Coalescence, Spreading, and Rebound of Two Water Droplets with Different Temperatures on a Superhydrophobic Surface.

This paper studied the coalescence, spreading, and rebound of two droplets with different temperatures on a superhydrophobic surface. When the temperature of the impacting droplet was the same as that of the stationary droplet, there was a large deformation of both droplets before the coalescence and the energy dissipation was also large. The coalescence happened at the time close to the maximum spreading. When the temperature of the impacting droplet increased, the deformation of both droplets became smaller before the coalescence and the coalescence happened at or even before the droplets started to spread. The energy dissipation and loss in the later situation is less than those in the previous case. The rebounding characteristics of the merged droplets were also found to be dependent on the temperature. There is an optimum temperature at which the merged droplets can rebound for more times due to the balance of energy loss and also the interaction of the merged droplets with the underlying superhydrophobic substrate. These findings may help further the fundamental understanding of droplet collision on a superhydrophobic surfaces and also offer an alternative strategy to remove droplets from the underlying surfaces for different industrial applications, including condensation heat transfer in steam power plants and phase-change-based thermal management systems.

Introduction

Droplet collision is a complicated process that involves droplet deformation, coalescence, spreading, rebound, and associated energy and mass transfer. Such a phenomenon happens frequently both in the natural processes,[1,2] such as in the rains and waterfalls, and in many industrial processes, such as spray cooling, anti-fog, and ink-jet printing.[3−5] This area of research attracted a large amount of attention, and some of those studies investigated the collisions of two droplets with different sizes to understand the droplet interaction in the rainfall.[6,7] In the 1990s, the focus of this research area was moved to the study of collisions involving hydrocarbon droplets[8−11] during the fuel spray process in combustion. Qian[11] and co-workers were able to map out the two-droplet collision states using Weber numbers and impact parameters. Their work provides a useful methodology to study the two-droplet collisions in free space.

Recently, droplet collisions on surfaces have also been studied extensively, which involve both experimental and theoretical studies.[12−25] The early study in this topic can actually be traced back to the 19th century when Lord Rayleigh first observed the collision of rain droplets onto the liquid surface of water pool, and he attributed the failure of coalescence between the rain droplets and the water pool to the trapped air layer between the interfaces of droplets and the pool surface.[1] He hypothesized that such an air layer prevented the contact of these droplets with the pool surface. As the droplet collision on solid surfaces plays a critical role in many industrial processes,[11,12,15,16] there is also an increased research effort in studying droplet collisions on solid surfaces,[17−24] especially on superhydrophobic (SH) surfaces due to the great implication of SH surfaces for a broad range of applications.[25−32] Li[23] experimentally studied two droplets impacting a solid surface and identified different coalescence mechanisms based on the comparison between the theoretical and experimental spread lengths. Wang et al.[24] studied the impact of nanodroplets on a solid surface and developed a new model to estimate the maximum spreading factor. They also numerically studied a double droplet impact on a moving liquid and analyzed the asymmetric heat transfer characteristics.[25] For the two droplets in free space, the collision process only involves the interaction between two droplets. For the collision between two droplets on solid surfaces, the process is more complicated, in which both the interaction between the droplets and the interaction between the surfaces and the droplets need to be considered.

Most studies of the two-droplet collision were focused on the collision of the two droplets at the same temperature. In many industrial processes, however, the temperatures of the colliding two droplets might not be the same. We studied previously the collision of two droplets with different temperatures on the SH surface. Our study revealed that temperature had a strong impact on the ratio of collision-coalescence to collision-separation during the two-droplet collision on SH surfaces.[32] When the temperature of one or both droplets increased, more coalescence phenomena were observed after the collisions between the impacting and stationary droplets. Whereas our previous effort focused on whether the two droplets were coalesced or separated after their collision with different temperatures of the droplets, this present study focuses specifically on the behavior of the droplets after their coalescence.

