Surfaces that cool before you think: From MIT

MIT researchers have come up with a way to cool hot surfaces more effectively by keeping droplets from bouncing. 

When an earthquake and tsunami struck Japan's Fukushima nucleur power plant in 2001, knocking out everything on the way, crews tried to spray seawater on the reactors. But everything was in vein. One possible reason can be droplets not able to fall or land on surfaces that are hot. Because they instantly begin to evaporate, forming a thin layer of vapour and then bouncing along it, juts as they would in a hot cooking pan. 

But now, MIT has come up with a new solution. Their solution is, decorate the surface with tiny structures and then coat it with particles about 100 times smaller. Using that approach, they produced textured surfaces that could be heated to temperatures at least 100 degrees Celsius higher than smooth ones before droplets bounced. 

"Our new understanding of the physics involved can help people design textured surfaces for enhanced cooling in many types of systems, improving both safety and performance," says Kripa Varansai, the Doherty Associate Professor of Ocean Utilization in MIT's Department of Mechanical Engineering and the lead author of the study. Their research was to find a way to increase the temperature at which water droplets start bouncing. Past research indicated tough materials would add more surface area to hold onto droplets, making it harder to them to bounce. But the research team discovered that not just any rough surface will do. They found that installing microscale silicon posts on a silicon surface raised the temperature at which droplets transitioned from landing to bouncing. But it worked best when the posts were relatively diffuse. As the posts got closer together, the transition temperature gradually dropped until it was no higher than that of a smooth surface. 

"The result was surprising," says Bird, who is now assistant professor of mechanical engineering at Boston University. "Common knowledge suggests that the closely spaced posts would provide greater surface are, so would hold onto the droplets to a higher temperature." 

Upon further analysis the researchers concluded that closely spaced posts do provide more surface area to anchor the droplets, but they also keep the vapour that forms from flowing. Trapped by adjacent posts, the accumulating vapour layer under a droplet builds up pressure, pushing the droplet off. When the force of the vapour exceeds the attractive force of the surface, the droplet starts to float. "Bringing the posts closer together increases surface interactions, but it also increases resistance to the vapour leaving," says Varanasi. 

Experiments confirmed their approach. when they sprayed water on their micro-nano surfaces at 400 degree Celsius (the highest temperature their experimental setup could provide), the droplets quickly wet the surfaces and boiled. Interestingly under the same conditions the droplets did not wet the surfaces of samples with either the microscale posts or nanoscale texture, but did wet the surfaces of samples with both. 

In addition to the nuclear systems, this work has a lot of future applications for steam generators, industrial boilers, fire suppression etc. The research was supported by a Young Faculty Award from the Defence Advanced Research Projects Agency, the MIT Energy Initiative and the MIT-Deshpande Centre. 

Micrographs  showing water droplets landing on specially designed silicon surfaces at different temperatures.
At higher temperatures, the droplets begin to exhibit a new behaviour" instead of boiling, they bounce on a layer of vapour, never really wetting and cooling the surface. At 400 degree Celsius the droplet continues to boil only on the surface  that combines microscale posts with a coating of nanoscale particles (last column). These results demonstrate that this micro-nano surface can be effectively cooled even at high temperatures.