A new superhydrophobic coating may help suppress explosive boiling reactions like those that can occur in nuclear reactors and other equipment exposed to water at high heat.
|As a hot 20mm steel ball cools in water, a vapor film that envelops the ball collapses, causing a transition from a gentle type of boiling (left) to an explosive one.|
The research, by an international team of scientists, focuses on controlling the reaction that causes water droplets to skitter across a hot frying pan.
Known as the Leidenfrost effect, the reaction “can be big trouble for equipment exposed to water at high temperatures, such as nuclear reactors, because sometimes the transition away from it leads to explosive boiling,” the researchers explain in a video demonstration of their research.
The team is led by Ivan U. Vakarelski, of Saudi Arabia’s King Abdullah University of Science & Technology, and Neelesh A. Patankar, of Northwestern University. Their work is published in Nature, DOI: 10.1038/nature11418.
Steel and Side Mirrors
The team started with steel spheres, then applied a patented coating that is normally used to keep rain from accumulating on car side mirrors. The textured coating renders the spheres super-water-repellent (superhydrophobic) and leads to smooth, nearly bubble-less boiling.
The researchers used high-speed photography to monitor the processes of the steel balls treated with a variety of coatings.
In one case, they found that immersing 20mm hydrophilic balls heated to more than 400°C in water caused the water to boil at the steel surface gently in a nearly bubble-free manner known as film boiling. As the surface temperature cooled to 275°C, the protective vapor layer collapsed, leading to an explosive transition to the “nucleate boiling” regime.
The textured superhydrophobic coating completely suppressed the violent transition, they report.
Watch the video.