This deceptively simple animation reveals a surprising new discovery out of MIT. Researchers say it could lead to more efficient power plants and new techniques for energy-harvesting.
Scientists have known for some time that when water droplets form on a highly water-repellant surface, they tend to catapult from that surface, propelled by excess energy. Now, in an unexpected twist, a team of researchers led by MIT engineer Nenad Miljkovic has discovered that when these droplets jump, they also become positively charged, causing them to veer away from one another once they've left their shared surface.
"We found that when these droplets jump, through analysis of high-speed video, we saw that they repel one another midflight,” said Miljkovic in a statement. “Previous studies have shown no such effect. When we first saw that, we were intrigued.”
In a series of experiments recounted in the latest issue of Nature Communications, Mikjovic's team generated electrical fields to verify that the droplets were, in fact, gaining a net positive charge and to make sense of how those charges accumulated in the first place. Using an electrode, the researchers verified that the same basic electrostatic phenomena that causes the droplets to avoid each other mid-flight (i.e. "like repels like") causes them to interact repulsively with a positively charged copper wire electrode. As is clear from their observations, these droplets wanted nothing to do with the electrode.:
Conversely, the GIF at the top of this post (which is sampled from the following video) illustrates the attractive interaction that was observed when the electrode was negatively charged (i.e. opposites attract):
As expected, increasing the voltage difference between the hydrophobic surface and the electrode made for stronger repulsive or attractive droplet-electrode interactions.
The researchers ultimately determined that the charging process begins when two neighboring droplets, still attached to some hydrophobic material, form a layer of paired positive and negative charges on their surfaces. When the neighboring droplets coalesce, it causes the newly formed drop to leap from the surface so quickly "that the charge separates," says Miljkovic. “It leaves a bit of charge on the droplet, and the rest on the surface.”
But so what does all of this have to do with the efficiency of power plants?
In a nutshell: power plants rely heavily on heat-exchanging apparatuses known as surface condensers. The condenser's main job is to convert steam produced by the operation of the plant into water, which is then shed and collected, often by a sump (labeled in the diagram below as the "hotwell"):
A condenser that sheds water droplets more quickly transfers heat more efficiently, and a more efficient condenser = a more efficient power plant. A superhydropohobic condenser can produce jumping droplets, thereby improving its water-shedding abilities; by running a negative charge through a nearby plate, engineers could conceivably guide jumping droplets away from the condenser, upping its efficiency all the more.
“Now we can use an external electric field to mitigate” any tendency of water droplets to return to the condenser, says Milkjovic, “and enhance the heat transfer.”
He says his team's findings could also allow for a new way of harvesting energy from the atmosphere. "You just need a cold surface in a moist environment," Miljkovic says. More specifically, he says you need two metal plates, “one surface that has droplets jumping, and another that collects them." By keeping the condenser surface cool – perhaps with water from a nearby lake – he says you "could generate some power" from naturally occurring condensation.
“We’re working on demonstrating this concept,” sayd Miljkovic.
The researchers' findings are published in the latest issue of Nature Communications.