In a new — and rather unexpected — finding, researchers at MIT have discovered that, under certain conditions, water droplets can actually “jump” away from a surface, rather than simply slowly slide off of one via gravity. Specifically, when tiny water droplets condense and form on a superhydrophobic surface, it creates a situation where, if any two of these droplets coalesce, then the newly formed droplet actually spontaneously “leaps” away from the surface — the “leap” being the result of a release of excess surface energy, and as the result of a net positive electrical charge forming on the droplets as they jumped away from the surface.
The researchers think that this new discovery could be utilized to improve the efficiency of conventional power plants, and also to develop entirely new ways of harnessing energy from the atmosphere.
MIT explains the findings in more detail:
In order to understand the reason for the repulsion between jumping droplets after they leave the surface, the researchers performed a series of experiments using a charged electrode. Sure enough, 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 drawn toward it. This established that the effect was caused by a net positive electrical charge forming on the droplets as they jumped away from the surface.
The charging process takes place because as droplets form on a surface they naturally form an electric double layer — a layer of paired positive and negative charges — on their surfaces. When neighboring drops coalesce, which leads to their jumping from the surface, that process happens so fast that the charge separates. It leaves a bit of charge on the droplet, and the rest on the surface.
The initial finding that droplets could jump from a condenser surface — a component at the heart of most of the world’s electricity-generating power plants — provided a mechanism for enhancing the efficiency of heat transfer on those condensers, and thus improving power plants’ overall efficiency. The new finding now provides a way of enhancing that efficiency even more: 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 either by gravity or by the drag created by the flow of the surrounding vapor toward the surface.
“Now we can use an external electric field to mitigate” the tendency of the droplets to gravitate to the condenser, “and enhance the heat transfer,” states researcher and MIT postdoc Nenad Miljkovic.
Interesting findings, especially when you consider some of the possibilities… As Miljkovic writes: “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 just from condensation from the ambient air. All that would be needed is a way of keeping the condenser surface cool, such as water from a nearby lake or river. You just need a cold surface in a moist environment.” Miljkovic and team are now working on demonstrating this concept.
The new research was just published in the journal Nature Communications.