Staying stuck when it’s wet; learning from the geckos

Researchers from the University of Akron have published another study on geckos and their ‘stickability’ in watery environments. