Experimental study on phenomena of single water droplet impacts on liquid surfaces: Pattern maps and correlations
Introduction
Droplets impacting surfaces of solids or liquids have been extensively investigated for diverse industrial applications such as spray cooling, inkjet printing, thermal spray coating, combustion engine, and fire extinguishment using water droplets [1], [2], [3], [4], [5], [6]. For example, the interaction between the impacting droplet and liquid surface can be closely related to the fire suppression behavior of a liquid pool. Water-based fire suppression systems (e.g., sprinkler systems) are commonly used in various situations because water, as a fire extinguishing agent, is economical, nontoxic, and eco-friendly and exhibits a good fire suppression performance. However, the use of sprinkler systems is limited in extinguishing liquid pool fires because the fire can spread out and expand owing to the water droplets. If relatively large water droplets penetrate the fire plume and reach the liquid fuel surface in fire, they can disturb the liquid surface, and in some cases, unexpected severe events of fire spreading and expansion can occur. On the contrary, if the water droplets gently reach the liquid surface, minimizing the disturbance of the liquid fuel surface, they may have an effective role in the fire suppression. Hence, understanding the detailed phenomena of droplets impacting liquid surfaces may be essential for the design and improvements in water-based fire suppression systems. In addition to fire suppression applications, the interactions between the impacting droplets and surfaces are an important factor determining their performances and efficiencies in various applications, mentioned above. However, the understanding of the interactions between the impacting droplet and liquid surface is still insufficient.
When the falling droplet reaches the liquid surface, complex interactions (e.g., floating, bouncing, coalescence, jetting, and splashing) occur, which are affected by various droplet and liquid surface factors such as the droplet impact velocity, liquid pool depth, physical properties of the droplets and liquid surfaces, and surfactants. Such investigations on a single droplet impacting a liquid surface have been performed [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. For example, Hasegawa and Nara [3] investigated experimentally the droplets impacting into the deep water pool. As the droplet fluids, the water, ethanol, and silicon oil were used. They reported that the impact behavior and cavity formation were affected by the liquid properties and impact parameters. They estimated and analyzed the energy balance from the droplet impact at the liquid surface to jet formation. Manzello and Yang [5] conducted the experiments on the water droplet impact on water and HFE7100 surfaces with different depths. They mentioned that the Weber number for the jet breakup was independent of liquid pool depth and in their experimental conditions, jet breakup was not observed in the case of water droplet impact on the HFE7100 surface. Kumar et al. [8] investigated experimentally the coalescence dynamics of an impacting droplet on a sessile droplet. As the impacting and sessile droplet fluids, the ethanol was used. The influences of sessile droplet volume, impacting droplet volume, and impacting height on the coalescence dynamics were explored. They presented the regime maps for the partial coalescence and spreading patterns, and the details on such patterns were analyzed and discussed. Xu et al. [13] performed the experiments of single water droplet impact on the burning or unburned liquid pool surface. As the liquid pools, the heptane, butanol, and ethanol were tested. They reported and discussed the details of impact behaviors and crater evolution observed on the burning and unburned liquid pool surfaces. Ray et al. [17] performed the numerical simulations to investigate the bubble entrapment and jet formation during liquid droplet impacting on the liquid pool. They constructed the regime map and reported the detailed characteristics of crater and jet. The additional previous studies on the phenomena and interactions between the impact droplets and liquid surfaces have been well reviewed and reported by Yarin [21] and Rein [22].
A few studies on the analysis of correlations to predict regime transitions have been carried out. Hsiao et al. [10] proposed the critical Weber number between the formation of vortex rings and jets when the liquid droplet impacted the liquid surface and compared it to experimental data of water and mercury. Okawa et al. [23] performed experiments on a single droplet impacting a liquid surface. They mainly focused on the secondary droplets generated from the crown rim structure. In addition, based on the experimental data, a correlation for the lower limit of central jet formation was proposed. Huang and Zhang [24] carried out experiments on droplet impingements on deep pool and film surfaces of water and oil for different droplet velocities and sizes, liquid types, and film thicknesses. Their experimental observations were compared to previous models. New correlations for the transitional boundaries between regimes of bouncing, coalescence, jetting, and splashing were then obtained and compared to the experimental data. Zhao et al. [25] investigated droplets impinging on liquid surfaces using water, ethanol, n-pentane, methanol, and 1-propanol as test fluids. They analyzed the coalescence and jetting regimes and developed two models for the transition from coalescence to jetting regimes based on experimental observations. Zou et al. [26] carried out experiments to analyze the behavior of droplets impacting a liquid surface restricted by the surrounding walls. The impact Weber number was in the range of 5–630. Glass tubes with inner diameters of 6, 8, 12, 17, and 26 mm were tested to control the restricted target liquid. They reported that the bubble entrapment was influenced by the droplet impact velocity and distance between the impact point on the liquid surface and surrounding wall. In addition, the restricted influences of surrounding walls on the critical velocity of canopy bubble formation and droplet bounce height were analyzed. They compared their experimental data to the previous correlations and proposed the threshold for the splashing regime.
Based on previous reports, experiments on droplets impacting liquid surfaces were carried out and a few correlations were proposed. However, most previous studies have been performed on limited liquid surface types and impact patterns. In addition, the judgment of impact patterns of the droplet onto liquid surfaces may be subjective. Hence, to understand and accurately predict the droplet impact on the liquid surface, experimental data regarding various impact patterns for different types of liquid surfaces should be obtained and consistently analyzed. Furthermore, based on the experimental data, the previous correlations for transitions between impact pattern regimes should be assessed.
In this experimental study, the phenomena of a single water droplet impact on liquid surfaces of water, ethanol, methanol, and n-heptane were investigated. Various impact phenomena were visualized using a high-speed camera. Based on the experimental observations, impact pattern maps were constructed for all liquid surfaces. The previous correlations were then compared and assessed using the present experimental data. New correlations for impact regime transitions were proposed considering the properties of the liquid surfaces.
Section snippets
Experimental methods
Fig. 1 shows the experimental setup, which consists of a syringe pump, moving stage, liquid container, support jack, high-speed camera, and computer. As the droplet fluid, the water was used because one of the motivations of this work is the application in the field of water-based fire suppression systems (e.g., sprinkler systems) using the water droplet as the fire extinguishing agent. A single water droplet was generated by a syringe pump and needles with three different inner diameters. The
Visualization of the droplet impact on liquid surfaces
The single water droplets impacting the different liquid surfaces were observed and visualized for the various single water droplet diameters and impact velocities. Based on the experimental observations and previous studies, in this study, the impact patterns were divided into coalescence, single jet, splash, and canopy patterns. Additionally, the splash patterns were classified into two types, splash pattern without secondary jet and splash pattern with secondary jet. The impact patterns are
Conclusions
In this experimental study, the impacts of a single water droplet on various liquid surfaces were investigated. The impacting single water droplets had diameters of 2.0, 2.5, and 3.2 mm and impact velocities of 0.26–3.73 m/s. Water, ethanol, methanol, and n-heptane were used as liquids. The impacts between the single water droplet and liquid surfaces were visualized. Based on the experimental observations, the previous correlations were assessed, and new pattern maps and correlations of regime
CRediT authorship contribution statement
Ji Hyun Yang: Conceptualization, Methodology, Validation, Investigation, Writing – original draft. Chi Young Lee: Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03932585). This manuscript was prepared by revising and improving the contents of the first author’s thesis for her M.S. degree.
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