WEDNESDAY, NOVEMBER 29, 2023
A team of researchers at Aalto University in Finland have reportedly been studying ways to alter surface roughness on a molecular scale to produce what they say could be the most slippery surface ever.
According to a release from the university, the new discovery could have applications in a range of fields, including plumbing, optics and the auto and maritime industries.
About the Research
Liquid-like surfaces are reportedly a new type of droplet-repellent surface that can offer certain technical benefits over past approaches.
According to the release, these surfaces contain highly mobile molecular layers that are covalently tethered to the substrates, giving solid surfaces a liquid-like quality to act like a layer of lubricant between the water droplets and the surface.
The slipperiness of surfaces is sensitive to topographic heterogeneity or roughness. Now, researchers have created the most slippery surface ever by tweaking its molecular-scale roughness.https://t.co/BvKfNuHMo7
— Chemistry World (@ChemistryWorld) November 26, 2023
A research team led by Robin Ras, a professor at Aalto University, utilized a specially designed reactor to produce a liquid-like layer of molecules, called self-assembled monolayers (SAMs), on top of a silicon surface.
The slickness of surfaces is reportedly altered by topographic variety or roughness, while some studies have also shown that this sensitivity can persist to the molecular level.
According to the team, droplets do not readily slide on most hydrophilic surfaces, with some exceptions like those based on polyethylene glycol.
“There are just a few reported, but [researchers] don’t really explain the origin,” Ras said to Chemistry World.
The report states that the team began their research by placing untouched hydrophilic silicon dioxide in a vacuum chamber, then using vapor deposition to create a mostly hydrophobic octyltrichlorosilane monolayer on the surface.
Through adjusting conditions like the temperature and water content inside the reactor, the team was able to alter how much of the silicon surface the monolayer covered.
“Our work is the first time that anyone has gone directly to the nanometer-level to create molecularly heterogenous surfaces,” said doctoral researcher Sakari Lepikko, lead author of the study.
By changing the deposition times, the team reportedly developed surfaces ranging from almost completely hydrophilic, to a jumble of hydrophilic and hydrophobic islands, to almost completely hydrophobic.
While testing the slipperiness of the surfaces, the team stated that water slid off surfaces that were covered densely with the hydrophobic octyltrichlorosilane. However, water droplets slid off with the same ease as highly hydrophilic surfaces with little coverage with this chemical.
Additionally, at intermediate coverage, water droplets reportedly exhibited much higher friction than either the mostly hydrophilic or fully hydrophobic surfaces. The researchers staed that they also tried testing molecular dynamics simulations to explain why.
In the case of highly hydrophobic surfaces, the water droplets touch the surface at a very steep angle, meaning that only a small area of the droplet would contact the surface before sliding off.
This method of texturing a surface to reduce a droplet’s contact area is reportedly commonly used in superhydrophobic surfaces.
However, in the low-coverage, hydrophilic case, water reportedly spread out to create a thin film, covering the surface. Other water droplets made large interfacial contacts with this layer, though as it was highly mobile, it lubricated their passage across the surface.
“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,” Lepikko stated.
Additionally, in the intermediate case, this hydrophilic surface effect was reportedly disrupted by the hydrophobic islands. Instead of sliding across the surface of a contiguous layer of water or skimming on a hydrophobic surface, water droplets were reportedly puddling over and over again.
“We actually had not expected this when we did the work, but looking back it fits very well with the idea that the heterogeneity is connected to friction, so it’s kind of logical,” said Ras.
Developing the New Surface
The researchers then reportedly lengthened their work to make a “super-slippery superhydrophobic surface.” The team reportedly did this by growing a hydrophobic monolayer on top of a layer of aluminum oxide-covered black silicon.
This silicon was reportedly textured at the micrometer scale to help reduce droplet contact area and create an exceptionally superhydrophobic surface.
"The usual approach is to reduce the area at which the droplet can make contact with the surface by having as fine a structure as possible, but people have not really focused on tuning the surface chemistry," stated Ras.
However, researchers blended both physical and chemical hydrophobicity. "The contact area is very small, and where it does contact the surface you have this lubricating effect," Ras explained.
The result, the team states, is what could be the most slippery surface ever created. Ras has said that the approach is scalable and that he has had “many discussions with companies that have expressed interest.”
"I think it’s an important paper," said Mathew Mate at SLAC National Accelerator Laboratory in the U.S., although he is curious about the researchers’ surprise.
"It wasn’t counter-intuitive to me," he said. "I like the scientific research that’s done in this paper, and it really shows on a molecular level what I thought would happen." He also adds that, "if the superhydrophobic surface is the slipperiest one to date then that’s a very important discovery in its own right.”
Next, the team plans to continue working with the self-assembling monolayer setup and to improve the layer itself. Lepikko stated that he is specifically excited about the information this work has given for future innovations.
"The main issue with a SAM coating is that it’s very thin, and so it disperses easily after physical contact. But studying them gives us fundamental scientific knowledge which we can use to create durable practical applications," he said.
The research was published in the journal Nature Chemistry.
Similar Research
In October of last year, a research team from the Institute of Oceanology of the Chinese Academy of Sciences reported a new mechanical robust superhydrophobic coating that can be applied via spray-coating.
The study, published in the journal Materials & Design, was led by IOCAS Professors Hou Baorong and Duan Jizhou.
According to the institute’s release, lotus-inspired superhydrophobicity attracted researchers due to interfacial non-wetting and unique multi-phase contact properties. However, properties such as fragile hierarchical structures, fluorine-containing chemical usage and strict requirements for substrate scopes can provide challenges.
As a result, the research team developed the superhydrophobic coating (ZnO@STA@PDMS) with fluorine-free reagents. Additionally, the coating utilized hierarchical rough micro-scale bump-porous structure, nano-scale particles and extremely low surface energy to repel water.
After spray-coated with the fabricated superhydrophobic coating, Q235 carbon steel’s ICorr saw decreased two orders of magnitude, suggesting a “superior” corrosion resistant performance. The |Z|10mHz value of the superhydrophobic coating was reportedly three orders of magnitude higher than the substrate.
Researchers utilized simulated marine atmospheric conditions with high relative humidity to test the coating, recording hygroscopic and deliquescence behaviors of NaCl salt particles. The results revealed that the corrosion damage in the edge of a saline droplet on bare Q235 carbon steel was more severe than interior because of faster ions transfer and abundant oxygen.
Based on this, researchers noted that the superhydrophobic coating possessed “promising” atmospheric corrosion inhibition performance based on the salt-deliquesce and instantaneous self-coalescence phenomenon observed.
The institute reported that the low interfacial adhesion force, low surface energy and air cushion-induced Cassie multi-phase contacts contributed to the saline droplet anti-wetting and remarkable anti-corrosion capability.
Tagged categories: Coating Materials; Coatings; Coatings Technology; Colleges and Universities; hydrophobic coatings; Program/Project Management; Research; Research and development; Surface roughness; Water repellents
