Hydrodynamics of droplet impingement on hot surfaces of varying wettability

Abstract

This work presents on the hydrodynamics of water droplet impingement on superheated solid surfaces across the entire wettability spectrum: superhydrophilic, hydrophilic, hydrophobic and superhydrophobic. While a large body of work exists on droplet impingement on hydrophilic and superhydrophilic surfaces, impingement on the latter two has been largely neglected and the present results show that dynamics are dramatically different. Experiments ranging in surface temperature from 125 • C to 415 • C and Weber numbers from 10 to 225 were performed and analyzed using high-speed imaging. Some of the most striking differences are as follows. While atomization is always present for impingement on the hydrophilic and superhydrophilic surfaces at temperatures below the Leidenfrost point, atomization is absent at low Weber numbers and at low excess surface temperatures on the hydrophobic surface. At high surface temperatures, the attraction of vapor bubbles on the hydrophobic surface allows a vapor blanket to form more readily thus leading to Leidenfrost behavior at a much lower temperature than classically observed on a hydrophilic surface. One of the most interesting phenomenon that will be discussed includes what will be described as a "pseudo-Leidenfrost" state for impingement on the superhydrophobic surface. Because water can be suspended at the peaks of the roughness on a superhydrophobic interface, vapor escapes from underneath the droplet thus mimicking Leidenfrost behavior for all excess temperatures. This results in minimal atomization for superhydrophobic impingement over the entire regime explored. Finally, maximum spread diameters for Leidenfrost impingement are tabulated as a function of the Weber number for all surfaces and are shown to be larger on the smooth surfaces than on the textured ones indicating that droplet spreading at the Leidenfrost point is not independent

The Impact of Single- and Multicomponent Liquid Drops on a Heated Wall: Child Droplets

“…However, there is still no agreement on the conditions for maximizing the number of child droplets. Therefore, it is expedient to analyze the latest achievements of the global scientific community in this area and to outline promising directions for future research [5][6][7][8][9][10].…”

“…For example, a surface slope angle of 67.4 • provides maximum droplet spreading. Also, the Weber numbers influence the drop spreading diameter [6]. The maximum drop diameter increases with the Weber number and decreases when the Weber number reaches 2.7.…”

“…A drop of the liquid with a lower surface tension broke up at lower Weber numbers, while an increase in roughness considerably reduced the critical Weber number. Clavijo et al [6] used lower Weber numbers because the drop diameter was 1.9 ± 0.03 mm, and the average roughness of surfaces was 0.025~6.3 µm. They showed that increasing the Weber number and the surface roughness promoted the drop breakup.…”

“…In addition to roughness and thermophysical properties of substrate materials, the surface hydrophilicity also had a governing effect on breakup. In particular, Clavijo et al [6] utilized ultrahydrophilic, hydrophilic, hydrophobic, and superhydrophobic surfaces and performed the experiments at a surface temperature ranging from 125 • C to 415 • C and Weber numbers from 10 to 225. There was some mean temperature range, at which the drop broke up to form a maximum number of child droplets.Fujimoto et al[7] generated a sequence of drops to impinge on a surface, so that each successive droplet contacted with a liquid film of a different thickness.…”

“…The behavior of drops on both treated and untreated surfaces had the same tendency: the drops expanded to the maximum diameter (stretch) and then narrowed. However, the treated surface provided a larger drop size, which is explained by the degree of adhesion between the drop and surface.Experimental findings [1][2][3][4][5][6][7][8] are the basis for developing the models of drop impact on solid walls. Some interesting results in this area were obtained, for instance, by Senda et al [9], who proposed a model of fuel film formation when impinging a diesel aerosol on a low-temperature wall.…”