Charged droplet discovery will improve heat transfer efficiency 04 October 2013

In what's being described as an unexpected finding, MIT (Massachusetts Institute of Technology) researchers have discovered that water droplets that form on a superhydrophobic surface and 'jump' away, carry an electric charge.

The significance: this could lead to more efficient power plants and a new way of drawing power from the atmosphere, say the researchers.

MIT postdoc Nenad Miljkovic says the work – an extension of earlier research by the MIT team – initially showed that, rather than simply sliding down and separating from a surface, droplets can leap away from it.

This occurs when droplets of water condense onto a metal surface with a specific kind of superhydrophobic coating and at least two of the droplets coalesce: They can then spontaneously jump from the surface, as a result of a release of excess surface energy.

Most important, however, is the new finding that the jumping droplets repel one another midflight – now established to be due their carrying a net positive charge.

"The charging takes place because, as droplets form on a surface they naturally form a layer of paired positive and negative charges. When neighbouring drops coalesce, which leads to their jumping from the surface, [it] happens so fast that the charge separates," says Miljkovic. "It leaves a bit of charge on the droplet, and the rest on the surface."

And he explains that the new finding provides the potential to enhance power station efficiency even more than had initially been suspected by the mere jumping of uncharged droplets from condenser surfaces.

"By applying the appropriate charge to a nearby metal plate, jumping droplets can be pulled away from the surface, reducing the likelihood of their being pushed back onto the condenser," says Miljkovic.

"Now we can use an external electric field to mitigate" any tendency of the droplets to return to the condenser, "and so enhance the heat transfer," he explains.

But the finding also suggests another application, suggests Miljkovic. By placing two parallel metal plates out in the open, with "one surface that has droplets jumping, and another that collects them … you could generate some power" simply by condensation from the ambient air.

"You just need a cold surface in a moist environment," he says. "We're working on demonstrating this concept."

Brian Tinham

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