Engineers are close to perfecting materials that can stay dry for months at a time even fully underwater, according to research published this week in Scientific Reports. Imagine a submarine that never gets wet, or a pipe that conveys liquids along without ever actually touching them. There are a lot of pretty obvious applications.
The Northwestern University-based researchers have, more specifically, identified the ideal amount of "roughness" a surface should have to completely repel water. This roughness comes as the result of minute valleys etched into the material less than a single micron (less than one millionth of a meter) in width, resulting in a fine nanoscale corduroy that works by encouraging the formation of water vapor.
"When the valleys are less than one micron wide, pockets of water vapor or gas accumulate in them by underwater evaporation or effervescence, just like a drop of water evaporates without having to boil it," offers lead investigator Neelesh A. Patankar in a statement. "These gas pockets deflect water, keeping the surface dry."
So, it's only the peaks of these nano-valleys that actually contact the liquid. Trapped gas keeps it from permeating further, but this is where the challenge lies: How do we keep the trapped gas from dissolving into the liquid and allowing it to successfully "wet" the surface? Moreover, having accomplished that, how do we keep the vapor from condensing enough (thanks to pressure or temperature) to allow the liquid to invade?
Superhydrophobicity isn't a brand new topic. Researchers have been chasing after this for some time now, with the ideal being the Cassie-Baxter state. Here, water droplets are able to chill out on the tops of these valleys without actually rolling down into them. It happens when the contact angle between the surface and the droplet exceeds 140 degrees (above). It's easier to just visualize it: more angle, less contact, less penetration.
Engineers can do some amazing things with non-submerged surfaces, but once we start dunking stuff, it gets tricky. Maintaining the Cassie-Baxter state will ensure a practically dry surface while immersed in a liquid," Patankar and his group write. "This is challenging, as air in the roughness valleys can dissolve into the liquid if the liquid is undersaturated with air. Thus, in order to keep a surface practically dry under water, the gas phase in the roughness valleys must be sustained."
So, they tested a range of roughness scales, chasing after the optimal dimensions of a surface's roughness valleys. They found that with widths of hundreds of nanometers or less, the materials are able to remain dry, but above 10 or so microns and and we're soaked.
As the authors note, this is a phenomenon already achieved by nature and can be seen in insects like water striders and water bugs. They have water-contacting surfaces akin to carpeting, with tiny spikes alternating with pores, with less than a single micron of spacing between them. The pores trap gas, and the bugs stay dry: built-in umbrellas.