The Leidenfrost transition of water droplets impinging onto a superheated surface

•Experiments are conducted on water droplets impinging on a sapphire surface heated up to 700 ∘C.•To characterize heat transfer, temperature of the solid surface is measured by IR thermography, and droplet temperature is determined by two-color laser-induced fluorescence imaging.•The dynamic LFP (Leidenfrost point) corresponds to the initial wall temperature for which the solid surface is cooled down to the temperature of the spinodal during the impact process.•Above the LFP, fingering boiling is observed while the liquid at the interface with the vapor film remains in a very high level of superheating.•The superheat of the liquid is an essential parameter for the modelling of heat transfer between liquid and solid surface in the film boiling regime. Water droplets impinge on a sapphire wall heated to a temperature ranging from 300∘C to 700∘C. Advanced measurement techniques are used to characterize the thermal processes associated with the drop impact. IR thermography, implemented by coating the impacted surface with an opaque and emissive material in the IR domain, makes it possible to measure the temperature of the solid surface during the impact process. Laser-induced fluorescence imaging is used to characterize the temperature field in the spreading droplet. At the onset of film boiling, the temperature distribution on the solid surface is marked by the formation of a fingering pattern. This latter corresponds to spatial fluctuations in the thickness of the vapor film. When a water droplet hits an overheated wall with a significant impact velocity, the thermal contact is so rapid and intense that the liquid temperature can largely overtake the saturation temperature and reach the spinodal temperature, i.e. the highest temperature at which water can exist in the liquid state. In this situation, experiments show that the dynamic Leidenfrost point is directly linked to the spinodal temperature. A superheating of the liquid by several hundred of ∘C and the subsequent homogeneous nucleation, have to be considered to describe the heat transfer in the film boiling regime.