Surfaces, the outermost layer of a material or substance, are probably not something you think about until you need to.
But try cleaning a nonstick pan after the nonstick surface has worn off. How about wiping down that once-smooth countertop that now has a few scratches and nicks?
What about medical devices such as knee implants? We definitely want them to be smooth. And it’s probably a good thing to have a fighter jet that doesn’t experience a lot of drag when it’s flying through the air.
You get the idea.
Now broaden your thinking a bit to consider “surface science,” the study of what happens at the interface of two phases, such as solid-liquid, solid-gas, etc.
But to understand what NC State scientists have discovered, you need to understand two terms: hydrophilic and hydrophobic.
If something is hydrophilic, water spreads and sticks easily to its surface.
If something is hydrophobic, it repels water. Water droplets tend to move and slide easily on this type of surface.
Remember that nonstick frying pan? Before its surface was damaged, it was hydrophobic. If you poured a small amount of water into it, its nonstick coating would repel it. Some water quickly ran off; what was left formed into small and large beads and slid off.
Now, as the catchphrase goes, “for something completely different.”
NC State researchers have recently engineered a surface that is both hydrophilic (makes water spread and stick) and slippery. Yep, that’s totally against conventional wisdom when it comes to the development of slippery materials. Think about that nonstick pan again.
Essentially, this is a new class of surfaces. And it could have multiple benefits in the manufacturing and biomedical fields.
“This finding is counterintuitive, since the longstanding view is that slippery surfaces tend to be hydrophobic,” said Arun Kumar Kota, associate professor of mechanical and aerospace engineering at NC State. “But we’ve demonstrated a way to engineer the surfaces of materials that makes them both very slippery and hydrophilic. We call them SLIC surfaces.”
The team has some specific applications where they think the material can be useful. But they admit there’s a lot more work that must be done to understand the scope of the application.
Until now, there were three ways to make a surface slippery.
The first was to texture the material to trap a layer of air against the surface. The air pocket became a lubricant.
The second method was to texture the surface to trap a layer of liquid lubricant against the material that would allow it to slide past other liquids or solids.
The problem is that in both cases, the textured surface of the material is damaged by repeated uses, which makes them less slippery. The gaseous or liquid lubricants also wear down over time.
The third approach has been to uniformly attach molecules to smooth, solid surfaces. The combination of uniformity and smoothness allows liquids to slide easily against the surface. That’s how a nonstick pan is made.
“But the molecules used to create these surfaces are hydrophobic, because it’s always been thought that slippery surfaces tend to be hydrophobic,” said Kota.
The new discovery shows that hydrophilic, slippery substances work.
The SLIC research shows that two things happen if water is poured onto a slippery, hydrophilic surface. First, the water will have a strong affinity for the surface, so it spreads out into a flattish bead. Then, because the surface is also slippery, the flattish bead of water slides off without a trace.
Researchers say the discovery of SLIC surfaces could have a dramatic impact in the biomedical field and in energy efficiency.
“Proteins cover almost the entire surface of hydrophilic materials and slippery, hydrophobic materials pretty quickly,” said Kota. “We’ve shown that virtually no proteins are found on a SLIC surface, even after 30 hours.”
That could have important applications for preventing infections or reducing the risk of clotting on implants.
SLIC technology could also make air conditioners more efficient. Most condensers have a component with a cool surface on which water vapor condenses. That water forms a film that serves as an insulator for the condenser, making it less efficient. On SLIC surfaces, the water vapor condenses quickly, but then slides off.
“When it comes to what SLIC technology could be used for, we’re really just scratching the surface,” said Kota, laughing. “There’s so much more to discover.”
NC State’s new findings were published in the journal Matter.
