video: 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. view more
Credit: Ivan U. Vakarelski of King Abdullah University of Science and Technology, Saudi Arabia
EVANSTON, Ill. --- Every cook knows that boiling water bubbles, right? New research from Northwestern University turns that notion on its head.
"We manipulated what has been known for a long, long time by using the right kind of texture and chemistry to prevent bubbling during boiling," said Neelesh A. Patankar, professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science and co-author of the study.
This discovery could help reduce damage to surfaces, prevent bubbling explosions and may someday be used to enhance heat transfer equipment, reduce drag on ships and lead to anti-frost technologies.
Published Sept. 13 in the journal Nature, the research outlines how a specially engineered coated surface can create a stable vapor cushion between the surface and a hot liquid and eliminate the bubbles that are created during boiling.
This phenomenon is based on the Leidenfrost effect. In 1756 the German scientist Johann Leidenfrost observed that water drops skittered on a sufficiently hot skillet, bouncing across the surface of the skillet on a vapor cushion or film of steam. The vapor film collapses as the surface falls below the Leidenfrost temperature. When the water droplet hits the surface of the skillet, at 100 degrees Celsius, boiling temperature, it bubbles.
To stabilize a Leidenfrost vapor film and prevent bubbling during boiling, Patankar collaborated with Ivan U. Vakarelski of King Abdullah University of Science and Technology, Saudi Arabia. Vakarelski led the experiments and Patankar provided the theory. The collaboration also included Derek Chan, professor of mathematics and statistics from the University of Melbourne in Australia.
In their experiments, the stabilization of the Leidenfrost vapor film was achieved by making the surface of tiny steel spheres very water-repellant. The spheres were sprayed with a commercially available hydrophobic coating --- essentially self-assembled nanoparticles --- combined with other water-hating chemicals to achieve the right amount of roughness and water repellency. At the correct length scale this coating created a surface texture full of tiny peaks and valleys.
When the steel spheres were heated to 400 degrees Celsius and dropped into room temperature water, water vapors formed in 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 the experiments, researchers completely avoided the bubbly phase of boiling.
To contrast, the team also coated tiny steel spheres with a water-loving coating, heated the objects to 700 degrees Celsius, dropped them into room temperature water and observed that the Leidenfrost vapor collapsed with a vigorous release of bubbles.
"This is a dramatic result and there are many applications in which a vapor-loving, water-hating surface is beneficial," Patankar said.
Journal
Nature