An artist's depiction of the liquid-like layer of molecules repelling water droplets. Credit: Ekaterina Osmekhina/Aalto University
Researchers from Aalto University have recently published their findings on a novel water-repellent surface that challenges existing ideas about the friction between solid surfaces and water droplets. The findings will create new pathways for studying how water droplets interact with solid surfaces at a molecular level.
The research, published in Nature Chemistry, utilizes a liquid-like surface as a novel droplet-repellent layer. The researchers utilized a reactor to create a liquid-like layer of molecules called self-assembled monolayers (SAMs) that were applied to a silicon surface. “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.
To fine-tune the amount of silicon surface covered by the SAMs, the researchers adjusted conditions such as temperature and water content within the reactor. “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,” said Lepikko. “It was counterintuitive that even low coverage yielded exceptional slipperiness.”
The researchers found that at low coverage the water became a film over the surface, a phenomenon previously 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. 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 added.
The new coating was highly effective and is believed to be the slipperiest coating in the world. The researchers believe the coating has potential applications across countless industries. “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. Our counterintuitive mechanism is a new way to increase droplet mobility anywhere it’s needed,” said Lepikko
The researchers plan to continue testing with the self-assembling monolayer to improve the layer itself. Due to the thinness of the coating, it is easily worn down during physical contact. The researchers believe that this study provides the foundational knowledge required to further research the coating and develop a more durable coating for practical applications.