Creating the new cool
Professor Zuankai Wang from The Hong Kong Polytechnic University (PolyU) and a Croucher Senior Research Fellow, has cracked the challenge posed by the Leidenfrost effect, a problem which had eluded resolution since its formal description in 1756.
What is the Leidenfrost effect?
The effect refers to the levitation of drops on a surface that is significantly hotter than the liquid’s boiling point. The insulating vapor layer produced from the evaporating liquid dramatically reduces heat transfer performances at high temperatures, which makes thermal cooling on the hot surface ineffective. This explains, for example, why people can, in certain circumstances, handle extremely hot materials without harm.
The effect was actually first recorded by a scientist in 1732, by Herman Boerhaave (1668–1738), a highly distinguished Dutch physician who gained a worldwide reputation in his time, and is sometimes referred to as the “father of physiology”.
But it was Johann Gottlob Leidenfrost who wrote the first paper about it and lent his name to the phenomenon. Amazingly, Professor Wang is the 15th generation disciple of Herman Boerhaave. Many scientists have tried to crack it and failed. This is a science problem with true pedigree.
Why it matters
Thermal cooling is a key issue for many of the technologies that we currently rely on or hope to develop. However, the Leidenfrost effect is a limitation on the usefulness of water-cooling systems beyond a certain temperature. Once you suppress the effect, you can use water to cool systems at higher temperatures, as heat transfer via the water can continue to occur at those higher temperatures. “This was one of the issues in the problem with the Fukushima reactor in 2011. In fact, issues stemming from the limitation of thermal cooling are widespread,” Wang told us. “We see them not just with nuclear reactors but computers and mobile phones, for example.”
The magical origins of Professor Wang’s own Leidenfrost story
When Wang was 8 years old, an itinerant magician visited his village in mainland China. As part of his show, the magician walked on hot needles, having first poured water on them first; “I was very scared because he was walking barefoot,” recalled Wang. But the magician escaped injury. How could this be? “Much later, I realised this was the Leidenfrost effect,” Wang told us.
The solution - and the value of counter-intuitive thinking
The solution that Wang and his team were to find came slowly. By 2016, after years of work, they had “50% of a solution”. But yet years more were ahead of them.
Eventually, they hit upon the Structured Thermal Armor (STA), which comes in 3 parts - pillars, membrane (mesh) and vapour channel.
“At first we had only the pillars and the membrane”, said Wang. “It worked, but not reliably”. Counter-intuitively, they then used some thermally-insulating material for the membrane. “This is not where you’d normally look for a solution to thermal cooling”, explained Wang. So, thinking outside the box was crucial to cracking the problem.
“What you normally seek is excellent conductivity, not insulation. But this membrane helps to spread out the water and prevent droplets forming. This suppresses the creation of the insulating Leidenfrost effect. The membrane has a very porous surface, which causes the water to disperse even at high temperatures. It kind of works as a sponge.”
The final breakthrough eventually came with the idea of adding the third element, the vapour channels. Crucially, Wang explained, “these allow the vapour to flow away - thus permitting the decoupling of water and vapour. Because of this, there’s no insulation effect from the vapour, no Leidenfrost effect. This is what allows us to achieve cooling at much higher temperatures than previously possible.”
What are the practical applications and when might we see them?
Wang thinks we’ll start to see practical applications soon, within 3-4 years. “Manufacture at scale and in a variety of shapes is already possible,” he said, adding, “we have several companies talking to us.”
He thinks STA will prove important in many fields. For example, they might help with solving the cooling problem in fusion reactors such as the one being built by ITER in Europe. The STA is also likely to have an application in the cooling of crucial modern-day technology such as batteries for electric vehicles and integrated computer chips.
The recipe for success
When asked why he and his team had been successful where so many others had failed, Wang cited “focus, persistence, the ability to think counter-intuitively and having a team of people with diverse skills and scientific backgrounds” as being key factors.
Finally, we wanted to know what advice Wang would give to other scientists struggling with similarly stubborn problems:
“First, define the problem very carefully. Second, focus on that problem, Third, keep going, even when you fail time after time. Finally, put together an interdisciplinary team that can look at the problem from many angles.”
To see Professor Wang's Croucher profile click here.