Abstract
By sufficiently heating a solid, a sessile drop can be prevented from contacting the surface by floating on its own vapour. While certain aspects of the dynamics of this so-called Leidenfrost effect are understood, it is still unclear why a minimum temperature (the Leidenfrost temperature TL) is required before the effect manifests itself, what properties affect this temperature, and what physical principles govern it. Here we investigate the dependence of the Leidenfrost temperature on the ambient conditions: first, by increasing (decreasing) the ambient pressure, we find an increase (decrease) in TL. We propose a rescaling of the temperature which allows us to collapse the curves for various organic liquids and water onto a single master curve, which yields a powerful tool to predict TL. Secondly, increasing the ambient temperature stabilizes meta-stable, levitating drops at increasingly lower temperatures below TL. This observation reveals the importance of thermal Marangoni flow in describing the Leidenfrost effect accurately. Our results shed new light on the mechanisms playing a role in the Leidenfrost effect and may help to eventually predict the Leidenfrost temperature and achieve complete understanding of the phenomenon, however, many questions still remain open.
Highlights
Once a drop is in the Leidenfrost state, for the vapour film thickness and profile, good agreement between observations and modelling is achieved.[5,15] The typical thickness h of the vapour layer scales with the superheat DT = (T À Tsat) as h p DT1/4, where T is the plate temperature and Tsat the saturation temperature.[5,6,7] the models predicting such scaling do not hold for vanishing superheat: no stableLeidenfrost drops are documented in literature for a plate temperature T - Tsat, whereas the models still predict a vapour layer in this limit e.g. in the order of micrometers for a superheat of DT = 1 K, much thicker than any long range forces
In contrast to the classical boiling curve for wetting drops, where a local maximum in the evaporation time is found at T = TL, here, a monotonic decrease in evaporation time was observed with increasing plate temperature
We have shown experimentally that the Leidenfrost phenomenon and the corresponding Leidenfrost temperature TL are influenced strongly by the environment
Summary
Once the vapour film of these unstable drops gets pierced they do not recover back to the Leidenfrost state. This regime was explored by utilizing superhydrophobic surfaces to prevent wetting.[20] In contrast to the classical boiling curve for wetting drops, where a local maximum in the evaporation time is found at T = TL, here, a monotonic decrease in evaporation time was observed with increasing plate temperature. We will find the existence of meta-stable Leidenfrost drops for smaller superheats when increasing the ambient temperature T0. We identify such metastable drops by the irreversibility towards the Leidenfrost state after disturbing them by vibration. The two sets of experiments provide new insight on the mechanism behind TL, which is of great importance for understanding and predicting the Leidenfrost effect
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