The discovery may aid Water Purification and Oil Refining. Researchers have observed droplets spontaneously fling themselves from thin fibers. As long as the strands are moderately hydrophobic and relatively thin, small droplets combining into one are apt to dance themselves right off of the tightrope.
“We were studying how insect wings with a hairy structure clean themselves, and an undergrad Adam Williams saw two droplets merge and suddenly leave a strand of hair,” said Chuan-Hua Chen, associate professor of mechanical engineering and materials science at Duke. “Since we couldn’t easily reproduce the effect, we thought it was just an artifact, perhaps due to the slight breeze created by the humidifier in the experiment.”
IMAGES: self-propelled removal of drops from a hydrophobic fiber, where the surface energy released upon drop coalescence overcomes the drop-fiber adhesion, producing spontaneous departure that would not occur on a flat substrate of the same contact angle. The self-removal takes place above a threshold drop-to-fiber radius ratio, and the departure speed is close to the capillary-inertial velocity at large radius ratios.
As a droplet grows larger, it stores energy on its expanding surface. When 2 droplets merge, the mass stays the same, but the surface area decreases. This causes a small amount of energy to be released. As long as the drops are only attached to a small solid area, the released energy is enough to fling them away. This proves true so long as the strand is reasonably hydrophobic, such as the Teflon-coated fibers in the experiment, and the diameter of the strand is a few times smaller than that of the droplet.
A potential application of the dancing phenomenon is in water purification technologies. Current methods use gravity or shearing forces to remove accumulated droplets from fibrous webs. If the droplets get too large, however, they can clog the gaps in the web. But with this new finding, fibrous woven materials could be engineered with Teflon-like coatings and large enough gaps to never clog before droplets jump off.
“Before we demonstrated this, people thought you’d never be able to get the self-propelled phenomenon on a moderately hydrophobic surface,” said Chen. “But now we’ve shown that you don’t need super-hydrophobicity to get this dancing effect. All you need are round fibers instead of flat surfaces.” http://pratt.duke.edu/news/dancing-droplets-launch-themselves-thin-fibers
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