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 $\gamma${} of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We ($\gamma${}$\sim${}We2/5), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces ($\gamma${}$\sim${}We1/4). We also interferometrically measure the vapor thickness under the droplet.

When a droplet impacts upon a surface heated above the liquid's boiling point, the droplet either comes into contact with the surface and boils immediately (contact boiling), or is supported by a developing vapor layer and bounces back (film boiling, or Leidenfrost state). We study the transition between these characteristic behaviors and how it is affected by parameters such as impact velocity, surface temperature, and controlled roughness (i.e., micro-structures fabricated on silicon surfaces). In the film boiling regime, we show that the residence time of droplets impacting upon the surface strongly depends on the drop size. We also show that the maximum spreading factor Γ of droplets in this regime displays a universal scaling behavior Γ ∼ We^3/10, which can be explained by taking into account the drag force of the vapor flow under the drop. This argument also leads to predictions for the scaling of film thickness and velocity of the vapor shooting out of the gap between the drop and the surface. In the contact boiling regime, we show that the structured surfaces induce the formation of vertical liquid jets during the spreading stage of impacting droplets.

At impact of a liquid drop on a solid surface, an air bubble can be entrapped. Here, we show that two competing effects minimize the (relative) size of this entrained air bubble: for large drop impact velocity and large droplets, the inertia of the liquid flattens the entrained bubble, whereas for small impact velocity and small droplets, capillary forces minimize the entrained bubble. However, we demonstrate experimentally, theoretically, and numerically that in between there is an optimum, leading to maximal air bubble entrapment. For a 1.8~mm diameter ethanol droplet, this optimum is achieved at an impact velocity of 0.25\,\,m/s. Our results have a strong bearing on various applications in printing technology, microelectronics, immersion lithography, diagnostics, or agriculture.