The activities of bantam water droplets in just one region of a power plant could make a significant difference in the output of power plants, scientists say.
A team of MIT researchers report this week that water droplets that “jump” from a water-repelling surface acquire a charge that prevents them from returning to that surface. The find makes an important leap in a burgeoning effort to harness “jumping” water droplets in making power plants more efficient.
“There has generally been a lot of buzz in this area of research,” says Nenad Miljkovic, a postdoctoral associate at MIT and a co-author on the paper, published in Nature Communications.
“The concept of jumping droplets is not new, and we have been working on it and publishing papers for a while now,” he said, referring to an earlier paper from his team, published this winter in Nano Letters. “However, none of the previous studies (including ours) had any idea that the droplets were charged.”
A superhydrophobe is a kind of hydrophobe, a molecule with water-repellant properties. Hydrophobic molecules include fats and oils — hence the adage, “oil and water don’t mix” and the apparent fruitlessness of trying to get a homogenous salad dressing out of oil and vinegar. But a superhydrophobe is "super" because, unlike just a hydrophobe, it has a rough either micro or nanoscale structure.
When a water droplet forms on a sheet of metal coated with a superhydrophobe, the droplet can camp there only so long as it does not merge with another droplet. As soon as it weds with another droplet, the energy produced is so great that the two will “jump” away from that surface, as if in urgent deference to the surface’s severe water phobia.
Scientists have proposed that this “jumping” could be incorporated into power plant design. In power plants, steam is produced in a boiler and is then turned into mechanical energy in a turbine. Excess steam from the turbine is next converted into water in a condenser and sent back to the boiler for reuse. Right now, in current condenser designs, water congeals in a thin film on the condenser’s surface. Before new water droplets can form there, this water must fall away from the surface and be conveyed back over the boiler. This can take time.
So, if the new water droplets could be quickly removed from the condenser’s surface, making room for new droplets, the condensation process would be much more efficient, scientists say. To do so, condensers could be fitted with superhydrophobic surfaces that encourage water forming there to make an urgent departure, they say.
“To have the most efficient condensing surface, you want to remove the droplets as early as possible,” says Dr. Miljkovic.
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.