Hydrodynamic pattern transition of droplet train impinging onto heated titanium substrates with or without nanotube coating

Highlights

• Droplet train impinges onto heated substrates with or without nanotube coating.

• Four hydrodynamic patterns are observed when droplet impinging onto substrates.

• Use quantitative measurements to examine transitions between four patterns.

• Leidenfrost point of the substrate with nanotube coating is significantly delayed.

Abstract

The impingement of ethanol droplet train on the heated titanium substrates without or with the titanium oxide nanotube coating has been experimentally investigated in close view. The coating makes the substrate with high wettability. Four distinct but steady hydrodynamic patterns are observed on both two surfaces, namely, liquid aggregation and crown periphery instability; sub-droplet splashing and crown periphery instability; splashing and stable crown; and splashing with stable angle. However, the more wetting nanotube coated substrates push the transition between the patterns to a higher temperature. The quantitatively analysis of spreading length, diameter and height of crown and stable splashing angle further proved the transitions between the hydrodynamic patterns on the two titanium surfaces. The instability at the crown periphery may result from the low surface tension of the working fluids in first and second patterns, while the crown becomes stable on the third pattern. In the fourth pattern, a sharp shift of the splashing angle from decreasing to increasing is found at the surface temperature of 323 ^∘C and 404 ^∘C for the bare titanium surface and the nanotube coated surface, respectively. The shift could be attributed to the emergence of Leidenfrost effect. It is found that the Leidenfrost point at the droplet train impingement on the nanotube coated surface has been remarkably delayed.

Introduction

The phenomena of droplet impinging onto heated and coated surface are kept receiving extensive attention, since they are related to many important industrial applications, such as coating, spray coating, spray cooling and combustion [1]. Many efforts have been undertaken to explore the heat transfer and hydrodynamics of single droplet impact [2], multiple droplet impingement simultaneously [3], [4] and droplet train impingement [5] on the substrate. The effects of impinged surfaces conditions, such as wettability [6], [7], stationary dry surface [8], stationary pre-wetted surface [9], surface with liquid film [10], [11], moving surface [12], etc., have been investigated and discussed.

The heat transfer associated with the droplet impingement can be divided into four regimes, including film evaporation, nucleate boiling, transition boiling, and film boiling [13]. The wall temperature between transition boiling and film boiling is Leidenfrost point, which indicates the onset of stable film boiling and is a crucial parameter for the heat transfer. A stable vapor layer can be generated from the droplet vaporization between the substrate surface and the impacting droplet after the Leidenfrost point. The existence of the vapor layer deteriorates the heat transfer performance dramatically. A large number of researches [14], [15], [16], [17], [18] have been taken on increasing the Leidenfrost point so as to postpone the regime change from transition boiling to film boiling and keep high heat transfer performance.

Nanotube coating on the substrate can be an effect way to defer the Leidenfrost point to a higher temperature. Kim et al. fabricated a nanotube zircaloy surface and investigated the effect of nanotube on the droplet impact at the heated wall [19]. The delayed Leidenfrost point and the cutback phenomena were observed in their experimental results. They also discussed the heat transfer performance of the water droplet on the over-heated micro- and nano-structured surfaces [20]. The experimental results suggested that the modified surfaces enhanced the nucleate boiling of the water droplet. The structured surfaces could increase the hovering points. Later, in Ref. [21], the nano-structured silicon surfaces were manipulated and prepared with the wettability from superhydrophilic to superhydrophobic state. The experimental results indicated that the Leidenfrost point was higher with more hydrophilic condition. The explosive transition boiling was observed on superhydrophilic substrate due to the cavities and capillary wicking on the nanotube surface. The required rebounding energy under the different wettability was estimated quantitatively. Tong et al. visualized the impact pattern of a single water droplet impinging onto the titanium oxide (TiO₂) nanotube coated titanium (Ti) surface [22]. A unique liquid film lift-off was observed and analyzed. The boiling regime maps of the nanotube coated surface and bare Ti surface were established and compared. The results revealed that the existence of the nanotube coating on the Ti surface noticeably delayed the Leidenfrost point. However, most of the studies are limited on the single droplet impacting.

In the industrial application such as coating and spray coating, the multiple and continuous droplets would impinge on the rigid surface. In the scenario, the following droplet would impact on the residual of the previous droplet on the surface. The droplets interact with each other among the impingements in form of collision and coalescence. The researchers have reported the hydrodynamics characteristics of the droplet train as the simplified model for the interaction among continuous droplets. Qiu et al. investigated the microscale droplet train impinging onto the heated copper surface [5], [24], [25], [26]. The transition phenomena were observed when the droplet train impinging on the substrate. It was found that an orderly rebound and splashing patterns were formed as the wall temperature was high, and the stable splashing angle varied with wall temperature until the substrate temperature reached the Leidenfrost point. The splashed secondary droplets were quantitatively analyzed as well. The influences of the Weber number, droplet frequency, impact angle, and surface temperature imposed on the transition of droplet splashing were discussed. Zhang et al. conducted the experimental and numerical analyses for the droplet train impinging onto the solid surface which was pre-wetted [27]. The crown propagation events were analyzed by the images captured by a high-speed camera. The computational fluid dynamics tool was also applied to simulate the cases. The result from the experiment and the simulation agreed with each other. A revised theoretical model of crown propagation was proposed. In Ref. [28], the hydrodynamics and heat transfer were investigated for the double, triple and hexagonally-arranged droplet trains impinging on the pre-wetted solid surface with coating. Li et al. presented the experiments to study the effects of the impact frequency, the Weber number of the droplet train, and the surface contact angle on the droplet train impingement [29]. By analyzing the experimental results, the empirical correlations were obtained for the maximum spreading factor and the average flatness factor. Fathi et al. investigated the behavior of the droplet train, jetted from a fixed nozzle, impinging onto the moving surface [30]. The effects of jetting frequency and surface linear velocity on the hydrodynamic behavior were investigated.

It can be found that a research gap exists on comparison of the droplet train impinging on the substrates with the difference wettability. The heat transfer performance between droplet train and surfaces with the various wetting behaviors has not been well studied yet. We will find that the TiO₂ nanotube coating will make the Ti substrate more hydrophilic, accordingly, The two substrates, one with the nanotube coating and the other without coating, will be applied for the droplet train impingement. Except for the surface properties, the working fluid also has great effect on the hydrodynamic behavior and heat transfer of droplet train. We will use organic working fluid, ethanol, instead of water in this study. Ethanol has lower boiling point, latent heat, and surface tension than water. The observation from the ethanol droplet train can be compared with that of water. Furthermore, the similarity and dissimilarity of hydrodynamic pattern and heat transfer of ethanol droplet drain impinging on the heated TiO₂ nanotube coated surface and a bare Ti surface will be analyzed and discussed.