MIT researchers have discovered that water droplets ejected from a hydrophobic surface carry an electric charge. The finding is unexpected and may lead to more efficient power plants, as well as entirely new methods of generating power from the atmosphere.
MIT postdoc Nenad Miljkovic and mechanical engineering professor Evelyn Wang have presented their new finding in the paper Nature Communications. When water comes in contact with most hydrophobic (water repelling) substances, it will simply slide off again, but according to the new research, certain super hydrophobic coatings, when applied to a metal surface, can have a different effect. When two tiny water droplets coalesce on the surface, they will literally leap into the air. The leap comes as a result of the two droplets combining: When they merge, the surface area becomes smaller and the energy needed for surface tension is reduced as well. The excess energy is released as kinetic energy and the droplets jump. However, when the research team looked closer, they discovered another curious phenomenon was occurring as well.
What the team found when studying the droplets on a high-speed camera, was that they repelled each other mid-jump. “We found that when these droplets jump, through analysis of high-speed video, we saw that they repel one another midflight,” Miljkovic says. “Previous studies have shown no such effect. When we first saw that, we were intrigued.” To investigate the repulsion effect, the team introduced a positively charged electrode. The droplets were repelled by the electrode as well, and that led to the conclusion that the droplets had a positive electrical charge. The team’s conclusion is that the hydrophobic coating when interacting with the droplets, creates a layer of paired negative and positive charges. When the droplets merge and jump from the surface, it occurs “so fast that the charge separates. It leaves a bit of charge on the droplet, and the rest on the surface,” explains Miljkovic.
An easy way to tell that a surface is at least somewhat hydrophobic, is that water tends to bead up on it.
Practically, there are a few promising applications for the discovery. First, droplets could be made to jump from a condenser’s surface. Condensers are at the heart of many power plants, and the research team believes their efficiency could be increased quite a bit with the new discovery, which would obviously also increase the efficiency of the plant itself. An equally exciting prospect however, could be a new type of power plant altogether. By placing two metal plates near each other, one to send off droplets and another to collect them, you could effectively generate electricity. All you would need is a cold surface in a moist environment. The team is working on creating a demonstration of this idea.