Researchers at Duke University and the University of British Columbia have discovered that droplets can launch themselves from thin fibers, a phenomenon that may find application in refining oil and purifying water.
Chuan Hua-Chen
When droplets grow in size, they store energy on their growing surface. When two droplets roll together, their surface area decreases. However, their mass remains constant, and this leads to release of a small amount of energy. This released energy would be sufficient to throw the drops from the fiber only when they were fixed to a small solid area. This phenomenon can take place only if the fiber or strand is at least moderately hydrophobic, and the strand’s diameter is a couple of times smaller than the droplet’s diameter.
In this experiment, fibers coated with Teflon were used.“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.”
Kungang Zhang, a graduate student in Chen’s group, helped discover that the “dancing droplets” were in fact real, and that they had a better chance of launching themselves of a thin fiber when they merged from opposite sides. This breakthrough enabled detailed study of the phenomenon.
Chen and his team had, in earlier studies, demonstrated a self-cleaning method similar to that which took place on cicada’s wings. These wings had a nanostructure that made their surfaces super-hydrophobic. This enabled the droplets to launch themselves from the flat surface of the wings.
“In engineering systems, these nanostructures are concerns for reliability,” said Chen. “Our new finding provides a solution without resorting to these super-hydrophobic surfaces.”
The dancing droplets phenomenon holds significant promise for water purification technologies. Presently, shearing forces or gravity is used in most methods for removing accumulated droplets from fibrous webs. However, when the droplets become very large, they can block the gaps in the web.
The findings made in this study could enable Teflon-like coatings to be used for fibrous woven materials wutg sufficiently large gaps, and that would prevent any clogging occuring before the droplets could fling themselves off the surface.
“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.”
The findings of this study have been reported in a paper entitled, “Self-Propelled Droplet Removal from Hydrophobic Fiber-Based Coalescers,” in Physical Review Letters.
The National Science Foundation has provided support for this study.