Researchers have discovered that the Leidenfrost point of water on hydrophobic surfaces shifts to about 130°C, a value that is higher than predicted, but lower than on hydrophilic materials. They also found that the drops of water on hydrophobic surfaces in a so-called cold Leidenfrost regime easily slide, even at extremely low tilt angles, because the drops only slightly adhere to the surface. What is more, the temperature-induced transition to Leidenfrost is continuous, which is the opposite to what happens on normal materials.
The researchers, led by Philippe Bourrianne and David Quéré of the Physique et Mécanique des Milieux Hétérogènes laboratory (UMR 7636 du CNRS, ESPCI, PSL Research University), obtained by studying water droplets evaporating off various water-repellent materials.
The Leidenfrost effect occurs when a liquid close to a surface that is significantly hotter than the liquid’s boiling pint produces an insulating vapour layer that keeps the liquid from boiling rapidly. The droplet levitates above the surface of the material rather than touching it, which results in a dramatic decrease in heat exchange and a frictionless liquid state. If the temperature of the surface lies between the boiling point and the Leidenfrost point (which is about 200°C for water), however, the water does boil.
Since the thermal properties of both liquid and solid affect the Leidenfrost temperature, choosing the right combination of both is thus important for applications. In some applications, it is preferable to generate the Leidenfrost levitation at a much lower energy cost than usual and recent studies have suggested that superhydrophobic textures may allow to generate the Leidenfrost point at the boiling point of water (100°C), a 100° drop – known as the cold Leidenfrost regime.
Read the research paper: The cold Leidenfrost regime. P. Bourrianne; C. Lv; D. Quéré. Science Advances 10.1126/sciadv.aaw0304