Their research began as a study on how animals interact with surfaces in nature, but their latest findings challenge one of our most fundamental suppositions about surface adhesion.
Ali Dhinojwala, W. Gerald Austen Endowed Chair and H.A. Morton Professor at Akron University, with his team of University of Pittsburgh’s Tevis Jacobs, University of Friedberg’s Lars Pastewka and Anirudha Sumant of the Argonne National Laboratory wanted to examine nature’s solutions to the problem of adhesion. What allows a gecko to run up a wall? How does a spider walk along its sticky silk?
The traditional view of adhesion is that water impedes the process — it would be laughable to try to put a Band-Aid on in the shower. But Dhinojwala says that when the team took a closer look at the interactions of wet and rough surfaces, the answer wasn’t so simple. Although the water made it more difficult to push two surfaces together, in some instances, the presence of water made it significantly harder to pull the two surfaces apart.
“That's where the surprise was — that when we look at the contact and we look at the adhesion, we see that there is 50% of that area still covered with water. So intuition would say, ‘hey, look, 50% of the area is covered with water. Adhesion should be low.’” Dhinojwala said. “But we started observing that when we pull them off, we actually get four to five times more adhesion than what we were expecting.”
The team is still narrowing down the exact reason why water can enhance adhesion under certain conditions, but associate professor and Whiteford Faculty Fellow at the University of Pittsburgh Tevis Jacobs says it all has to do with the scales of roughness.
After the surfaces were chemically prepared at the Argonne National Laboratory in Illinois, Jacobs’ group characterized their roughness down to the atomic scale at the University of Pittsburgh. There are large variations in classifying roughness — small-, intermediate- and large-scale bumps on the surface that are in charge of different properties.
“You're going for a hike, right? There's bumpiness that are the pebbles on the ground, there's bumpiness that are the boulders, and then there's bumpiness that are the mountains,” Jacobs said. “You can imagine two places that have the same pebble scale roughness, but, you know, one is the Swiss Alps and one is Kansas.”
When it comes to adhesion, the large- and intermediate-scale roughness — the mountains and the boulders — control the drainage of water that typically gets in the way of adhering two objects together. The very small scale, however, can control the retention of water in tiny nano-pockets that prevent the surfaces from pulling apart.
Jacobs says that these findings open up new possibilities for improving underwater adhesion, challenging old ideas that we should always squeeze out as much water as possible in order to treat underwater adhesion the same way we treat dry adhesion.
“You can't just say, ‘rougher is better underwater’ or ‘smoother is better underwater,’” Jacobs said. “It unlocks this whole new pathway to optimizing surfaces where, yes, you want drainage at some scales — at larger scales — but you actually might not want drainage at the very small scales.”
The team plans to continue looking into applications of these new findings, but the research could have a significant impact on the biomedical industry in the form of bandages, health monitoring sensors on moist skin, advanced adhesives that could replace sutures and other fluid transfer technologies like syringes and o-rings.
While they have their work cut out for them exploring proposed explanations for the behavior of trapped water and developing practical applications for their findings, Jacobs says these kinds of unexpected developments are great catalysts for more exciting work.
“In my opinion, the best science is not, ‘Here's the answer,’” he said. “It's, ‘Wow! Here's a bunch of new interesting questions.’”
After years of data collection and research and six months of writing, Dhinojwala’s team published their full findings in the multidisciplinary journal Science Advances on Aug. 7.