In this work, we intend to investigate the change of the coalescence, spreading, and also the rebounding behaviors of the coalesced two droplets with the impacting droplets set at different temperatures. We used a high-speed camera to capture the images of the coalescence, spreading, and rebounding processes on SH surface with the temperature of the stationary droplet (TS) kept constant and the temperature of the impacting droplet (TI) changed (Figure ). In the study, the stationary droplet was doped using a red dye whereas the impacting droplet was not doped so the change of the interface between two droplets during these processes can be clearly captured. Through both the experimental investigation and the analysis of the key factors, including the contact time, restitution coefficient, and energy change, we hope to help reveal the details of the collision process. The findings provide a further understanding on the collision of two droplets with different temperatures on the SH surface and also offer new insight in designing applications involving droplet collisions with different temperatures.

Figure 1

Schematic of the coalescence, spreading, and rebound of two droplets on the SH surfaces, with TI > TS. (a) Head-on interaction between a hot impacting droplet (top) and a cold stationary droplet (bottom). (b) Coalescence of two droplets on the SH surface. (c) Spreading of the merged droplets on the SH surface. (d) Rebound of the merged droplet from the surface.

Experimental Section

Materials

The following chemicals and substrates were used in the fabrication of superhydrophobic Si nanowire (SH Si NW) surfaces: acetone (99.5%, Sinopharm Chemical Reagent Co. Ltd, China); ethanol (99.7%, Changshu Hong Sheng fine chemical Co. Ltd, China); sulfuric acid (H2SO4, 98%, Sinopharm Chemical Reagent Co. Ltd, China); hydrogen peroxide (H2O2, 30%, Shanghai Ling Feng Chemical Reagent Co. Ltd, China); silver nitrate (AgNO3, 99.8%, Aladdin, China); hydrofluoric acid (HF, 40%, Sinopharm Chemical Reagent Co. Ltd, China); Perfluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (fluorinated silane, 97%, Alfa Aesar); P type silicon wafers (Zhejiang Li Jing Photoelectric Technology Co. Ltd, China).

Fabrication of the Superhydrophobic Surface

The SH Si NW surface was generated via a silver-assisted etching process[33,34] followed by silanization with fluorinated silane[35] to make the surface hydrophobic. Briefly, the Si wafer was first cleaned with acetone, ethanol, and deionized water and then treated with a mixture of H2SO4/H2O2 (v/v = 4:1) and diluted HF solution (1 wt %) to remove the surface contaminants and the oxide layer. The cleaned wafer was then dipped into the solution of 0.01 mol/L AgNO3 and 4.60 mol/L HF for 1 min at room temperature to deposit silver on the surface through electroless deposition. The wafer was then immediately transferred into the solution containing 0.01 mol/L AgNO3, 0.44 mol/L H2O2, and 4.60 mol/L HF. After 1 h of reaction at 50 °C, the wafer was taken out from the solution, washed with deionized water, and dried at room temperature. The fabricated nanostructured wafer was further surface-modified with fluorinated silane through vapor deposition.[36]

