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Technique Furthers Water-Harvesting Possibilities

Water droplet
Researchers at the U.S. Department of Energy’s Oak Ridge National Laboratory have developed a method to create air-stable water droplet networks that could prove valuable in developing more efficient methods of harvesting water from fog or air. Kyle Kuykendall

A new method identified by a team of scientists in Tennessee has implications for water droplet-collection techniques worldwide.

May 27, 2014—Collecting water from fog is possible, but not terribly efficient. Prior to research released last year, most systems operated at a rate of a mere 2 percent efficiency, using large sheets of mesh to capture condensing water droplets and shed them into collection basins. Research by a team led by scientists from MIT and released in 2013 revealed a way to increase that efficiency to approximately 10 percent by adjusting the size of the mesh and the material used to create it. (See “Researchers Improve the Efficiency of Fog Harvesting,” Civil Engineering online, October 8, 2013). And research published by the American Chemical Society in Washington. D.C., revealed that coating a water-harvesting system with oil can increase its efficiency by up to 35 percent over a similar system that is not oil- infused.

Now a new technique that enables entire networks of air-stable water droplets to form, discovered by a team led by scientists at the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) in Tennessee, is adding to this body of knowledge—and its method may push the efficiency of water-harvesting systems even further.

“Our current manuscript is not directly related to water harvesting at all, but rather has implications for the future design of oil-infused water harvesters,” said Jonathan Boreyko, Ph.D., a research scientist at ORNL’s Bredesen Center. Boreyko is the first author of the study, which was published in the paper “Air-Stable Droplet Interface Bilayers on Oil-Infused Surfaces” in the Proceedings of the National Academy of Sciences. He wrote in response to written questions posed by Civil Engineering online.

“Oil-infused surfaces are super slippery to droplets,” Boreyko said. “For this reason, there has been a lot of interest in using oil-infused surfaces as more efficient condensers or to harvest fog from the air.” However, the ORNL research ascertained that there is a point at which an oil-infused surface will actually hinder the efficiency of water shedding, he explained.

The study determined that by manipulating the thickness of an oil coating upon a surface, the coalescence—or noncoalescence—of water droplets could be manipulated. By creating an environment in which the water droplets coalesce, scientists can force water to be shed from a surface t much faster, increasing potential water-harvesting efficiency.

“Our discovery of the noncoalescence phenomenon for droplets on oil-infused surfaces, and that it can be switched on or off depending on the thickness of the oil coating, has important implications for these systems that naturally exhibit droplet-[to]-droplet interactions,” Boreyko explained. This includes water-droplet harvesting systems. “Coalescence is usually desirable in order to grow and shed the water off the surface as quickly as possible,” he explained.

“Our findings suggest that oil-infused water harvesters should be designed to have very thin oil layers in order to prevent the noncoalescence …and maximize droplet shedding,” Boreyko said. “A lot of media outlets are confused about this fact,” he noted. “We are not suggesting that our droplet networks themselves will necessarily help condensation or fog harvesting; rather, it is the understanding that droplets can either exhibit coalescence or noncoalescence depending on the oil thickness that should be helpful in the design of water-harvesting systems on oil-infused surfaces.”

Finding that so-called “sweet spot,” at which an oil-infused surface can create a surface slippery enough to encourage the water droplets collected from fog, for example, to move into a collection point—but not so slippery that the droplets fall before they can coalesce—is the key to understanding how the technique works, he said. “If the oil layer is too thick, the droplets won’t coalesce,” Boreyko said. “But if the oil layer is too thin, the droplets are no longer slippery on the surface.”

While the implications of the current technique on the design of oil-infused water-harvesting systems could be significant, the point of the current research was to look into how the noncoalescence of water droplets could be controlled. The scientists will be pushing their research further in this direction in the future.

The technique developed by Boreyko and his team determined that drops of water placed on an oil-infused surface could contact each other, yet not coalesce for minutes or even hours, depending upon the viscosity of the oil. “As the droplets collide together, a microscopically thin film of oil gets squeezed up between the droplets to prevent coalescence,” Boreyko said.

Pure water droplets eventually merge together once the intermediate oil films drain out. However, it was discovered that the droplet networks could persist even beyond this time if lipids are mixed into the water to create something called “lipid bilayer membranes” between droplets. “Once the thin oil film drains out, the stable bilayer forms and is all that separates the two droplets from each other,” Boreyko said. “It is well known that lipid monolayers can assemble at oil-water interfaces, such that when two droplets are brought together, a stable bilayer membrane forms between them to prevent coalescence,” Boreyko explained. “But droplets brought together in an air environment [typically] just coalesce together, even with lipids at the interface,” he said. “The oil film that gets squeezed between colliding droplets on an oil-infused surface provides the water-oil interface required to form the stable bilayer membrane.

“What is significant about our method is that we assemble these droplet-interface bilayers in air instead of in oil, which is the first time that a synthetic bilayer has ever been suspended in an ambient environment,” Boreyko noted.

The fact that the noncoalesce of the water droplets can be manipulated in this manner “is remarkable because the bilayer is only about 5-7 nm thick and yet this membrane is able to withstand the pressure of the two droplets pressing on each other in an air environment,” Boreyko said. “We envision that these air-stable droplet-interface bilayers will facilitate the robust fabrication, manipulation, and utilization of functional droplet networks.”

Such networks could be used in synthetic biology and for the creation of biological batteries or biological circuits in an air environment, according to Boreyko.

The coauthors of the study are Georgios Polizos, Ph.D., a researcher, and Panos Datskos, Ph.D., a distinguished scientist, both of the energy and transportation science division at ORNL; Charles Patrick Collier, Ph.D., a research scientist in the Center for Nanophase Materials Sciences at ORNL; and Stephen A. Sarles, Ph.D., an assistant professor in the department of mechanical, aerospace, and biomedical engineering at the University of Tennessee, Knoxville.



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