Hydrodynamic and thermal behavior of drops impacting hot surfaces near the Leidenfrost point

Film-to-nucleate boiling transition is paramount to droplet cooling applications. However, it is difficult to observe this phenomenon experimentally in detail due to the small temporal and spatial scales. Therefore, open questions still remain around the mechanisms of the transition in boiling regimes.

In this study, we employed a combination of temperature sensitive paint (TSP) and high-speed cameras, along with an infrared camera, to capture simultaneously the hydrodynamic and thermal behavior of drops impinging on heated surfaces near the Leidenfrost point. We used millimeter-sized drops of FC-72 as the test fluid and sapphire glass as the substrate.

So far, we have been successful in measuring drop impact behavior and accompanying heat transfer above and below the Leidenfrost point. This has enabled us to observe some interesting behavior of a drop which detached after a while with wavy motion. At the initial impact, we measured transient temperature distributions corresponding to nucleate boiling while no apparent thermal variation was visible once the vapor cushion was developed to hinder heat transfer. This was only observed in the presence of a TSP layer, which had a low thermal diffusivity, and not for a bare sapphire glass. Moreover, we found that even above the Leidenfrost point, the drop eventually came in contact with the substrate when it became sufficiently small. Further investigations are undergoing to elucidate the effects of thermal response of the substrate and drop size.