When the heat is on, water travels uphill

Physicists at the University of Bath have been investigating a strange phenomenon that allows water droplets to levitate and even climb uphill.

When droplets of water on a heated surface reach a certain temperature, the droplet surface starts to boil rapidly allowing it to float or levitate on the evaporated gas vapour. This is known as the Leidenfrost effect and is commonly seen during cooking – when sprinkling water onto a hot pan which is above the Leidenfrost point, droplets skitter across the pan and take longer to evaporate.

The researchers, undergraduate physics students Alex Grounds and Richard Still, looked at how droplets travel on different textured surfaces, heated at varying temperatures.

Their research, published in prestigious Nature Group journal Scientific Reports, found that they could change the direction of the droplets’ movement by changing the temperature of the ratcheted surface.

They also found that droplets can be made to climb up a steep incline – the sharper the teeth of the surface, the steeper incline they were able to climb.

Alex Grounds, who is now studying for an MSc Innovation and Technology Management at Bath, said: “The Leidenfrost effect has been known for centuries, but we still don’t really completely understand the physics behind all the remarkable consequences.

“We think the droplets change direction depending on how fast the gas evaporates from the surface of the droplet and how much the droplet is levitating, combined with the effect of the textured surface that allows it to be propelled along and even go uphill.

“We found that the sharper the teeth on the surface, the steeper the droplets could climb, which we believe is due to more efficient heat transfer of the ratcheted surface.”

Dr Kei Takashina, a lecturer in Physics at Bath, supervised the project. He added: “This project has really captured the students’ imaginations. This year’s final year students are now continuing this work, trying to understand the complex physics surrounding this well-known phenomenon.

“In the future, this knowledge could be used to develop more sophisticated methods for controlling small droplets and heat transfer, for example designing cooling systems without moving parts.”

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