Drop Impact on Superheated Surfaces

Abstract

At the impact of a liquid droplet on a smooth surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surface ("contact boiling"), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back ("gentle film boiling"), or both forms the Leidenfrost layer and ejects tiny droplets upward ("spraying film boiling"). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading γ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We (γ~We(2/5)), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces (γ~We(1/4)). We also interferometrically measure the vapor thickness under the droplet.

Levitation of a drop over a moving surface

“…To measure the thickness of this air film, we use a color interferometry method, which was described in details by van der Veen et al (2012), and successfully applied to measure the air layer thickness under droplets impacting on either solid or liquid surfaces (van der Veen et al 2012;Tran et al 2012Tran et al , 2013. A color high-speed camera (SA2, Photron Inc.) is connected to a long working distance microscope and aims perpendicularly at the cylinder's wall, as shown in figure 1(a).…”

“…A typical image of this pattern is shown in figure 1(c). From a small strip across the fringes, we extract the absolute thickness of the air film (see van der Veen et al 2012) and reconstruct the whole 3D air film profile by following the fringes with known thickness. In figure 2(a) and (b), we show the 3D profile and the contour plot extracted from the interference pattern shown in figure 1(c).…”

Contactless prompt tumbling rebound of drops from a sublimating slope

“…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. A sublimating surface is different from aerodynamically assisted surface levitation [23][24][25] and from the Leidenfrost effect [12][13][14][26][27][28], in the sense that it is independent from liquid properties, such as boiling temperature, and there is no loss of drop mass due to its own boiling (as in the Leidenfrost phenomenon). Of course, in both cases an intervening layer is generated between the drop and the substrate.…”

“…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.…”

Hole growth dynamics in a two dimensional Leidenfrost droplet

“…15 In that case, the mechanism is rather different: the droplets spread upon impact while partial contacts with the surface enhance the local evaporation and trigger the formation and growth of a vapor bubble in the droplets' center.…”