Droplet Collision Experiment

As shown in Figure , droplets with a diameter of about 2.2 mm and size variance of ∼1% were generated using a syringe pump attached with a needle that has an inner diameter of ∼0.3 mm. The temperature of the droplets was controlled through controlling the temperature of the droplet-delivering needle using a heating system (DIYCH401, Shanghai Hua Jian Electric Heating Appliance Co. Ltd, China). The heating system includes a coil heater, which was wrapping around the needle, and a thermal controller. The water was preheated for ∼3 min in the needle before being released as the impacting droplet. The temperature can be adjusted from room temperature to 100 °C with the temperature resolution of 1 °C. An IR camera (A300S, FLIR Systems Inc) was used to measure the temperature of the released droplet. The volume of the released droplet was ∼5.6 mL, whereas the volume of the water inside the heated needle is ∼19.6 mL, all of which was preheated to the set temperature. The detachment of the droplet from a needle observed in the experiment would induce oscillations during the subsequent falling.[37] In this work, we assumed that the effect of the oscillations during the falling process is the same for all impacting experiments. During the study, a cold stationary droplet was placed on the SH Si NW substrate first (Figure b) and the impacting droplet was preheated inside the needle and released down toward the stationary droplet. The position of the stationary droplet needs to be adjusted carefully using the three-dimensional microstage to ensure the alignment of the two droplets and therefore a head-on collision between the two droplets (Figure c). In this work, the releasing height between the tip of the needle and the substrate was set to 13 mm. The collision process of the two droplets was captured by a high-speed camera (X-Stream XS-4, IDT) with a zoom lens (XDS-0745, JANUS, China) of 3× magnification. A planer LED light source (C-F1230, Nikon, Japan; 12 V, 100 W; 51 mm in diameter) was placed behind the droplets and used to illuminate the droplets during the imaging acquisition process, as shown in the optical image in Figure S1. The camera was operated at 2000 frames per second with a resolution of 512 × 512 pixels. All experiments were conducted with controlled humidity of 50% to minimize the impact of humidity. The data of impacting parameter and impacting velocity were obtained through analysis of the high-speed video images using Motion Studio software.

Figure 2

Experimental setup: (a) Schematic of the instruments used in the experiment, which includes the heating system and the visualization system. (b) Scanning electron microscopy of SH surface of Si NWs in clusters. (c) Optical image of the impacting droplet (hot) in contact with the stationary droplet (cold) on the SH Si NW surface.

Results and Discussion

The experimental setup used in this study consists of a micropositioning system, a droplet generator, a heating system, and an imaging acquisition system. In this work, the stationary droplets were doped with 0.03 wt % Rhodamine B, which was a red dye, and were kept at room temperature (∼27 °C). The impacting droplets had no doping, and their temperatures were adjusted through the heater attached to the delivering needle. Figure shows the coalescence, spreading, and rebound of the two colliding droplets on the SH Si NW surfaces with TI changing from 27 °C (Figure a) to 40 °C (Figure b) and 70 °C (Figure c). The images show that the influence of temperature is quite significant in the process of droplet coalescence. As the temperatures of the two droplets were both set at 27 °C, the head-on collision proceeded with a large deformation of both droplets and a clear interface maintained during the deformation. The rapid contact and deformation of both droplets trapped some air in the interfaces. Due to the air layer trapped between the two droplets, both droplets deformed as elastic balls and did not merge together.[13,19,21,22,32] During this process, the kinetic energy was converted to the surface energy. When the droplets spread close to the maximum spreading length (spreading length is defined as the spreading diameter of a water droplet in contact with the substrate) and the largest deformation, the air layer at the interface disappeared and the two droplets started to coalesce. Many small air bubbles can be seen inside the merging droplets due to the dissolved air (the last image in Figure a). Previous studies showed that higher impact speeds, lowering the pressure of the surrounding air and increasing the roughness of the solid surface can help minimize the effect from the air layer trapped at the interface,[18,20,38] whereas in this study, we showed that temperature may also help reduce the amount of the air trapped at the interface.

Figure 3

Droplet coalescence, spreading and rebound on SH Si NW surfaces with the stationary droplet set at 27 °C and impacting droplet set at (a) 27 °C; (b) 40 °C; and (c) 70 °C. The sizes of both droplets were ∼2.2 mm in diameter.

