Superhydrophobic coating allows water to boil without bubbles
By Nick Gilbert
September 14, 2012
You know that thing that water does when it boils? The thing with the bubbles? Turns out, it doesn't really need to do that at all, with scientists finding a way to make boiling water a completely bubble-free zone. Researchers from Northwestern University, King Abdullah University of Science and Technology in Saudi Arabia and Melbourne University in Australia teamed up to prevent water from bubbling when it boils by using tiny spheres coated with a hydrophobic material.
The breakthrough is thanks to a little thing called the Leidenfrost effect. When a liquid (say, water) comes into contact with a surface above the Leidenfrost threshold temperature, instead of simply bubbling and evaporating completely away, an insulating layer of vapor is created that protects the majority of the liquid from the searing surface temperatures. Because this vapor forms a layer between the water and the heating surface, the effect also reduces drag and has previously been shown to reduce in-water resistance by up to 85 percent.
Until now, it's been scale that has prevented the effect being used more widely. While it's easy to demonstrate the effect with small droplets of water, trying to do it with a large amount of liquid in a container is much more difficult, as the Leidenfrost temperature is usually much higher than what your kitchen kettle can achieve. The bubbles that occur during boiling are actually a multitude of Leidenfrost layers, collapsing as quickly as they form.
This is where the research comes in. By coating the textured surface of small steel balls with a commercially available hydrophobic coating, essentially consisting of self-assembled nanoparticles, and then heating them to 400 degrees Celsius (752° F), the team was able to replicate the Leidenfrost effect when dropping the spheres into room temperature water. This allowed the the vapor to steadily disappear as the material cooled, with no explosive bubbling in sight. The peaks and valleys on the surface of the steel balls created by the hydrophobic coating helped to maintain the vapor layer at lower temperatures.
For comparison, the team also heated tiny spheres that, rather than water-repelling nanoparticles, had been coated with a water-loving coating to 700° C (1,292° F). Dropping this second collection of spheres into the water caused violent bubbling, the Leidenfrost layer collapsing almost instantly.
The breakthrough has potential applications in improving the efficiency of heat transfer, anti-frost technology, and reducing drag on aquatic devices or vehicles. The lack of an explosive boil-to-bubble transition would also improve safety in large industrial applications, such as the heating and cooling of metals and the water cooling used in nuclear power plants.
The team's findings are published in the journal Nature
Check out the short video below from Nature to see water boiling sans bubbles.
Sources: Northwestern University