The Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces

Abstract

Droplets impacting on a superheated surface can either exhibit a contact boiling regime, in which they make direct contact with the surface and boil violently, or a film boiling regime, in which they remain separated from the surface by their own vapor. The transition from the contact to the film boiling regime depends not only on the temperature of the surface and the kinetic energy of the droplet, but also on the size of the structures fabricated on the surface. Here we experimentally show that surfaces covered with carbon-nanofibers delay the transition to film boiling to much higher temperatures compared to smooth surfaces. We present physical arguments showing that, because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, thus permitting contact boiling up to much higher temperatures than on smooth surfaces. We also show that as long as the impact is in the film boiling regime, the spreading factor of impacting droplets is consistent with the We(3/10) scaling (with We being the Weber number) as predicted for large We by a scaling analysis.

Dynamic Leidenfrost Effect: Relevant Time and Length Scales

“…However, this model, combined with a force balance on the levitating droplet base (shear stress of vapor ∼ capillary pressure), does not correctly describe the dependence of T L on U because it predicts a scaling T L − T b ∼ U −2 , which is neither consistent with the current experimental results [see Figs. 3(b) and 3(c)] nor with previous ones [8,[11][12][13].…”

“…Next to the impact velocity U, a key process that significantly affects T L is the cooling of the substrate due to its exposure to the cold liquid. T L thus strongly depends on the thermophysical properties of both the liquid and the substrate used [12,25,26]. For example, a gently deposited ethanol droplet can achieve the Leidenfrost state at T L;static ¼ 157°C on polished aluminum, whereas on pyrex glass a temperature as high as 360°C is required.…”

“…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.…”

“…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.…”

The dynamics of Leidenfrost drops

“…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 micro-structured silicon [50], FC-72 on carbon-nanofiber [67], water on aluminium [68], and ethanol on sapphire [69]. All these phase diagrams show a weakly increasing behaviour of T L with U .…”