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Controlling the Leidenfrost effect

Turn on a skillet and let it heat up until it is well above the boiling point of water. Then sprinkle a teaspoon of water on the skillet and watch. Water droplets will bounce up, form spheres and scurry across the surface.

What you have just observed is an example of the Leidenfrost effect, named for Johann Gottlob Leidenfrost, an 18th-century German physician and scientist. The phenomenon occurs when a liquid, upon approaching an object that is much hotter than the liquid’s boiling point, produces a vapor which insulates the liquid from the surface of the object.

This repulsive force, say scientists, has two consequences. It prevents droplets of the liquid from making physical contact with the surface, causing them instead to hover over the surface. And it causes the droplets to boil off more slowly than they would on a surface with a lower temperature that is still above the liquid’s boiling point.

Researchers in Hong Kong and at Lehigh recently demonstrated that it is possible to exploit the Leidenfrost effect to control the direction and destination of liquid droplets on a surface and thus to cool it more efficiently. They achieved this by lithographically patterning a surface with microscale features that convert excess surface tension into a kinetic energy that propels droplets to “hot spots” on the surface.

The discovery, say Zuankai Wang of the City University of Hong Kong and Manoj Chaudhury, professor of chemical and biomolecular engineering at Lehigh, has the potential to improve technologies that involve microfluidics, heat transfer, heat exchange, micro-heat exchange, water management and thermal management.

Wang, Chaudhury and their colleagues reported their results in Nature Physics, a journal of Nature magazine, in an article titled “Directional transport of high-temperature Janus droplets mediated by structural topography.”

The article’s lead author is Jing Li, a Ph.D. candidate in the department of mechanical and biomedical engineering at City University. The article was also co-authored by Yahua Liu, Chonglei Hao and Minfei Li of the department of mechanical and biomedical engineering of the City University of Hong Kong, and by Youmin Hou of the department of mechanical and aerospace engineering of the Hong Kong University of Science and Technology.

Read the full story at the Lehigh University News Center.

-Kurt Pfitzer is the Editorial Services Manager in Office of Communications and Public Affairs with Lehigh University Media Relations.

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February 25, 2016

Manoj Chaudhury

Manoj Chaudhury, Franklin J. Howes Jr. Distinguished Professor, Chemical Engineering

Manoj Chaudhury

The image shows how droplets making impact on a silicon wafer are attracted to regions of the wafer that are topographically altered to have higher Leidenfrost points. Researchers say the Leidenfrost effect can be exploited to cool hot surfaces more efficiently. (Image courtesy of Zuankai Wang and Lehigh University)