Method controls whether freezing droplets bounce off or stick

When freezing droplets impact a surface, they generally either stick to it or bounce away. Controlling this response is crucial to many applications, including 3-D printing, the spraying of some surface coatings, and the prevention of ice formation on structures such as airplane wings, wind turbines, or power lines.

Now, MIT researchers have found a surprising new twist to the mechanics involved when droplets come in contact with surfaces. While most research has focused on the hydrophobic properties of such surfaces, it turns out that their thermal properties are also crucially important -- and provide an unexpected opportunity to "tune" those surfaces to meet the exact needs of a given application. The new results are presented today in the journal Nature Physics, in a report by MIT associate professor of mechanical engineering Kripa Varanasi, former postdoc Jolet de Ruiter, and postdoc Dan Soto.

"We found something very interesting," Varanasi explains. His team was studying the properties of a liquid -- in this case, drops of molten metal -- freezing onto a surface. "We had two substrates that had similar wetting properties [the tendency to either spread out or bead up on a surface] but different thermal properties." According to conventional thinking, the way droplets acted on the two surfaces should have been similar, but instead it turned out to be dramatically different.

On silicon, which conducts heat very well, as most metals do, "the molten metal just fell off," Varanasi says. But on glass, which is a good thermal insulator, "the drops of metal stuck and were hard to remove."

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