In a completely unexpected finding, MIT researchers have discovered that tiny water droplets that form on a super-hydrophobic surface, and then 'jump' away from that surface, carry an electric charge. The finding could lead to more efficient power plants and a new way of drawing power from the atmosphere, they claim.
Previous work by the MIT team showed that under certain conditions, rather than simply sliding down and separating from a surface due to gravity, droplets can actually leap away from it. This occurs when droplets of water condense onto a metal surface with a specific kind of super-hydrophobic coating and at least two of the droplets coalesce. They can then spontaneously jump from the surface, as a result of a release of excess surface energy.
"We found [in the new work] that when these droplets jump, through analysis of high-speed video, we saw that they repel one another mid-flight," post-doctoral researcher, Nenad Miljkovic says. "Previous studies have shown no such effect. When we first saw that, we were intrigued."
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. 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 (layer of paired positive and negative charges) on their surfaces. When neighbouring 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," adds Miljkovic.
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 a power plant's 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 vapour toward the surface.
This suggests that an external electric field might be used to mitigate any tendency of the droplets to return to the condenser, thus enhancing the heat transfer.
But the finding also suggests another possible new application. By placing two parallel metal plates out in the open, with one surface that has droplets jumping, and another that collects them, it may be possible to generate some power as a result of 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.
The team is currently working on a demonstration of this concept.