Water droplet networks can harvest water from fog

Researchers have developed a method to create air-stable water droplet networks that are valuable for applications in biological sensing and harvesting water from fog.
Researchers at the US Department of Energy's Oak Ridge National Laboratory (ORNL) developed the method to create air-stable water droplet networks known as droplet interface bilayers.
These interconnected water droplets have many roles in biological research because their interfaces simulate cell membranes. Cumbersome fabrication methods, however, have limited their use.
"The way they've been made since their inception is that two water droplets are formed in an oil bath then brought together while they're submerged in oil," said ORNL's Pat Collier, who led the new study published in the Proceedings of the National Academy of Sciences.

"Otherwise they would just pop like soap bubbles," Collier said.
Instead of injecting water droplets into an oil bath, the ORNL research team experimented with placing the droplets on a superhydrophobic surface infused with a coating of oil. The droplets aligned side by side without merging.

To the researchers' surprise, they were also able to form non-coalescing water droplet networks without including lipids in the water solution.
Scientists typically incorporate phospholipids into the water mixture, which leads to the formation of an interlocking lipid bilayer between the water droplets.
"When you have those lipids at the interfaces of the water drops, it's well known that they won't coalesce because the interfaces join together and form a stable bilayer," ORNL coauthor Jonathan Boreyko said.

"So our surprise was that even without lipids in the system, the pure water droplets on an oil-infused surface in air still don't coalesce together," Boreyko added.
The team's research found the unexpected effect is caused by a thin oil film that is squeezed between the pure water droplets as they come together, preventing the droplets from merging into one.
With or without the addition of lipids, the team's technique offers new insight for a host of applications.
Controlling the behaviour of pure water droplets on oil-infused surfaces is key to developing dew- or fog-harvesting technology as well as more efficient condensers, for instance.

The ability to create membrane-like water droplet networks by adding lipids leads to a different set of functional applications, Collier noted.
"These bilayers can be used in anything from synthetic biology to creating circuits to bio-sensing applications," he said.
"For example, we could make a bio-battery or a signalling network by stringing some of these droplets together. Or, we could use it to sense the presence of airborne molecules," he added.