Scientists have developed what they believe is the most hydrophobic surface in history
Have you ever seen drops of water slide on a duck’s feathers? These birds cover their feathers with an oily, waxy substance that they secrete in a gland near the tail. Some species spend a quarter of their day grooming their feathers in this way to stay dry, warm, and afloat.
The ability to repel water is important for many materials, particularly in the automotive, marine and aerospace industries. Many superhydrophobic surfaces work by trapping a layer of air or liquid, which causes water that falls on them to clump together into droplets and roll off more easily. But in this case, it’s a new technology that creates so-called liquid-like surfaces (LLS), which have layers of highly mobile molecules that act like liquids but stick to substrates so they don’t escape. The end result is like a lubricated surface that water slides on.
In the new study, scientists at Aalto University in Finland developed a new LLS made from molecules called self-assembled monolayers (SAMs) that coat a silicon substrate. By adjusting conditions such as temperature and water content in the reactor during production, the team was able to control the amount of silicon covered by the SAMs.
As SAMs covered much of the surface, it became superhydrophobic, causing water to bead up and roll like ducks’ backs. This was expected, but to the researchers’ surprise, the low SAM coverage also made the surface slippery. And this without the formation of water droplets, which was long considered necessary for superhydrophobicity.
According to Sakari Lepikko, lead author of the study, “It was counterintuitive that even low coverage resulted in exceptional smoothness.” “Instead, we found that when SAM coverage is low, water flows freely between the SAM molecules and over the surface slides.” And when the SAM cover is high, water stays on the SAM and slides off just as easily. Only between these two states does water adhere to the SAM and the surface.”
The team claims that some versions of their SAM surfaces are the most water-repellent materials ever: Superhydrophobic surfaces typically have sliding angles (the angle at which the surface must be tilted for water to slide off) of just 5°. However, according to Aalto’s team, the slope could be 0.01°, meaning water will slide off any surface that isn’t perfectly flat.
The most common measure of hydrophobicity is the contact angle, which results from the pronounced curvature that water droplets form on the surface. However, in this case, it is difficult to apply this measure because although water on SAM surfaces can spread and form a film, it still rolls easily.
As intriguing as the SAM coating is, the researchers admit that it is still quite thin and easy to spread. However, they plan to continue working to improve it so that it can be useful in a range of industrial use cases over time.
“Heat transfer in pipes, de-icing and anti-fog are possible applications,” explains Lepikko. “It will also be useful in microfluidics, where tiny droplets need to move smoothly, and in creating self-cleaning surfaces.” “Our counterintuitive mechanism is a new way to increase droplet mobility where it is needed.”
REFERENCE
Droplet slipperiness despite surface heterogeneity at the molecular level