Drop impact on superheated surfaces

At 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 surfaces (``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 $\gamma$ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number \We (\gamma\sim\We^{2/5}) -- regardless of surface temperature and of liquid properties -- which is much steeper than for the impact on non-heated (hydrophilic or hydrophobic) surfaces (\gamma\sim\We^{1/4}). We also intereferometrically measure the vapor thickness under the droplet

Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces

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.Comment: 10 pages, 6 figure