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John O | November 2018

Researchers develop novel approach to overcome condensation issues in omniphobic materials


By Josh Perry, Editor
[email protected]

 

Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. designed a novel surface design for omniphobic materials to overcome the issue of condensation, which can limit the ability of the material to repel liquid.

 


This photo shows water droplets placed on the nanostructured surface developed by MIT researchers. (Kyle Wilke/MIT)

 

The solution, according to a report from MIT, was a very specific surface geometry that included ridges shaped like the letter ‘T’ or a ‘T’ with serifs. The shape of the ridges and the spacing between them were both critical to enable the surface to drastically reduce the effects of condensation and be more robust.

 

“Creating a surface that can shed virtually all liquids requires a precise kind of texture that creates an array of microscopic air pockets separated by pillars or ridges,” the report explained. “These air pockets keep most of the liquid away from direct contact with the surface, preventing it from ‘wetting,’ or spreading out to cover a whole surface. Instead, the liquid beads up into droplets.”

 

The surface geometry needs to be at the microscopic level because when droplets form they are initially at the nanometer-scale and the space between droplets can be less than a micrometer.

 

According to the report, “The shapes are designed to use the surface tension of the liquid to prevent it from penetrating the tiny surface pockets of air, and the way the ridges connect prevents any local penetration of the surface cavities from spreading to others nearby — as the team has confirmed in laboratory tests.”

 

To create the ridges, researchers etched spaces between them and coated the edges of the pillars. They etched the coatings to create indentations in the sides of the ridges. While this was just a proof of concept, researchers believe that this demonstrates potential for omniphobic materials in numerous applications, including thermal management.

 

The research was recently published in ACS Nano. The abstract stated:

 

“Omniphobic surfaces based on reentrant surface structures repel all liquids, regardless of the surface material, without requiring low-surface-energy coatings. Although omniphobic surfaces have been designed and demonstrated, they can fail during condensation, a phenomenon ubiquitous in both nature and industrial applications. Specifically, as condensate nucleates within the reentrant geometry, omniphobicity is destroyed.

 

“Here, we show a nanostructured surface that can repel liquids even during condensation. This surface consists of isolated reentrant cavities with a pitch on the order of 100 nm to prevent droplets from nucleating and spreading within all structures. We developed a model to guide surface design and subsequently fabricated and tested these surfaces with various liquids.

 

“We demonstrated repellency to 10 °C below the dew point and showed durability over 3 weeks. This work provides important insights for achieving robust, omniphobic surfaces.”

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