Dynamic Leidenfrost Effect: Relevant Time and Length Scales

Abstract

When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost temperature for droplet impact on an isothermal substrate. DOI: 10.1103/PhysRevLett.116.064501 Boiling and spreading of droplets impacting on hot substrates have been extensively studied since both phenomena strongly affect the heat transfer between the liquid and the solid. Applications include spray cooling [1], spray combustion [2], and others [3].At room temperature, an impacting droplet spreads on a solid surface and entraps a bubble under it [4][5][6]. At temperatures higher than the boiling temperature T b , vapor bubbles appear which disturb and finally rupture the free surface, resulting in the violent spattering of tiny droplets [7,8]. On even hotter surfaces, however, beyond the socalled Leidenfrost temperature T L , the droplet interface becomes smooth again without any bubbles inside it. In this regime the droplet lives much longer, as now it levitates on its own vapor layer: the well-known Leidenfrost effect [9,10].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. When theoretically deriving T L , one needs to determine the vapor thickness profile. In the case of a gently deposited droplet, this can be accomplished since the shape of the droplet is fixed except for the bottom surface, which reduces the problem to a lubrication flow of vapor in the gap between the substrate and the free surface [15][16][17][18][19][20]. For impacting droplets on an unheated surface at high Weber number We ≡ ρU 2 D 0 =σ (here, D 0 is the equivalent diameter of the droplet and ρ and σ are the density and the surface tension of the liquid, respectively), it is known that the neck around the dimple beneath the impacting droplet rams the surface. In this cold impact case, the neck propagates outwards like a wave [21]. For impact on a superheated s...

Levitation of Liquid Microdroplets Above A Solid Surface Subcooled to the Leidenfrost Temperature

“…Under the certain conditions, heat in these systems is removed due to intensive evaporation of liquid microdroplets. However, due to the Leidenfrost effect [4] the liquid microdroplets cannot reach the heating surface [5], and this may reduce the efficiency of apparatus cooling and cause its failure. The behavior of liquid microdroplets falling onto a solid surface, subcooled to the Leidenfrost temperature (about 200°C for water), is studied insufficiently.…”

The dynamics of Leidenfrost drops

“…In the case the drop has made contact with the plate, the contact line dynamics will be strongly affected by the hot plate, since strong evaporation is expected here [45]. The local heating of the contact line will induce Marangoni flows in the drop as a result of the temperature difference with the remainder of the drop, which is still at ambient temperature.…”

“…This causes a deceleration of the drop and the formation of a dimple in the drop before touchdown [30,39]. Touchdown of the drop on the substrate can be prevented completely in the case of a heated substrate, because the escaping air is replenished by the evaporation of the drop [8,36,45].…”

“…As a consequence, Kolinski, Rubinstein and co-workers [53][54][55] recently developed frustrated total internal reflection (FTIR) imaging for their study of thinner films. We have adopted this powerful technique [45,56,57]. Here, a transparent wall is illuminated at an angle larger than the critical one.…”

“…The energy loss of the light beam is thus a measure for the distance between the wall and the other medium. Further applications of this technique other than the film thickness measurement include particle image velocimetry in a near-wall flow [58], studies of moving contact lines and wetting/drying behaviour of heated surfaces in nucleate boiling studies [13,45,56,57].…”