New method can create aquatic levitation at much lower temperatures, impacting the cooling of nuclear reactors

Splash a few drops of water on a hot pan and when the pan is hot enough, the water will hiss and the water droplets will appear to roll and float, floating above the surface.

The temperature at which this phenomenon, called the Leidenfrost effect, occurs is predictable and usually occurs above 230 degrees Celsius. The team of Jiangtao Cheng, associate professor in the Virginia Tech Department of Mechanical Engineering, has discovered a method to create aquatic levitation at a much lower temperature, and the results have been published in Natural physics.

In addition to first author and Ph.D. student Wenge Huang, Cheng’s team collaborated with Oak Ridge National Lab and Dalian University of Technology for parts of the research.

The discovery has great potential in heat transfer applications such as the cooling of industrial machines and the cleaning of surface contamination for heat exchangers. It could also help prevent damage and even disasters to nuclear machines.

Today, there are more than 90 licensed nuclear reactors in the U.S., powering tens of millions of homes, anchoring local communities, and essentially producing half of the country’s clean energy electricity production. Resources are required to stabilize and cool these reactors, and heat transfer is crucial for normal operations.

The physics of floating water

For three centuries, the Leidenfrost effect has been a well-known phenomenon among physicists that determines the temperature at which water droplets float on a bed of their own vapor. While it’s commonly documented to start at 230 degrees Celsius, Cheng and his team set that limit much lower.

The effect occurs because two different water states live together. If we could see the water at droplet level, we would observe that not the entire droplet is boiling on the surface, but only part of it. The heat evaporates the bottom, but the energy does not travel through the entire drop. The liquid portion above the vapor receives less energy because much of it is used to cook the bottom. That liquid part remains intact, and this is what we see floating on its own vapor layer. Since its discovery in the 18th century, this has been called the Leidenfrost effect, named after the German physician Johann Gottlob Leidenfrost.

That hot temperature is well above the boiling point of 100 degrees Celsius, because the heat must be high enough to immediately form a vapor layer. Too low and the drops will not float. Too high, and the heat will vaporize the entire drop.

New work on the surface

The traditional measurement of the Leidenfrost effect assumes that the heated surface is flat, allowing heat to be transferred evenly to the water droplets. Working in the Virginia Tech Fluid Physics Lab, Cheng’s team has found a way to lower the starting point of the effect by producing a surface covered in micropillars.

“Like the papillae on a lotus leaf, micropillars do more than just decorate the surface,” says Cheng. “They give the surface new properties.”

The micropillars designed by Cheng’s team are 0.08 millimeters high, about the width of a human hair. They are arranged in a regular pattern with a mutual distance of 0.12 millimeters. A drop of water contains 100 or more. These small pillars press into a water droplet, releasing heat into the inside of the droplet and causing it to boil more quickly.

Compared to the traditional view that the Leidenfrost effect occurs at 230 degrees Celsius, the fin array-like micropillars push more heat into the water than a flat surface. This causes microdroplets to float and jump off the surface within milliseconds at lower temperatures, as the boiling rate can be controlled by changing the height of the pillars.

Lowering the borders of Leidenfrost

When the textured surface was heated, the team found that the temperature at which the floating effect was achieved was significantly lower than that of a flat surface, starting at 130 degrees Celsius.

This is not only a new discovery for understanding the Leidenfrost effect, it is a twist on the limits previously proposed. A 2021 study from Emory University found that the properties of water actually caused the Leidenfrost effect to fail when the temperature of the heated surface drops to 140 degrees. Using the micropillars created by Cheng’s team, the effect is durable even 10 degrees below.

“We thought the micropillars would change the behavior of this well-known phenomenon, but our results defied even our own imagination,” said Cheng. “The observed interactions between bubbles and droplets are a major discovery for boiling heat transfer.”

The Leidenfrost effect is more than an intriguing phenomenon to watch; it is also a crucial point in heat transfer. When water boils, it removes heat from a surface in the most efficient way possible. In applications such as machine cooling, this means that matching a hot surface to Cheng’s team’s texturing approach allows heat to dissipate more quickly, reducing the chance of damage if a machine gets too hot.

“Our research can prevent disasters such as vapor explosions, which pose a significant threat to industrial heat transfer equipment,” Huang said. “Vapor explosions occur when vapor bubbles in a liquid expand rapidly due to the presence of an intense heat source nearby. An example of where this risk is particularly relevant is in nuclear power stations, where the surface structure of heat exchangers can influence the growth of vapor bubbles and potentially cause such explosions. Through our theoretical exploration in the paper, we investigate how surface structure influences the growth mode of vapor bubbles, providing valuable insights into controlling and reducing the risk of vapor explosions.”

Another challenge the team faces is the impurities that liquids leave behind in the textures of rough surfaces, posing a challenge for self-cleaning. Under spray cleaning or rinsing conditions, neither conventional Leidenfrost nor cold drops at room temperature can completely remove particles deposited from the surface roughness.

Using Cheng’s strategy, generating vapor bubbles can release these particles from the surface roughness and allow them to hang in the droplet. This means that the boiling bubbles can remove both heat and impurities from the surface.