Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces

TL;DR

This study demonstrates that carbon-nanofiber surfaces significantly increase the Leidenfrost temperature for impacting droplets by delaying the transition to film boiling, due to their small-scale structure cooling vapor flow.

Contribution

It provides experimental evidence and physical explanation for how nanostructured surfaces alter boiling regimes and droplet impact behavior.

Findings

Carbon-nanofiber surfaces delay the transition to film boiling.

Spreading factor follows the same scaling law as smooth surfaces in film boiling.

Small-scale fibers are cooled by vapor flow, enabling higher Leidenfrost temperatures.

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 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 temperature 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 follows the same $\We^{3/10}$ scaling (with $\We$ the Weber number) found for smooth surfaces, which is caused by the vapor flow underneath the droplet.