The Leidenfrost transition of liquid droplets impinging onto a superheated surface

Abstract

Heat transfer during the impact of a droplet on a sapphire substrate is investigated by means of infrared thermography. For that, the sapphire is coated with a very thin layer of TiAlN having a high emissivity in the IR domain. Spatially and time resolved measurements of the temperature at the front face of the solid wall are obtained. Results obtained for water droplets show that the dynamic Leidenfrost point (LFP) is close to 450°C and coincides with the onset of a fingering pattern. Approaching the dynamic LFP, despite the cooling by the droplet, the wall surface temperature never decreases below 310°C which is about the temperature of the spinodal for water, i.e. the maximum temperature at which water can still exist in the liquid state. Considering that a wetting contact is taking place below the dynamic LFP, wall temperature measurements demonstrate that the drop impact comes with a very strong superheating of the liquid. The liquid touching the wall is heated up to the spinodal temperature. Based on the idea that the dynamic LFP could correspond to the wall temperature, for which the contact temperature at a solid/liquid interface is equal to the spinodal temperature, a model is proposed for the dynamic LFP. This model considers the thermal effusivities of the liquid and the wall, as well as the liquid flow in the spreading lamella.