Dynamic characteristics of thin vapor layers at substrate temperatures below the Leidenfrost point

Volatile drops can levitate on hot substrates by forming a thin vapor layer beneath them. The minimum temperature for stable vapor layer formation is called the Leidenfrost point, but the transition mechanism remains unclear. To investigate this, we studied the liquid–solid contact of Leidenfrost drops below the Leidenfrost point. Steady-state, pancake-like water drops were generated by sandwiching water between a hot substrate and a glass capillary, forming nearly flat vapor films while maintaining constant volume. Vapor film shapes (with thicknesses on the order of $10^0$ $\mu m$, confirmed experimentally and by a heat conduction model) were observed by interferometry. Although the films were generally stable for several minutes, microscopic perturbations consistently appeared and stochastic, sudden liquid–solid contacts, likely induced by van der Waals interactions, occurred at a peak of the perturbations. The selection rules for the wavelength and amplitude of the perturbation (measured as $\sim$300–400 $\mu m$ and $\sim$2 $\mu m$, respectively) were revealed by scaling analysis, derived from the balance between the vapor flow pressure drop and local Laplace pressure. Because the wavelength and amplitude were nearly insensitive to substrate temperature for a given drop radius, liquid–solid contact likely occurs when the vapor thickness approaches the perturbation amplitude. The liquid–solid contact is then driven by the natural resonance of the drops, which induces van der Waals interactions. Additionally, the effects of high (sapphire) and low (quartz) conductivity substrates were examined using an unsteady heat-conduction model, revealing that low-conductivity substrates exhibit larger local cooling, potentially promoting contact events.