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Abstract:
When a liquid drop impacts on a heated substrate, it can remain deposited, or violently boil in contact,
or lift off with or without ever touching the surface. The latter is known as the Leidenfrost effect. The
duration and area of the liquid–substrate contact are highly relevant for the heat transfer, as well as
other effects such as corrosion. However, most experimental studies rely on side view imaging to determine contact times, and those are often mixed with the time until the drop lifts off from the substrate.
Here, we develop and validate a reliable method of contact time determination using high-speed X-ray
imaging and total internal reflection imaging. We exemplarily compare contact and lift-off times on flat
silicon and sapphire substrates. We show that drops can rebound even without formation of a complete
vapor layer, with a wide range of lift-off times. On sapphire, we find a local minimum of lift-off times
that is much shorter than expected from capillary rebound in the comparatively low-temperature
regime of transition boiling/thermal atomization. We elucidate the underlying mechanism related to
spontaneous rupture of the lamella and receding of the contact area.