Compared with the collision at 27 °C, when TI increased to 40 °C, there was a change in the coalescence process (more snapshots from 3 to 7 ms for Figure b,c are included in Figure S2). As shown in Figure b, the most obvious change was the coalescence time. The two droplets started to merge together at around 3 ms, which was during the spreading process rather than close to the end of the spreading process. The increase of TI led to the increase of water vapor around the water droplet.[39] The increased amount of water vapor at the interface between the impacting droplet and the stationary droplet helps facilitate the formation of water bridges between the two droplets and accelerates the coalescence process.[40] What needs to be pointed out is that the heating up of the stationary droplets will help form the water bridge to shorten the coalescence time as well. During the experiment, we found that the heating up of the stationary droplets led to the pinning of the stationary droplet to the substrate, which affected the spreading and rebounding of the merged droplets and increased the complexity in analyzing the interaction between the two droplets. In this study, we thus only focused on the heating up of the impacting droplets. The decrease of surface tension and viscosity with the increase of the droplet temperature will also help the coalescence process.[13,32] The decrease in surface tension will lead to the increase of the Weber number, which enhances the coalescence between the two droplets.[32] The decrease in viscosity helps the formation of a large contact area during the collision, which also will facilitate the coalescence between the two droplets.[13] The deformation of both droplets before the coalescence with TI at 40 °C was smaller than in the case when both droplets were set at 27 °C. As the process moved forward, the merged droplet deformed as an integrated droplet with the subsequent spreading and rebounding processes similar to the processes of a single droplet impacting on a SH surface. When the spreading area expanded to the maximum value, the droplet was retracted for the rebound. Through the high-speed camera we observed that after the initial deformation of both droplets and the start of the coalescence of the droplets, the hot impacting droplet penetrated into the middle of the stationary droplet at the point of the head-on collision. The penetration continued downward to the solid substrate due to the kinetic energy, which was converted from the gravitational potential energy of the impacting droplet. The contacting interfaces of both droplets merged together during the penetration and the two droplets eventually coalesced into a single droplet, with an anisotropic distribution of liquid at two different temperatures. The kinetic energy in the impacting droplet caused the merged droplet to deform, and the merged droplet behaved like a single impacting droplet but with the top portion warmer than the bottom portion. As the merging of the interface started earlier than in the case when the impacting droplet was set at 27 °C, there was less deformation for both droplets before the merge and thus less air trapped at the interface, which led to less air bubble formation inside the merged droplet (the last image in Figure b). The less air trapped at the interface may also lead to the penetration of the impacting droplet into the stationary droplet during the impacting process.

As TI was raised further to 70 °C, the coalescence process started even earlier due to the increase of the amount of water vapor at the interface between the impacting droplet and the stationary droplet. The coalescence happened almost immediately when the two droplets started to contact and continued to the spreading stage. The impacting droplet penetrated into the stationary droplet, with the contact line merged outward from the initial contact center. The two droplets merged together to form a larger anisotropic droplet. Such a droplet went through the spreading and rebound as a single droplet, except that there was a temperature gradient within the rebounding droplet, with the top (transparent) at higher temperature and bottom (red) at lower temperature through the video image analysis. With much shorter time and much smaller contacting interface before the coalescence of the two droplets than both previous cases, there was much less air trapped between the merging interface and there was almost no air bubbles observed in the merged droplet (the last image in Figure c). Such an observation through the high-speed camera shows that the internal mass exchange, which was induced by the droplets mixing and internal liquid flow, was not fast enough to heat up the interface before the first rebound of the merged droplet from the surface. A similar trend of accelerated coalescence with the increase of TI was observed with the change of the sizes of the droplets and the releasing heights of the impacting droplets (Figures S3 and S4). When the size or velocity of the impacting droplet increased, the coalescence process started as early as the two droplets started to contact with a high impacting temperature (TI = 70 °C).

We also carried out the study with impacting droplet at other temperatures between 27 and 70 °C. Figure a shows the images for the impacting droplets with different impacting temperatures at the moment when the two droplets just started to coalescence, which was defined as the moment when a clearly merged interface was observed in the video images. Figure shows similar trends to what were observed in Figure . When TI was set at less than 40 °C, the starting moment of coalescence was close to the maximum spreading due to the existence of the trapped air layer. With the increase of the temperature of the impacting droplet, the increasing amount of water vapor at the interface between the impacting droplet and the stationary droplet reduced the amount of trapped air and enhanced the coalescence of the droplets. As shown in Figure , when the impacting droplet was set at 40 °C, the coalescence started during the early stage of spreading. As TI increased from 40 to 60 °C, the starting moment of coalescence moved to the beginning of the spreading, whereas no obvious spreading was observed during the coalescence of the droplets when TI reached 70 °C.

