BU, MIT researchers find new way to repel droplets from surfaces

Engineers have long been interested in creating surfaces that can repel water—materials so waterproof that droplets bounce right off. Such technology could help prevent icing on airplane wings, make steam turbines more efficient, and be used in a range of industrial applications.

The basic principle is one that can be observed in a simple dinner table experiment. Use a match to heat up a portion of a knife, creating a blackened spot where soot is deposited. Then, drip water onto the knife. A drop of water on the unblackened part will simply sit on the blade. But a drop will splash off the blackened bit, because the fine pattern of soot left behind has created a textured surface that causes the drop to bounce off.

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Scientists have used high-speed cameras and electron microscopes to learn the rules of droplet behavior at this scale. They have even figured out how to minimize the amount of time a droplet is in contact with a surface, making the bounce as short as possible. But that research has also revealed a limit—some amount of time that a droplet would have to be in contact with a surface as it splashes down, spreads out, and then recoils.

A team from the Massachusetts Institute of Technology and Boston University described a way to break that minimum bounce time in the journal Nature on Wednesday. Their innovation is startlingly simple—they create ridges on the surface that break a drop into two. That can reduce the amount of time the droplet touches the surface by more than a third.

The researchers have patented the technology and have already shown they can get the phenomenon to work on some metals used in industrial applications, such as copper and aluminum. They also showed that a similar property helps gossamer butterfly wings repel water and helps explain why nasturtium leaves are so waterproof. They hope that the insight could help in the design of materials for industrial purposes, where keeping a surface dry is paramount. But they also wonder if the simple insight into how surfaces repel water could help provide new insights into biology, too.

James C. Bird, an assistant professor of mechanical engineering at BU who began the work as a post-doctoral researcher in Kripa Varanasi’s laboratory at MIT, said that it was the kind of insight that seems extremely simple in hindsight. One of the comments he received from an outside scientist who reviewed the work compared the study’s simplicity and elegance to Columbus’ egg—a discovery that seems obvious once it’s been explained.

Bird looked up the story online and took it as a compliment. As the lore goes, after Christopher Columbus returned from America, with evidence the world was indeed round, people declared his discovery obvious. A frustrated Columbus is said to have then asked his critics to balance an egg on its end. They could not do it, and then Columbus showed them how—by lightly crushing the end of the egg and setting it upright on the table.

Critics are said to have protested that they didn’t know that they were allowed to break the shell. Columbus said that was the point.

“The point is now you know you can do it, it’s really obvious how to do it,” Bird said. “And yet you didn’t try that way.”