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Dry Advance in Underwater Coatings

Tuesday, September 15, 2015

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A team of engineers from Northwestern University is the first to determine the specific “roughness” needed to keep a surface dry even when submerged in water for a long period of time.

Paul Jones

A research team has determined the specific “roughness” needed to keep a surface dry even when submerged in water for a long period of time.

According to research at Northwestern University's McCormick School of Engineering, the key is that the surface roughness must have valleys that measure less than one micron in width, the university announced in an Aug. 18 release.

“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,” said Neelesh A. Patankar, a theoretical mechanical engineer who led the research.

“These gas pockets deflect water, keeping the surface dry.

“The trick is to use rough surfaces of the right chemistry and size to promote vapor formation, which we can use to our advantage,” he added.

Peaks and Valleys

The team published its findings in a recent issue of Scientific Reports through an article titled “Sustaining dry surfaces underwater.”

According to the researchers, recognizing what makes a surface deflect water so well means the property could be reproduced in other materials on a mass scale.

There, the team acknowledges that there has been much study of the mechanisms that keep water from invading the valleys of rough surfaces. However, it found that additional mechanisms had to be considered in order for a surface to remain “practically dry” underwater.

The researchers explained that rough surfaces immersed under water remain practically dry if the liquid-solid contact is on roughness peaks, while the roughness valleys are filled with gas. However, the trapped gas can dissolve in the water, and when that happens, the surface will no longer be dry.

Water vapor, too, can fill the valleys, but when it condenses, it also leads to water “invasion.”

In their work, however, the team members pinpointed the ideal roughness scale, below which the vapor phase of water and/or trapped gases in roughness valleys can be sustained and the immersed surface will remain dry.

According to the researchers, recognizing what makes a surface deflect water so well means the property could be reproduced in other materials on a mass scale. They foresee purposes ranging from antifouling surfaces for ships to coatings that would reduce drag on pipes or submarines, which they predict could save billions of dollars for a variety of industries.

Inspired by Insects

The team was inspired by the ability of certain insects to survive in water. Outside of the team’s research, others have also studied “air-retaining” insect surfaces.

Team members cite research by A. Balmert and others titled “Dry under water: Comparative morphology and functional aspects of air-retaining insect surfaces.”

Paul Jones

In earlier research, the parts of gas-retaining insects that remained dry the longest were found to have hair spacing of hundreds of nanometers or less.

Balmert’s experiments indicated that the surface roughness on such insects involves hair spacing. The parts of the insect that remained dry the longest were found to have hair spacing of hundreds of nanometers or less.

“These gas-retaining insects have surface properties consistent with our predictions, allowing them to stay dry for a long time,” said Paul R. Jones, the study’s first author. He is a Ph.D. student in Patankar’s research group.

At Work in the Lab

In their research, the team members focused on the nanoscopic structure of surfaces, which at such as scale resemble the texture of a carpet, with spiky elevations separated by valley-shaped pores. 

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

The researchers submerged a variety of materials with and without the key surface roughness during their testing.

Those samples that featured the nanoscale roughness were found to stay dry for the duration of the experiment (four months).

Konrad Rykaczewski

The researchers submerged a variety of materials with and without the key surface roughness during their testing.

Other samples were placed in harsh environments, where dissolved gas was removed from the ambient liquid, and they also remained dry.

“When we looked at the rough surfaces under the microscope, we could see clearly the vacant gaps—where the protective water vapor is,” Patankar said.

Prior to this breakthrough, scientists were unable to keep water vapor from succumbing to condensation. But the key, according to the Northwestern team, is that valleys of less than one micron in width can sustain the trapped air as well as vapor in their gasified states, strengthening the seal that thwarts wetness.

Supporting the Research

Since 1909, The McCormick School of Engineering has sought to produce new knowledge and to engage and educate students. According to its website, McCormick builds a culture that both encourages and expects innovation: "That culture still thrives today, driving new initiatives, research discoveries, and superior education."

The article includes authors from ETH Zürich, Switzerland; Arizona State University; University of Illinois at Chicago; Massachusetts Institute of Technology; and the University of Denmark.

The Initiative for Sustainability and Energy at Northwestern (ISEN) supported the research.

   

Tagged categories: Coatings Technology; Nano and hybrid coatings; Nanotechnology; Research and development; Water repellents

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