Figure 4

Starting moment of coalescence and the restitution coefficient for different TI: (a) images of the merging moment between the two droplets. (b) The restitution coefficient of the two droplets with different impacting temperatures.

To further understand the difference of the droplets’ interaction when the impacting droplets are set at different temperatures, we also calculated the restitution coefficient (ε) at different TI. The restitution coefficient is a key parameter used to evaluate the kinetic energy loss during the coalescence process. Conventionally, ε is defined as the ratio of impacting velocity (v0) to rebounding velocity (vr).[41] It is quite straightforward to calculate the velocity of the impacting droplet, which can be derived from the initial gravitation potential energy. It is challenging, however, to calculate the exact velocity value of the merged droplets, which most of the time have a distorted shape. In this work, we used gravitation potential energy at the maximum rebounding height to estimate the rebounding velocity. What needs to be pointed out is that there is still internal flow and kinetic energy inside the merged droplets even at the maximum rebounding.[42] Such an internal flow is complex and difficult to estimate, so in this work we did not include such flow in the estimation of the restitution coefficient. The estimated restitution coefficient can be calculated through eqs and 2in which m is the mass of the droplets, h1 represents the maximum rebounding height of the merged droplet after the coalescence, h0 represents the original height of the impacting droplet, and g is the gravitational constant. The height (h) was measured between the center of the droplets at the top and also the center of the droplets at the maximum spreading length. In this study, the mass of the merged droplets doubled and the total surface energy would reduce after coalescence. In this study, we used the average temperature between the stationary and the impacting droplets as the temperature for the merged droplets. With the temperature range we studied, the average temperature for the merged droplets was in the range of 27–48.5 °C. Within such range, the change of surface tension is only 5%.[43] We calculated the change of the total surface energy for the two droplets before the impact and for the merged droplets just before rebounding (Table S1 in the supporting information). Figure b shows that there is a large jump in the ε value between 35 and 40 °C. When TI was low, the ε value was about 0.30–0.35 and the energy loss was large compared with the theoretical restitution coefficient, which might be due to the chaotic mixing process observed in the video images. When TI was higher than 40 °C, the energy loss was small with a relatively larger coefficient of ∼0.5, which might be due to the rather smooth merging process and relatively small energy loss during the coalescence. Therefore, the restitution coefficients (Figure b), which were all smaller than the theoretical restitution coefficients (Table S1), indicate that there was loss of kinetic energy during the droplets collision. Compared with the value of ∼0.35 at lower TI, the value of ∼0.5 at higher TI indicates the smaller energy dissipation during the collision processes with higher TI.

We also studied the contact time of the collision process. For droplets impacting solid surfaces, the contact time is defined as the time duration between the time of droplets in contact with surfaces and the time just before the droplets leaving the surfaces. It is often a crucial parameter for the study of the interaction between a droplet and a surface. In this study, we defined the contact time as the time duration between the time of the contact of the two droplets and the time of the separation of the liquid bottom interface of the merged droplets from the SH Si NW surfaces. All contact times were analyzed from the video images. Figure provides the contact time values before the first rebound for the impacting droplets set at different temperatures. The schematic inserted in the figure shows the states of the merged droplets just before their departure from the SH Si NW surfaces for the first rebound. As shown in Figure , the contact times stay relatively constant (within the experimental error) for collisions at different TI. Even as the impact velocity increased with the increase of the releasing heights of the impacting droplets (Figure S4), the contact time remained constant (∼22 ms). The relatively stable contact time was also observed during the impact of a single droplet onto the SH substrate.[44−46] The contact time of a single impacting droplet on a solid surface can be estimated by balancing the inertia with the capillarity (44)where τ is the contact time, γ is the coefficient of surface tension of the impacting droplet, ρ is the density of the liquid, and R is the diameter of the droplet. Here, we also applied the same equation to explain the realtivecontact time observed in our experiment. The R of the merged droplets for different TI is the same since there was no loss of mass during the coalescence process. With the same ρ, eq can be simplified to eq With the average temperature in the range of 27–48.5 °C, the change of surface tension is only 5%[43] and the contact time thus remained relatively constant, which was consistent with the experimental observation in Figure b,c. Certainly, here we ignored the interaction between the droplets and the SH Si NW surface, which was negligible during the first contact since the temperature of the water interface interacting with the SH Si NW surface was maintained at 27 °C.

