Droplet impact on superheated micro-structured surfaces

Abstract

When a droplet impacts upon a surface heated above the liquid's boiling point, the droplet either comes into contact with the surface and boils immediately (contact boiling), or is supported by a developing vapor layer and bounces back (film boiling, or Leidenfrost state). We study the transition between these characteristic behaviors and how it is affected by parameters such as impact velocity, surface temperature, and controlled roughness (i.e., micro-structures fabricated on silicon surfaces). In the film boiling regime, we show that the residence time of droplets impacting upon the surface strongly depends on the drop size. We also show that the maximum spreading factor G of droplets in this regime displays a universal scaling behavior G $ We 3/10 , which can be explained by taking into account the drag force of the vapor flow under the drop. This argument also leads to predictions for the scaling of film thickness and velocity of the vapor shooting out of the gap between the drop and the surface.In the contact boiling regime, we show that the structured surfaces induce the formation of vertical liquid jets during the spreading stage of impacting droplets.

Dynamic Leidenfrost Effect: Relevant Time and Length Scales

“…In the Leidenfrost regime, R w ¼ 0 during the whole spreading process, i.e., no wetting at all. Note that the present classification of the boiling regimes is based on the direct observation of wet respective dry areas, and is thus different from the classification in previous studies [8,11], where the boiling regimes were more superficially classified based on the smoothness of the droplet surface or the ejection of tiny droplets from the impacting droplet. On the basis of the definitions described above, the boiling regimes can be classified into contact, transition, and Leidenfrost for impact velocities ranging from about 0.5 to 4 m=s for both liquids.…”

“…In order to determine the Leidenfrost temperature T L and its dependence on the impact velocity U, phase diagrams have been experimentally produced for various impacting droplets with many combinations of substrates and liquids: water on smooth silicon [8], water on microstructured silicon [11], FC-72 on carbon nanofiber [12], water on aluminium [13], and ethanol on sapphire [14]. All of these phase diagrams show a weakly increasing behavior of T L with U.…”

“…At low impact velocities, the upper T L boundary is also lower than for ethanol, but it increases with U and reaches almost the same level as that for ethanol at U ≈ 4 m=s. Several models for the spreading diameter D max of an impacting drop have been developed [9,11,29,30]. For the so-called pancake model [11], D max and D 0 =U are considered to be the relevant length and time scales [11].…”

Contactless prompt tumbling rebound of drops from a sublimating slope

“…These include splash [6][7][8][9][10], phase-change-induced surface levitation [11][12][13][14][15], skating on a film of trapped air [16][17][18], and rebounding [19][20][21][22]. Recently [11], it was demonstrated that drops can rebound after impact on an extremely cold solid carbon dioxide surface (at -79°C, well below the limit of even homogeneous nucleation of water), because of the formation of a sublimated vapor layer acting both as impact cushion and thermal insulator, enabling drops to hover and rebound without freezing.…”

Kelvin–Helmholtz instability in an ultrathin air film causes drop splashing on smooth surfaces

“…Splashing is crucial in many important fields, such as the sprinkler irrigation and pesticide application in agriculture, ink-jet printing and plasma spraying in printing and coating industries, and spray cooling in various cooling systems; therefore its better understanding and effective control may make a far-reaching impact on our daily life. Starting in the 19th century, extensive studies on drop impact and splashing have covered a wide range of control parameters, including the impact velocity, drop size, surface tension, viscosity, and substrate properties (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12), and various splashing criteria have been proposed and debated (13)(14)(15)(16)(17)(18). Nevertheless, at the most fundamental level the generation mechanism of splashing remains a big mystery.…”