Discovery of charged droplets could lead to more efficient power plants

That research showed that in certain environments, as opposed to sliding down and separating from a surface because of gravity, droplets can actually jump away from it.

Researchers from the Massachusetts Institute of Technology (MIT) have discovered that tiny water droplets that form on a superhydrophobic surface, and then “jump” away from that surface, have an electric charge. The finding could lead to more efficient power plants, as well as a new technique for harnessing power from the atmosphere.

MIT postdoc Nenad Miljkovic explained that this finding was a continuation of previous research led by the MIT team. That research showed that in certain environments, as opposed to sliding down and separating from a surface because of gravity, droplets can actually jump away from it. This happens when water droplets condense onto a metal surface with a particular type of superhydrophobic coating and at least two of the droplets combine: they are then primed to automatically jump from the surface because of the dispensation of an overabundance of surface energy.

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

To figure out the cause for the repulsion between jumping water droplets after they depart from the surface, the MIT team turned to several experiments involving a charged electrode. They found 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 showed that the effect was the consequence of a net positive electrical charge forming on the drops as they jumped away from the surface.

The charging procedure takes place due to the fact that as droplets form on a surface they easily develop an electric double layer — a layer of paired positive and negative charges — on their surfaces.  When nearby drops combine, which leads to their jumping from the surface, that process happens so quickly that the charge divides. According to Miljkovic, some of the charge stays on the droplet, and the rest remains on the surface.

The finding that droplets could jump from a condenser surface provided a procedure for enhancing the efficiency of heat transfer on those condensers, and thus enhancing power plants’ general efficiency.

The findings also suggests another new application: by placing two parallel metal plates out in the open, with one surface that has water droplets jumping, and another that collects them “you could [produce] power” just from condensation from the atmosphere.