&Cartridge; physics 14, p107
With water on hot surfaces, the Leidenfrost effect persists at temperatures much lower than what is required to set it on, regardless of surface or liquid properties.
Place a drop of water on a sufficiently hot surface and it will float on a cushion of water vapor. This “Leidenfrost effect” has been known since 1756, yet the reported values of the exact temperature at which the steam is formed vary widely. Now, a new study shows that the vapor layer in water can withstand temperatures well below those required for its formation, regardless of the salinity of the water, the volume of vapor or the type of material being heated  .
Leidenfrost experiments with actually floating droplets result in many difficult-to-control variables. So Dana Harvey and colleagues from Emory University in Georgia took a new approach: They immersed a heated metal cylinder with a rounded tip in a water bath and monitored the electrical impedance between the cylinder and an electrode at the bottom of the bath. When a layer of vapor formed under the tip, it introduced a capacitance that varied with the thickness of the vapor. By changing the temperature of the cylinder, the team was then able to locate the start of steam formation.
On average, a stable vapor layer was formed at around
, the exact temperature will vary depending on the type of metal used for the cylinder. But regardless of the type of metal or the salinity of the water, the steam persisted until the temperature rose. had fallen
causing an “explosive and audible” vapor collapse. This consistency suggests that the minimum temperature is determined by the stability of the gas flow within the vapor layer, rather than the properties of the water or the heated surface, the team says. What exactly triggers the collapse remains open.
Christopher Crockett is a freelance writer based in Arlington, Virginia.
- D. Harvey et al., “Minimum temperature Leidenfrost on smooth surfaces”, Phys. Rev. Lett.127, 104501 (2021).