Figure 5

Contact time of droplet collision with impacting droplets set at different temperatures. The schematic insets in the plot show the merged droplets before their departure from the SH Si NW surface when the temperature of impacting droplets was set at 27, 45, and 70 °C.

Besides the study of the interaction of the two droplets during their first impact, spreading, and rebound, we also investigated their subsequent impact, spreading, and rebound processes. Figure a shows the schematic of the 1st and 2nd maximum spreading of two droplets when TI and TS are both at room temperature, and Figure b shows the schematic of the 1st and 2nd maximum spreading of two droplets when TI > TS. The plots in Figure c,f represent the relationship between the dimensionless spreading length ψ (ψ = l/D0; l is the measured spreading length of the droplet, and D0 is the initial diameter of the droplet) and dimensionless time t (t = tmv0/D0; v0 is the initial impact velocity of the droplet; and tm is the real time measured in the experiment, which starts from the maximum spreading during the first impact). As the TI was set at 27 °C, the maximum spreading length, which represented the remaining energy of the system in the form of surface energy after coalescence, was about 1.3 (Figure c). When TI increased to 45 °C, the maximum spreading length increased to ∼1.6 (Figure d) and then stayed close to ∼1.6 with the increase of TI (Figure e,f). The increase in the maximum spreading length indicated the increase of the surface energy after the coalescence process, which also implied that there was less energy loss or less energy dissipation during the coalescence process for TI = 45 °C than for TI = 27 °C. Another interesting observation is the number of rebounds at each temperature. Figure c shows that the merged droplet after coalescence at TI = 27 °C only rebounded twice due to the relatively low energy remained after coalescence. When TI increased to 45 °C (Figure d), however, the merged droplet rebounded five times due to the reduced energy loss during the coalescence process. With the further increase of TI, the number of rebounds for the merged droplets decreased to four times and three times at 60 and 70 °C. Such decrease in rebounds might be due to the increased interaction of the merged droplets with the substrate surface at elevated temperature.

Figure 6

Spreading and rebounding of the two droplets with different TI. The schematics in (a) and (b) represent the first spreading at maximum length and the 2nd spreading at maximum length: (a) TI = TS = 27 °C. (b) TI > TS = 27 °C. The plots represent the change of the spreading length with the time for different TI: (c) 27 °C, (d) 45 °C, (e) 60 °C, and (f) 70 °C.

To understand the spreading and rebounding behavior, we also analyzed the change of system energy during these processes. Figure S5 showed such analysis for TI = 27 and 45 °C. Before the release of the impacting droplet, the energy at the initial state was the sum of surface energy of the two droplets and the gravitational potential energy of the impacting droplet. During the falling of the impacting droplet, the gravitational potential energy converted to kinetic energy. When the two droplets collided, the droplets began to deform and part of the kinetic energy was transformed to the surface energy. The surface energy reached its maximum value at the maximum spreading and the kinetic energy decreased to zero at that moment. During the deformation and spreading process, a part of the kinetic energy was lost due to the following three possible energy loss or dissipation mechanisms: the viscous loss, the loss due to internal vibration of the droplet, and the loss due to the interaction with the underlying surface. The viscous dissipation is one of the most common energy dissipation mechanisms during the droplet coalescence. The viscous dissipation varies with the change of the droplet shape and internal liquid flow during the coalescence process.[47,48] Reynolds number (Re = ρVR/η, ρ is the density of the liquid, V is the velocity of the droplet, R is the radius of the droplet, and η is the viscosity of the liquid) represents the ratio between the inertia and the energy dissipated in viscous loss. As Reynolds number increases, the viscous dissipation tends to decrease.[47,48] Besides the viscous loss, the loss related to the internal vibration and liquid movement is also needed to be considered in the process. A part of the translational kinetic energy could be possibly transferred into internal vibration during the shock of the merging process,[49,50] especially at the low temperature. As shown in Figure , the droplets went through a large deformation before merging when the impacting droplets were set at TI < 40 °C. The time of their merging was usually close to the moment of maximum spreading length of stationary droplet. At that moment, the stationary droplet was going to bounce but the impact droplet might still have the kinetic energy. The difference of the velocity direction between the fluid within the two droplets produced the velocity gradient inside the merged droplets, which resulted in the loss of kinetic energy through internal rotation and inner mixing.[51] Compared with the merging process at higher TI, the merged droplet at 27 °C showed a relatively more internal fluid movement during the rebound from the surface at first time. The red dye within the merged droplet showed the chaotic mixing clearly. This chaotic mixing is the main factor of energy loss when TI was low.

