Could tiny water droplets lower your electric bill? (+video)
Researchers report that a water droplet acquires a charge when it 'jumps' from a water-repellant surface, a find that could make power plants more efficient.
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“To have the most efficient condensing surface, you want to remove the droplets as early as possible,” says Dr. Miljkovic.Skip to next paragraph
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But, in prototypes, this “jumping” design is not as efficient as engineers believe it could be. Some of the "jumping" droplets will just fall back to the condenser’s surface, recoating it and slowing the process down. This is either because gravity tugs them down, or because they get pulled back to the surface in a vapor drag.
“Both of these return mechanisms are detrimental to condensation performance,” says Miljkovic.
But a newly discovered component to the “jumping” process might allow scientists to eliminate this fall back. In an accidental find, the MIT team found that droplets don’t just spring from the surface – they also rebound from each other. In high-speed camera footage, the researchers saw that water globes ping away from each other mid-flight, much like two fleeing, disoriented crooks smacking into each other and then taking off in opposite directions.
Next, in experiments using a charged electrode, the researchers found that this mid-air rebounding happens because an electrical charge forms on the droplets as they flee the hydrophobic surface. This charge happens as the droplets on the metal surface naturally form a layer of paired positive and a layer of paired negative charges. But when two drops slip together just before “jumping,” the charge separates, leaving some charge on the water and the rest on the surface, says Miljkovic.
So, if a charge is applied to the condenser system, the water droplets can be electrically prevented from returning to the surface, he said.
“If you utilize the fact that these droplets are charged, you can now create an external electric field, which can attract the droplets away from the surface, and make sure they don’t return,” says Miljkovic.
At the moment, these are lab results, but the scientists say that they are confident that the charged "jumping" can be reproduced on a macro-scale suitable for commercial purposes — those possible applications include not just use in power plants, but also in de-icing technologies for airplanes and wind turbines.
"We emphasize the scalability of jumping droplet surfaces so that they can be easily implemented at large scales for competitive costs," says Daniel Preston, a graduate student at MIT and a co-author on the paper.
"We are also extensively testing the robustness of these surfaces to make sure they hold up over time," he said.
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