Leidenfrost effect studied by video analysis

Eur. J. Phys. 40 (2019) 065101

M. Zeuner, K. Schwark, C. Hanisch, and M. Ziese

When liquid droplets touch a sufficiently hot surface the Leidenfrost effect leads to levitation of the droplets on a steam layer. At the same time the heat flux from hot surface to droplet is strongly reduced and the droplet lifetime is increased. The understanding of this effect, which can readily be observed on a standard stove, requires knowledge of fluid dynamics, basic mechanics and thermodynamics. It therefore combines appealing experiments with interdisciplinary physics and is well-suited for teaching physical principles, experimentation, modelling and data analysis in schools and in the early stages of university physics. The aim of this work is to present some guidance for projects in this area. Video analysis was used to quantitatively measure droplet sizes and lifetimes of water and liquid nitrogen droplets placed on a hot plate. A simple model for a levitating droplet was developed and compared to the data. The findings indicate that video analysis is a suitable way to examine the Leidenfrost effect for droplet diameters between about 3 and 10 mm. From the analysis the effective thermal conductivities of the steam layers were determined and were found to be two to three orders of magnitude smaller than the thermal conductivities of the corresponding gases at rest. By measuring several different droplets under varying parameters it was concluded that the precision of the experimental setup is sufficient to provide a practical tool for pupils and undergraduate students to explore this fascinating everyday phenomenon.