I don’t profess to be a culinary genius by any means, but I do my fair share of cooking and the results are usually pretty damned tasty. My mother and Alton Brown have taught me well. And for all of you out there who also do a bit of cooking, you’re probably familiar with the Leidenfrost effect, though you probably don’t know it.
If you’ve ever tested the heat of a pan by sprinkling some water on top of it to see if it dances across the surface, then you’ve seen the effect in action. The extreme heat difference between the pan and the water droplet creates a thin layer of water vapor that insulates the rest of the water droplet from the heat. Consequently, the dancing droplet takes longer to evaporate than the same drop on a mildly hot pan would take.
While the Leidenfrost effect is useful tool in the kitchen, Northwestern University researchers are putting to use in other ways. An international team including Neelesh Patankar, professor of mechanical engineering at Northwestern’s McCormick School of Engineering and Applied Sciences, has developed a way to use this knowledge to stop water from bubbling while boiling.
When water reaches 212 degrees Fahrenheit, small bubbles of vapor begin to form and head towards the surface. When they first begin to form, they dissipate on the surface rather unimpressively because the surrounding atmospheric pressure is much greater than that of the vapor bubble. But as the temperature rises, the pressure inside the vapor bubbles rises, until at 212 degrees Fahrenheit, they begin to explode violently at the surface.
While the new study has nothing to do with the explosive surface bubbles in a rolling boil, I thought it was a good teaching moment. Instead, the team has discovered a way to stop the small vapor droplets from forming in the first place.
The key is to stabilizing the Leidenfrost effect at the surface of the heat source, insulating the rest of the liquid in the surround area. The team made the surface of tiny steel spheres extremely water-repellant by spraying them with commercially available hydrophobic (literally, afraid of water) coating combined with other water-fearing chemicals to achieve the right amount of roughness and repellency. When balanced correctly, the chemicals produced a surface texture full of tiny peaks and valleys.
Then, when the tiny steel spheres were heated to 400 degrees and dropped in water, vapors formed around the valleys of the textured surface, creating a stable Leidenfrost vapor film that did not collapse once the spheres cooled to the temperature of boiling water.
In contrast, steel spheres coated with water-loving chemicals also were heated and dropped into water. Once the heat of the sphere dissipated enough to lose the Leidenfrost vapor, it collapsed and a chorus of vapor bubbles was created.
Besides being kind of cool, the technique could be used to reduce damage to surfaces, prevent bubbling explosions, enhance heat transfer equipment, reduce drag on ships and produce anti-frost technologies.
Check out the video below.
This movie shows the cooling of 20 mm hydrophilic (left) and superhydrophobic (right) steel spheres in 100 C water. The spheres’ initial temperature is about 380 C. The bubbling phase of boiling is completely eliminated for steel spheres with superhydrophobic coating.
Credit: Ivan U. Vakarelski of King Abdullah University of Science and Technology, Saudi Arabia