September 14, 2012
Leidenfrost Effect Inspires Researchers To Boil Water Without Bubbles
Watch the Video: Boiling Water Without Bubbles
April Flowers for redOrbit.com - Your Universe Online
How do you tell if water is boiling? You look for the bubbles, because every cook knows boiling water bubbles. But now, research from Northwestern University is turning 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.
This discovery has many practical applications, including reducing damage to surfaces, and preventing bubbling explosions. Future applications could include enhancing heat transfer equipment, reducing drag on ships and new anti-frost technologies.
The research, published in Nature, 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 new breakthrough is based on the Leidenfrost effect. Johann Leidenfrost, in 1756, noticed 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 collapses as the surface falls below the Leidenfrost temperature. If water hits 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. 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.
Stabilization of the Leidenfrost vapor film was achieved by making the surface of tiny steel spheres very water-repellant by spraying them with a commercially available hydrophobic coating — essentially self-assembled nanoparticles — combined with other water-hating chemicals to get 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, thereby 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.