New method can create water levitation at much lower temperature, has implications for cooling nuclear reactors

Sprinkle a few drops of water on a hot pan and if the pan is hot enough, the water will sizzle and the water droplets will appear to swirl and float, hovering above the surface.

The temperature at which this phenomenon, called the Leidenfrost effect, occurs is predictable, usually occurring 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 water levitation at a much lower temperature, and the results are published in Nature Physics.

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

The discovery has great potential in heat transfer applications such as cooling industrial machinery and surface fouling cleaning for heat exchangers. It can also help prevent damage and even catastrophe to nuclear machinery.

Currently, there are more than 90 operating nuclear reactors licensed in the US that power tens of millions of homes, anchor local communities and currently account for half of the nation’s clean energy electricity generation. It requires resources to stabilize and cool those reactors, and heat transfer is essential for normal operation.

The physics of hovering water

For three centuries, the Leidenfrost effect has been a well-known phenomenon among physicists that determines the temperature at which water droplets hover over a bed of their vapor. While it is widely documented to start at 230 degrees Celsius, Cheng and his team have pushed that limit much lower.

The effect occurs because there are two different states of water living together. If we could see the water at the droplet level, we would observe that not the entire droplet boils on the surface, only a part of it. The heat vaporizes the bottom, but the energy does not pass through the entire point. The liquid part above the vapor is receiving less energy because most of it is used to boil the bottom. This liquid part remains intact, and this is what we see floating in its vapor layer. This has been referred to since its discovery in the 18th century as the Leidenfrost effect, named after the German physician Johann Gottlob Leidenfrost.

This hot temperature is well above the 100 degree Celsius boiling point of water, because the heat must be high enough to immediately form a layer of steam. Too low, and the dots don’t hover. Too high and the heat will vaporize the entire drop.

New works on the surface

The traditional measurement of the Leidenfrost effect assumes that the heated surface is flat, which causes the heat to strike the water droplets uniformly. Working at 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 with micropillars.

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

The micropillars designed by Cheng’s team are 0.08 millimeters long, about the same width as a human hair. They are arranged in a regular pattern of 0.12 millimeters. A drop of water contains 100 or more of them. These tiny pillars are pressed into a drop of water, releasing heat inside the drop and causing it to boil faster.

Compared to the traditional view that the Leidenfrost effect turns on at 230 degrees Celsius, the string-like micropillars of the edges compress more heat into the water than a flat surface. This causes the microdroplets to fly and bounce off the surface within milliseconds at lower temperatures because the boiling rate can be controlled by changing the height of the pillars.

Lowering the limits 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.

Not only is it a new breakthrough in understanding the Leidenfrost effect, but it is a twist on previously imagined limits. A 2021 study from Emory University found that the properties of water actually caused the Leidenfrost effect to fail when the temperature of the hot surface drops to 140 degrees. Using the micropillars created by Cheng’s team, the effect is stable even 10 degrees below.

“We thought that micropillars would change the behavior of this well-known phenomenon, but our results challenged even our imaginations,” Cheng said. “Observed bubble-droplet interactions are a breakthrough for heat transfer in waves.”

The Leidenfrost effect is more than an intriguing phenomenon to observe, it is also a critical point in heat transfer. When water boils, it more efficiently removes heat from a surface. In applications such as car cooling, this means that adapting a hot surface with the textured approach presented by Cheng’s team dissipates heat more quickly, reducing the chance of damage caused when a car overheats .

“Our research can prevent disasters such as steam explosions, which pose significant threats to industrial heat transfer equipment,” Huang said. “Vapor explosions occur when vapor bubbles within a liquid expand rapidly due to the presence of an intense heat source nearby. An example of where this risk is particularly important is in nuclear power plants, where the surface structure of heat exchangers can affect the growth of vapor bubbles and potentially cause such explosions Through our theoretical exploration in the paper, we investigate how surface structure affects the growth pattern of vapor bubbles, providing valuable insights into the control and mitigation of the risk of steam explosions.

Another challenge addressed by the team is the dirt that liquids leave behind on rough surface textures, presenting challenges for self-cleaning. Under spray cleaning or rinsing conditions, neither conventional Leidenfrost nor cold drops at room temperature can completely eliminate particles deposited by surface roughness.

Using Cheng’s strategy, the generation of vapor bubbles is able to remove those particles from the surface roughness and suspend them in the droplet. This means that boiling bubbles can remove heat and impurities from the surface.