Is this the most water-repellent surface ever created?

Cooking, transportation, optics and hundreds of other applications are affected by how water sticks to surfaces or slides off them, so understanding the molecular dynamics of these microscopic droplets helps scientists find ways to improve many household and industrial technologies. Researchers at Aalto University have developed a new mechanism to make water droplets slip off surfaces, which they have described in the journal Nature Chemistry.

Liquid-like surfaces are a new type of droplet-repellent surface that offer many technical benefits over traditional approaches. They have molecular layers that are highly mobile yet covalently tethered to the substrates, giving solid surfaces a liquid-like quality acting like a layer of lubricant between the water droplets and the surface itself.

Researchers led by Professor Robin Ras have now used a specially designed reactor to create a liquid-like layer of molecules, called self-assembled monolayers (SAMs), on top of a silicon surface. According to doctoral researcher Sakari Lepikko, lead author of the new study, “Our work is the first time that anyone has gone directly to the nanometre-level to create molecularly heterogenous surfaces.”

By carefully adjusting conditions such as temperature and water content inside the reactor, the team could fine-tune how much of the silicon surface the monolayer covered. The new method proved exceptionally effective, as the team believe they have created the slipperiest liquid surface in the world.

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

At low coverage, the water becomes a film over the surface, which had been thought to increase the amount of friction. “Instead, water flows freely between the molecules of the SAM at low SAM coverage, sliding off the surface,” Lepikko explained. “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 discovery promises to have implications wherever droplet-repellent surfaces are needed, with Lepikko stating, “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.”

Next, the team plans to continue experimenting with their self-assembling monolayer set-up and improve the layer itself. Lepikko is particularly excited about the information this work has provided for future innovations.

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

Image caption: Liquid-like surfaces offer many technical benefits over traditional approaches. Image credit: Ekaterina Osmekhina.