Micro-texture inspired by nature makes surfaces liquid repellent Tuesday, 01 August 2017

Researchers at the King Abdullah University of Science and Technology (KAUST) have taken a cue from nature to develop a micro-texture that makes surfaces liquid repellent.

Omniphobicity is the property of surfaces that are repellent to liquids, and is very useful in industrial applications, helping reduce biofouling, underwater drag, improving membrane distillations, waterproofing and oil-water separation processes.

Conventionally, omniphobicity can be generated by applying perfluorinated coatings. But these degrade under harsh physical and chemical environments, increasing costs and resulting in health and environmental impacts.

In 2014, a US-based research team demonstrated micro-textures comprising of doubly re-entrant pillars, which were omniphobic in air, even when the base materials themselves were intrinsically wetting.

According to Himanshu Mishra from the KAUST Water Desalination and Reuse Centre, the team took inspiration from those results.

“At first, these results seemed to defy conventional wisdom as roughening intrinsically wetting surfaces makes them even more wetting,” said Mishra. “So we decided to investigate these microtextures for ourselves.”

The team confirmed the effect of the doubly re-entrant micropillars micro-texture, but found that the effect could not be sustained in the presence of localised physical defects or damage or upon immersion in wetting liquids.

“These were serious limitations because real surfaces get damaged during use,” said Mishra. “This inspired us to look to nature and investigate the skins of springtails.”

Springtails are tiny soil-dwelling insects that live in moist conditions. On their skin are patterns that keep them dry.

The KAUST researchers found that the textures contain doubly re-entrant cavities. They used photolithography and dry-etching tools at the KAUST Nanofabrication Core Lab to recreate those doubly re-entrant microcavities on silica surfaces.

The microcavities trapped air and prevented penetration of liquids, even when subjected to elevated pressures. And because they are compartmentalised, localised damage or defects, and immersion into wetting liquids did not result in the loss of omniphobicity.

“Having demonstrated the proof of concept, we now plan to translate the fabrication process from the lab to the Workshop Core Lab in KAUST to create doubly reentrant cavities on common materials, such as polyethylene terephthalate and low-carbon steels,” said Mishra. “This may help to unlock their potential for applications to reduce hydrodynamic drag and antifouling.”

[Image: Wetting surfaces with micropillars (left) lose their omniphobicity when damaged, while microtextures with doubly reentrant cavities (right) exhibit omniphobicity despite localised damage. Reproduced with permission from ref 1.© 2017 American Chemical Society. Created by Ivan Gromicho © 2017 KAUST]