Omniphobic liquid-like surfaces are a new type of liquid-repellent surface that offer many technical benefits over traditional approaches. They have molecular layers that are highly mobile yet covalently tethered to the substrate, giving solid surfaces a liquid-like quality that acts like a layer of lubricant between the water droplets and the surface itself. In new research, a team of scientists led by Aalto University used a specially-designed reactor to create a liquid-like layer of molecules, called self-assembled monolayers, on top of a silicon surface.
An artist’s depiction of the liquid-like layer of molecules repelling water droplets. Image credit: Ekaterina Osmekhina / Aalto University
Anti-wettability, the ability of a surface to repel liquids, is crucial to many aspects of daily life and industry.
Advanced anti-wetting surfaces that can passively remove liquids have attracted increasing interest because of their potential applications in anti-fouling, anti-icing, drag reduction, membrane separation and enhanced heat transfer.
Inspired by biological surfaces and benefiting from progress in nanofabrication techniques, a large number of advanced liquid-repellent surfaces have been developed since the 1990s, including air-mediated superhydrophobic surfaces (SHPSs), superoleophobic surfaces (SOPSs) and liquid-mediated slippery lubricant-infused porous surfaces (SLIPSs).
Air-mediated liquid-repellent surfaces (such as SHPSs and SOPSs) rely on a trapped air layer in the structure of the surface to repel liquids.
Stable trapping of the air layer generally requires the combination of low-surface-energy surface chemistry and specific surface structures, such as hierarchical micro/nanostructures for SHPSs or reentrant structures for SOPSs.
Alongside the progress of air-mediated surfaces and liquid- mediated surfaces, a new research field has bloomed: the development of omniphobic liquid-like surfaces by covalently grafting highly flexible polymer brushes or alkyl monolayers onto smooth solid surfaces.
“Our work is the first time that anyone has gone directly to the nanometer-level to create molecularly heterogenous surfaces,” said Sakari Lepikko, a doctoral researcher at Aalto University.
“By carefully adjusting conditions such as temperature and water content inside the reactor, we could fine-tune how much of the silicon surface the monolayer covered.”
“I find it very exciting that by integrating the reactor with an ellipsometer, that we can watch the self-assembled monolayers (SAMs) grow with extraordinary level of detail.”
“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.”
At low coverage, the water becomes a film over the surface, which had been thought to increase the amount of friction.
“We found that, instead, water flows freely between the molecules of the SAM at low SAM coverage, sliding off the surface,” Lepikko said.
“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.”
The new method proved exceptionally effective, as the team created the slipperiest liquid surface in the world.
“Things like heat transfer in pipes, de-icing and anti-fogging are potential uses,” Lepikko said.
“It will also help with microfluidics, where tiny droplets need to be moved around smoothly, and with creating self-cleaning surfaces.”
“Our counterintuitive mechanism is a new way to increase droplet mobility anywhere it’s needed.”
The team now plans to continue experimenting with their self-assembling monolayer setup and improve the layer itself.
“The main issue with a SAM coating is that it’s very thin, and so it disperses easily after physical contact,” Lepikko said.
“But studying them gives us fundamental scientific knowledge which we can use to create durable practical applications.”
The team’s work appears today in the journal Nature Chemistry.
_____
S. Lepikko et al. Droplet slipperiness despite surface heterogeneity at molecular scale. Nat. Chem, published online October 23, 2023; doi: 10.1038/s41557-023-01346-3
