Massachusetts Institute of Technology researchers have found that small water droplets that develop on a superhydrophobic surface (surfaces that are extremely difficult to wet), and then “jump” away from the surface, carry an electric charge. The discovery could reportedly turbocharge power plants, and lead to a new method of obtaining power from the atmosphere.

Nenad Miljkovic, a postdoc at MIT, said that this discovery was an addition to earlier work performed by the MIT researchers. Their work revealed that under specific conditions, instead of sliding down and separating from a superhydrophobic surface due to gravity, water droplets can actually jump away from it. This occurs when droplets condense onto a metal surface with a certain kind of superhydrophobic coating and two or more of the droplets mix: they are then ready to jump from the surface, because of the discharge of excess surface energy.

“We found that when these droplets jump, through analysis of high-speed video, we saw that they repel one another midflight,” Miljkovic professed. “Previous studies have shown no such effect. When we first saw that, we were intrigued.”

To determine the reason for the repulsion between jumping water droplets after they leave the surface, the researchers tried several tests utilizing a charge electrode. As they expected, the researchers discovered that when the electrode had a positive charge, droplets were repelled by it as well as by each other; when it had a negative charge, the droplets were attracted to it. This revealed that the effect was the result of a net positive electrical charge developing on the drops as they jumped away from the surface.

Why do the droplets become charged? As droplets develop on a surface they easily form an electric double layer — a layer of paired positive and negative charges — on their surfaces. When nearby droplets coalesce, which results in their jumping from the surface, that procedure takes place so rapidly that the charge separates. However, some of the charge remains on the droplet, while the rest stays on the surface.

The discovery that droplets could jump from a condenser surface offered a process for improving the efficiency of heat transfer on those condensers, and thus improving power plants’ overall efficiency.

The results are discussed in greater detail in the journal Nature Communications.

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