New Delhi: Can you imagine ice being used to boil water? Well, a researcher from Virginia Tech, a research university in Virginia, United States, has just done that.
Montaba Edalatpour, a graduate fellow, along with Jonathan Boreyko, Associate Professor at Virginia Tech, has made a discovery about the properties of water that could add to a phenomenon established over 200 years ago, in the 18th century.
The study was recently published in the journal, Physical Review Fluids.
The findings hold interesting possibilities for cooling devices and processes in industrial applications using only the basic properties of water, the study said.
There are three phases in which water can exist: a frozen solid, a liquid, and a gas. Frozen solid becomes a liquid when heat is applied to it. On applying heat to the liquid, it becomes vapour.
However, water’s behaviour changes dramatically when the heat source is enough. For instance, a water droplet deposited onto an aluminium plate heated to 150 degrees Celsius or above will no longer boil, according to Boreyko. Instead, the vapour formed when the droplet approaches the surface will become trapped beneath the droplet. This will create a cushion that prevents the liquid from making direct contact with the surface, the study said.
What Is The Leidenfrost Effect?
Due to the trapped water, the liquid is levitated, and slides around the heated surface. This phenomenon is known as the Leidenfrost effect, which occurs when a liquid comes into contact with a solid that is at a temperature well above the liquid’s boiling point. A layer of vapour forms between the liquid-solid interface upon contact, and creates a barrier between the two.
Also known as film boiling, the Leidenfrost effect is named after the German doctor and theologian Johann Gottlob Leidenfrost, who first described it in a 1751 publication.
The Leidenfrost effect is a commonly accepted scientific principle, and applies to water as a liquid, floating in a bed of vapour. Boreyko’s team had a question in mind whether ice could perform in the same way, a statement issued by Virginia Tech said.
The Leidenfrost effect has been a two-phase phenomenon to date, the study said. It is either an evaporating liquid or a sublimating solid that levitates on its vapour.
How Ice Exhibits A Three-Phase Leidenfrost Effect
The researchers demonstrated that an ice disk placed on a sufficiently hot surface exhibits a three-phase Leidenfrost effect, where both liquid and vapour films emanate from under the levitating ice, the study said.
The critical Leidenfrost effect was about 400 degrees Celsius hotter for ice than for a water drop, the authors noted in the study.
Therefore, the effective heat flux is larger when quenching aluminium with ice rather than water over a wide temperature range of 150 to 550 degrees Celsius.
Boreyko said that there are many papers out there about levitating liquid, but the team of researchers wanted to ask the question about levitating ice, according to the statement.
He added that the question of whether or not it was possible to have a three-phase Leidenfrost effect with solid, liquid, and vapour was what drove their research.
Delving Deep Into Ice
Five years ago, Daniel Cusumano, a researcher at Virginia Tech, made a fascinating discovery. He observed that ice did not levitate on vapour as liquid does, even when the aluminium was heated above 150 degrees Celsius, according to the statement.
Cusumano continued raising the temperature, and observed the behaviour of the ice as the heat increased. He found the threshold for levitation to be dramatically higher, that is 550 degrees Celsius, rather than 150 degrees Celsius.
The meltwater beneath the ice, up until that threshold, continued to boil in direct contact with the surface, rather than exhibit the Leidenfrost effect, the statement said.
What Prolonged The Boiling & What Was Going On Beneath The Ice?
Edalatpour picked back up the project a short time later, to solve the mystery. He had been working with Boreyko to develop novel methods of heat transfer and used that knowledge to work in approaching the ice problem.
Edalatpour found that the temperature differential in the meltwater beneath the ice was what prolonged the boiling, the study said.
The meltwater layer has two different extremes. Its bottom is boiling, which fixes the temperature at about 100 degrees Celsius. However, its top is adhered to the remaining ice, which fixes it at about 0 degrees Celsius.
The maintenance of this extreme temperature differential consumes most of the surface’s heat, Edalatpour’s analytical model revealed.
This explained why levitation was more difficult for ice. The mechanism for the delayed boiling is that the majority of the surface’s heat is conducted across the levitating meltwater film due to its 100 degrees Celsius differential. This left little heat for evaporation, the study said.
Boreyko elaborated that the temperature differential the ice is uniquely creating across the water layer has changed what happens in the water itself, according to the statement. This is because most of the heat from the hot plate has to now go across the water to maintain that extreme differential, he explained. Therefore, only a tiny fraction of the energy can be used to produce vapour anymore.
The study said that the elevated temperature of 550 degrees Celsius for the icy Leidenfrost effect is practically important.
The difficulty in levitating ice is actually a good thing, because the larger temperature window for boiling will result in better heat transfer, compared to when a liquid is used alone, according to the study.
Quoting Boreyko, the statement said that it is much harder to levitate ice than it is to levitate the water droplet. He explained that heat transfer plummets as soon as levitation begins, because when liquid levitates, it does not boil anymore.
He said that the liquid is floating over the surface rather than touching, and touching is what causes the liquid to boil away.
Boreyko said that levitation is terrible for heat transfer, while boiling is incredible.
Ice Can Be Used For Heat Transfer Applications In Nuclear Power Plants, Metallurgy, Firefighting
Heat transfer plays an important role in cooking off things like computer services or car engines. Heat transfer requires a substance or mechanism that can move energy away from a hot surface.
Ice is used in nuclear power plants to induce rapid cooling. This could become an easily-deployed emergency measure of power fails, or a regular practice for servicing power plant parts, according to the study.
Also, ice can be used for heat transfer applications in metallurgy. The application of ice would allow the heat from metals used for the production of alloys to be offloaded rapidly through the three water phases. This would quickly cool the metal, according to the study.
Boreyko said that ice could also be used for applications in firefighting.
He proposed a specially made hose to spray ice chips, instead of a jet of water.
Boreyko said that this is not science fiction. He added that he had visited an aerospace company that has an icing tunnel and they already have technology where a nozzle sprays ice particles as opposed to water droplets, according to the statement.
The researchers are excited about the new contribution that has come to the science world, with a myriad of possibilities.