April 1, 2013
Anyone who lives in or has visited a tropical country is likely familiar with the chipper chirping of the gecko. These friendly little lizards inhabit homes and jungles stretching from Indonesia to Tanzania to the Dominican Republic. They emerge after sunset, taking advantage of their night vision eyesight—which is 350 times more powerful than a human’s—and are welcome guests in homes and hotels since they gobble up mosquitoes and other insect pests.
In addition to the locals, scientists also love these colorful lizards. Geckos possess the unique ability among lizards to run up flat walls and scamper across ceilings, even if the surface is very smooth. Researchers have been puzzling over this ability for years, and dozens of labs have tested gecko adhesion in the hopes of harnessing this superpower for potential use in everything from robotics to space technology to medicine to “gecko tape.”
Gecko toes, it turns out, contain hair-like structures that form a multicontact interface, meaning geckos grip with thousands of tiny adhesive structures rather than what appears to be a single uniform foot.
Gaps remain, however, in researchers’ understanding of how gecko feet interact with surfaces in their natural environment, especially in dry versus wet conditions. Scientists know that gecko toe pads are superhydrophobic, or water repelling, yet geckos lose their ability to cling to glass when it becomes wet. Why don’t they just repel the water and cling to the glass surface below? Similarly, scientists wonder how geckos deal with wet leaves in the forest during rain storms.
A new paper published in Proceedings of the National Academy of Sciences investigates these mysteries. The authors decided to test gecko grip on a range of wet and dry materials that both attract and repel water. To perform their experiments, they outfitted six tokay geckos with gecko-sized harnesses. They placed the geckos onto four different types of materials, such as glass, plastic and a substance designed to mimic waxy tropical leaves. After giving the lizards some time to adjust to their new surroundings, the researchers applied a uniform tugging pressure onto the geckos’ harnesses, pulling in the opposite direction of where the animals were walking. Eventually, the geckos could cling no longer and lost their grip. This allowed the team to measure the adhesive force required to displace the animals. They repeated the same experiments under very wet conditions, too.
The authors found that materials that are more “wettable”—an indication of the degree to which a surface attracts water molecules—the less force it took to disrupt the clinging geckos’ grips. Glass had the highest wettability of the surfaces the researchers tested, and geckos easily slipped from wet glass compared to dry glass. When that material gets wet, water forms a thin, attractive film that prevents the gecko’s tiny toe hairs from coming into contact with the surface.
The low wettability properties of waxy leaves, on the other hand, allow geckos to establish a sturdy grip, even in rain storms, because leaves actively repel water. Geckos performed equally well in wet and dry conditions on the leaf-mimicking surface, the researchers found.
How the geckos interact with surfaces depends upon a thermodynamic theory of adhesion, the authors conclude. These features are dictated by Van der Waals force, or the sum of attractive and repulsive interactions between gecko toes and the characteristics of the surfaces they come into contact with. So long as those attractive forces jibe, geckos are in luck for getting a grip on whatever surface they come into contact with, regardless of whether it’s wet or dry.
Using our whole-animal adhesion results, we found that wet surfaces that are even weakly [water repulsive] allow the gecko adhesive system to remain functional for clinging and likely locomotion as well.
Our findings suggest a level of versatility in the gecko adhesive system that previously was not accounted for and calls into question interesting evolutionary, ecological, and behavioral predictions.
In addition to shedding light on how gecko adaptations help the lizards cope with their natural environment, the authors think their findings may contribute to designing new synthetic gecko robots that may overcome real-life geckos’ wet glass Achilles’ heel, useful perhaps for cleaning skyscraper windows, spying on suspected terrorists, or simply changing a hard-to-reach light bulb.
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