Abstract
Droplets can be levitated by their own vapour when placed onto a superheated plate (the Leidenfrost effect). It is less known that the Leidenfrost effect can likewise be observed over a liquid pool (superheated with respect to the drop), which is the study case here. Emphasis is placed on an asymptotic analysis in the limit of small evaporation numbers, which indeed proves to be a realistic one for millimetric-sized drops (i.e. where the radius of the drop is of the order of the capillary length). The global shapes are found to resemble ‘superhydrophobic drops’ that follow from the equilibrium between capillarity and gravity. However, the morphology of the thin vapour layer between the drop and the pool is very different from that of classical Leidenfrost drops over a flat rigid substrate, and exhibits different scaling laws. We determine analytical expressions for the vapour thickness as a function of temperature and material properties, which are confirmed by numerical solutions. Surprisingly, we show that deformability of the pool suppresses the chimney instability of Leidenfrost drops.
Highlights
A drop can be prevented from merging with a liquid bath when the bath is heated above the saturation temperature
Leidenfrost drops on a superheated liquid pool were studied in the limit of small evaporation numbers E, the latter proportional to the superheat T and determined by both the thermal and hydrodynamic properties of the system
The pool surface being deformed under the drop, the vapour gap was found to be of quite a different morphology as compared to that of Leidenfrost drops deposited on a superheated flat plate
Summary
A drop can be prevented from merging with a liquid bath when the bath is heated above the saturation temperature. Note that the case by Maquet et al (2016) is slightly different from the boule case by Hickman (1964b) in the following regard In the former (Leidenfrost) case, the evaporative heat flux is limited by heat conduction across the vapour gap from the superheated pool surface (non-volatile) to the drop surface being maintained at the saturation temperature, evaporation proceeding from the drop surface. A baseline consideration will explicitly be adapted to the former case (Maquet et al 2016), whereas the boule case will be only mimicked by choosing equal densities and surface tensions of the two liquids While it is just a numerical solution of the problem that was provided by Maquet et al (2016) in the theoretical part of their work, here we aim at a detailed exploration of the physics and structure of the phenomenon by means of asymptotic methods.
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