When TI increased, the interaction of the merged droplets with the underlying substrate surface, which was related to the wetting property of surface, also played an important role in the energy loss during the spreading and rebounding processes. The stationary water droplets showed a Cassie state on the SH Si NW surfaces in this study, with an advancing contact angle of ∼162° and receding contact angle of ∼147° (Supporting Information). As the temperature of the water droplet in contact with the SH surface increases, the contact angle will normally decrease and the sliding angle will increase.[52] When the impacting droplet was set at room temperature, the merged droplet had the same temperature as TS and the interaction with the underlying SH surface was the same as the original stationary droplet (Figure a). Even when TI was higher than 27 °C, with the observation of the separation of the cold water (bottom of the merged droplet) and hot water (top of the merged droplet) and the theoretical analysis of the heat diffusion length (Supporting Information), there was little heat transfer from the hot impacting droplet to the cold stationary droplet before the first rebound (Figure b). After the first rebound, however, there were fluid mixing and also heat transfer inside the merged droplets. Such mixing and heat transfer increased the temperature of the merged droplet and also the water interface in contact with the SH surface, which also resulted in the increase of the vapor pressure at the interface. The water interface with increased temperature could transfer some of the heat to the underlying surface (Figure b). The increased vapor pressure at the water–Si NW interface could also lead to the condensation of water vapor on the relatively cold SH surface. Both the heat transfer to the solid surface and also the condensation would increase the attractive interaction between droplets and the surface and also the pinning of the liquid to the surfaces.[53−55] As the temperature of the merged droplet increased, the droplet gradually transferred from the initial Cassie state to a partial Wenzel state.[36,56] The combination of these factors made the droplet prone to be stuck on the surface. As shown in Figure d, the merged droplet at TI = 45 °C rebounded 5 times while the merged droplet stuck on the surface only after the third rebound when TI = 70 °C (Figure f). Besides, as shown in Figure d,e, the contact time of the merged droplet became larger during the subsequent rebound, which indicated a slight increasing trend attributed to the factors mentioned above.

Conclusions

In summary, this work studied the coalescence, spreading, and rebound processes of two colliding droplets, with the stationary droplet sitting on a SH surface, and the impacting droplet at elevated temperatures. Through the use of dye and also high-speed color video imaging, the relatively detailed observation of the change of the droplet interface, and the internal movement of the merged droplet were observed. The energy conversion during the coalescence, spreading and rebound was also discussed. The increase of the impacting droplet temperature resulted in less energy loss during the coalescence process. Optimizing the impacting droplet temperature can lead to more rebound after the coalescence. Such findings will help further the fundamental understanding of the two droplets interaction at different temperatures, which will be helpful in generating different engineering approaches for relevant applications involving droplet interactions, such as thermal spray, ink jet printing, and microfluidics. The efficient retaining of kinetic energy with the high impacting temperature also provides a strategy for the efficient separation of the merged droplets from the underlying substrate, which could help the removal of hot droplets from solid surfaces, such as the condensed droplets in dropwise condensation systems.