If you put ice on a hot metal plate you very quickly don’t have any ice anymore. However, scientists have discovered that before it disappears, the ice makes unexpected things occur at the boundary where hot and cold meet. This discovery could lead to better ways to temper metals and, should the worst occur, prevent a nuclear accident.
Sprinkle water droplets on an object far above water’s boiling point and, rather than quickly turning to vapor, they will dance around for a while, skating frictionlessly over the surface. Known since 1751 as the Leidenfrost effect, this behavior appears in forms as various as liquid nitrogen on room temperature surfaces, screeching hydrogels, and (arguably) firewalkers strolling across hot coals without coming to harm.
Virginia Tech’s Dr Jonathan Boreyko and then undergraduate student Daniel Cusumano were curious about Leidenfrost’s frosty limits – such as a solid block of ice replacing liquid water. In the journal Physical Review Fluids, they report findings that could be useful as well as surprising.
When the liquid in contact with a hot surface boils with vapor trapped beneath, the Leidenfrost effect is created, insulating the rest of the droplet since gas conducts heat far more poorly than liquid. Counterintuitively, it makes water drops last much longer on a very hot metal plate than one only slightly above 100º C (212º F), where the effect has yet to kick in.
In the classic Leidenfrost Effect the vapor is trapped by the curved drop causing the liquid to levitate. Imae Credit: Vystrix Nexoth at the English Wikipedia, CC BY-SA 3.0,
“There are so many papers out there about levitating liquid, we wanted to ask the question about levitating ice,” Boreyko said in a statement. “It started as a curiosity project. What drove our research was the question of whether or not it was possible to have a three-phase Leidenfrost effect with solid, liquid, and vapor.”
Cusumano filmed the melting of an ice cube on a 150º C (302º F) aluminum plate using slow-motion cameras, finding it does not levitate as a water drop does. Instead, as it melts the water swiftly boils.
When Cusumano continued raising the temperature, however, he found that even ice experiences a Leidenfrost effect – the temperature just has to be a lot higher. For an aluminum plate, temperatures around 550º C (1022º F) were required, although this will vary slightly with the metal used.
Graduate student Mojtaba Edalatpour took over the quest to explain the unexpected findings, attributing them to heat transfer within the meltwater layer. Where the water touches the plate its temperature is held at 100º C – anything hotter boils – but where it is in contact with the ice it must be close to 0º C.
“The temperature differential the ice is uniquely creating across the water layer has changed what happens in the water itself,” Boreyko said. “Now most of the heat from the hot plate has to go across the water to maintain that extreme differential. So only a tiny fraction of the energy can be used to produce vapor anymore.”
The tiny amount of vapor can’t form a cushioning layer. Ironically, because boiling is initially slowed, the ice melts faster, eventually leading to increased boiling.
Only at extreme temperatures does heat enter the water fast enough to boil it rather than transfer upwards.
The finding could add an extra twist to those who use the Leidenfrost effect in physics demonstrations, but also has practical implications. The Leidenfrost effect interferes with efforts to cool nuclear reactors after disasters like Fukushima. Ice coolants might circumvent that.
Metals tools become brittle if cooled too slowly after shaping, so swift quenching has been a challenge since the Iron Age. For millennia, people have created legends around swords stronger than ordinary steel – ice baths might be the secret to making them real.
