A heat-seeking slingshot: Liquid droplets show ability to cool extremely hot surfaces !

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 University 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 of Lehigh, has the potential to improve technologies that involve microfluidics, heat transfer, heat exchange, micro-heat exchange, water management and thermal management.

"Many applications, such as power plant reactors, require the management and control of the movement of water droplets at very high temperatures," says Wang, an associate professor of mechanical and biomedical engineering at City University. "Typically, the cooling of extremely hot surfaces has been accomplished with spray cooling. You spray a lot of water droplets onto a surface and as they boil, they take away the heat.

"At a high temperature, however, this doesn't work because the Leidenfrost effect prevents the droplets from making sufficient contact with the surface to cool it. Thus it takes too long to cool a surface by boiling off water."

Wang, Chaudhury and their colleagues reported their results today (Feb. 1) 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.

Contrasting topographies

Scientists in the last 20 years have learned to control the movement of liquid droplets on a solid surface by breaking the wetting symmetry that results from the impact of a droplet on a surface. They have accomplished this by harnessing gradients of surface energy and by utilizing light, temperature, electric force and mechanical vibration.

Chaudhury, the Franklin J. Howes Jr. Distinguished Professor of Chemical and Biomolecular Engineering at Lehigh, for example, has published articles with his students in Science and Langmuir describing their successful efforts to direct the movement of water droplets on surfaces.

But scientists have not yet achieved this control on surfaces heated to Leidenfrost temperatures and above, or on surfaces with extremely hot local spots.