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New Technique Creates Extremely Water-Repellent Materials

An artist's depiction of the liquid-like layer of molecules repelling water droplets.
An artist's depiction of the liquid-like layer of molecules repelling water droplets. Credit: Ekaterina Osmekhina / Aalto University.
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Researchers have developed a new mechanism that could hold the key to creating exceptionally water-repellant surfaces, according to a new study published in Nature Chemistry.

The technique hinges on the liquid-like behavior of self-assembled monolayers (SAMs) on a material surface. According to the researchers, the new discovery challenges existing ideas about friction and how water glides over different surfaces at the molecular level.

The researchers say that this new technique could also have significant implications for a range of other fields, including plumbing, optics and the auto and maritime industries.

Water slides straight off of liquid-like surfaces

Studying how water sticks to or slides off of different surfaces may seem like a very niche research interest at first, but understanding the molecular dynamics behind this behavior is essential. Cooking, transportation, aviation, optics and hundreds of other industries could be improved by the development of more water-repellant materials.

In their investigation of new, more slippery materials, researchers from Aalto University began to look at a new type of droplet-repellant surface. These “liquid-like” surfaces are coated with a highly mobile layer of molecules that are covalently bonded to the material’s surface. The liquid-like layer of molecules – the self-assembled monolayer (SAM) – effectively acts as an additional lubricant layer between any water droplets and a surface, increasing its slipperiness and water-repellant action.

In their new work, the researchers report the creation of a specially designed reactor that creates a SAM layer on top of a silicon surface, which they used to study how different amounts of SAM surface coverage would affect the material’s hydrophobic nature.

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By carefully adjusting the conditions inside the reactor, including temperature and water content, the team found that they could fine-tune how much of the silicon surface was covered. Using ellipsometry – a technique that uses polarized light to characterize very thin films on a surface – the researchers were able to study the function of these liquid-like SAMs at the nanometer level for the first time.

“I find it very exciting that by integrating the reactor with an ellipsometer, we can watch the self-assembled monolayers grow with an extraordinary level of detail,” said Professor Robin Ras, senior author of the study and head of the Department of Applied Physics at Aalto University.

“The results showed more slipperiness when SAM coverage was low or high, which are also the situations when the surface is most homogeneous. At low coverage, the silicon surface is the most prevalent component, and at high, SAMs are the most prevalent.”

“It was counterintuitive that even low coverage yielded exceptional slipperiness,” Sakari Lepikko added. Lepikko is a doctoral researcher in the Department of Applied Physics at Aalto University and the lead author of the new study.

“We found that, instead, water flows freely between the molecules of the SAM at low SAM coverage, sliding off the surface. And when the SAM coverage is high, the water stays on top of the SAM and slides off just as easily. It’s only in between these two states that water adheres to the SAMs and sticks to the surface.”

Is better de-icing and self-cleaning tech on the horizon?

The Aalto University team believes that their SAM-coated silicon material is the “slipperiest liquid surface” in the world. Similar surfaces could have a multitude of use cases in daily life, they say.

“Things like heat transfer in pipes, de-icing and anti-fogging are potential uses. It will also help with microfluidics, where tiny droplets need to be moved around smoothly, and with creating self-cleaning surfaces,” Lepikko said. “Our counterintuitive mechanism is a new way to increase droplet mobility anywhere it’s needed.”

That being said, the researchers are upfront with the fact that their SAM coating is not yet ready for real-world use.

“The main issue with a SAM coating is that it’s very thin, and so it disperses easily after physical contact,” Lepikko noted. “But studying them gives us fundamental scientific knowledge which we can use to create durable practical applications.”

Reference: Lepikko S, Jaques YM, Junaid M, et al. Droplet slipperiness despite surface heterogeneity at molecular scale. Nat Chem. 2023:1-8. doi: 10.1038/s41557-023-01346-3

This article is a rework of a press release issued by Aalto University. Material has been edited for length and content.