynamics

Upon impacting the heated wall, the atomization effect significantly affects the evaporation characteristic of the droplet, which in turn impacts the heat-transfer process. This section addresses the characteristics and formation processes of satellite droplets. Davis V8.0 software is utilized for the identification and analysis of satellite droplets. The satellite droplets included in the statistics must occupy at least 15 pixels and should have a contour gradient exceeding 25% in order to minimize severe identification errors. Figure 6 illustrates the satellite droplet size distribution of the two types of binary droplets at T_s = 120 °C. The diameters of satellite droplets originating from ethanol/propanol binary droplets are relatively small and do not change much with the increase in ethanol content. In contrast, the diameters of satellite droplets originating from ethanol/water binary droplets show a wider distribution, which tends to decrease with the increase in ethanol content.

The two types of binary droplets primarily generate satellite droplets through mechanisms of puffing and ejection. Figure 7a,b and Figure 8a,b show the puffing and ejection behaviors of the two types of binary droplets at an ethanol content of 25 vol.% and a surface temperature of 120 °C. Huang et al. [40] have compared and analyzed these two secondary atomization modes and the corresponding satellite droplet diameters. Puffing is characterized by the rapid formation of small bubbles, which leads to the generation of secondary droplets with smaller diameters through the rupture of the liquid film. When the heat continues to transfer to the droplet and the bubble grows, a cavity is formed when the bubble breaks up. Under the action of surface tension, the cavity retracts, ejecting a liquid jet outward. Then, the tip of the liquid jet is pinched off to produce multiple secondary droplets due to the Plateau–Rayleigh instability. Supplementary Videos S1 and S2 show the complete binary droplet impact process under conditions corresponding to Figure 7 and Figure 8. It can be observed that puffing occurs more frequently during the impact of ethanol/propanol binary droplets, whereas ejection is more prevalent in ethanol/water binary droplet impacts. The higher surface tension of water enables ethanol/water binary droplets to generate larger bubbles than ethanol/propanol droplets, resulting in larger satellite droplet diameters.

In the later stage of evaporation, the generation of large bubbles is often observed. This phenomenon is attributed to the decreasing ethanol content, which consequently leads to an increase in the surface tension of the binary mixtures. When multiple bubbles are formed, the bubbles undergo coalescence to form a bubble with a larger diameter. Comparing Figure 7c and Figure 8c, for ethanol/propanol binary droplets, the bubbles cannot grow significantly due to lower surface tension, and the liquid film of the bubbles is thick. In contrast, for ethanol/water binary droplets, the bubbles can grow considerably due to the higher surface tension, and the liquid film of the bubbles is thin. As the amount of vapor continues to increase, the bubble ruptures, as shown in Figure 7c at t = 830.3 ms and Figure 8c at t = 1926.2 ms. The internal pressure within the bubble (P) can be obtained using the Young–Laplace equation, $P=P_0+\frac{2\sigma }{R}$, where P₀ is the atmospheric pressure and R is the radius of curvature of the bubble. It is shown that the larger a bubble grows, the lower its internal pressure, and the weaker the bubble rupture. In droplet combustion research, the rupture of large bubbles can lead to micro-explosions [40,41]. Following bubble rupture, a liquid ligament is formed and then breaks into several secondary droplets. However, under droplet impact conditions, the rupture of large bubbles usually leads to the contraction of the liquid film and reintegration with the attached droplet body. Thus, micro-explosions cannot be generated. As a result, the production of satellite droplets at the late stage of evaporation is observed to decrease.

3.3. Observation and Preliminary Analysis of Novel Bubble Shrinkage Phenomenon

In the case of 25E/75W binary droplets’ impact on a heated wall at T_s = 120 °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 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. Huang et al. [41,42] 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 Huang et al. [41,42], 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_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_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.

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 °C to 400 °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:

• 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.
• 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_s > 0.3 mm). Both types of binary droplets generate satellite droplets with smaller diameters (D_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.
• 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_s = 120 °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.