How to keep surfaces dry underwater

Northwestern University engineers have examined a variety of surfaces that can stay dry underwater, which could have implications from reducing drag on submarines to improving nucleate boiling.

The research team has identified the ideal 'roughness' needed in the texture of a surface to keep it dry for a long period of time when submerged in water. The valleys in the surface roughness typically need to be less than 1µm in width.

"The trick is to use rough surfaces of the right chemistry and size to promote vapour formation, which we can use to our advantage," said Neelesh A. Patankar, a theoretical mechanical engineer who led the research.

"When the valleys are less than 1µm wide, pockets of water vapour or gas accumulate in them by underwater evaporation or effervescence, just like a drop of water evaporates without having to boil it. These gas pockets deflect water, keeping the surface dry," he continued.

The researchers focused on the nanoscopic structure of surfaces, which, at the nanoscale, are somewhat akin to the texture of a carpet, with tiny spike-like elevations separated by valley-shaped pores in between.

When submerged, water tends to cling to the top of the spikes, while air and water vapour accrue in the pores between them. The combination of trapped air and water vapour within these cavities forms a gaseous layer that deters moisture from seeping into the surface below.

"When we looked at the rough surfaces under the microscope, we could see clearly the vacant gaps where the protective water vapour is," Patankar said.

Historically, scientists had not understood how to keep water vapour from succumbing to condensation within the pore, which can cause water to wet the surface. But the Northwestern team demonstrated that when the valleys are less than 1µm in width, they can sustain the trapped air as well as vapour in their gas states, strengthening the seal that repels wetness.

Tom Austin-Morgan

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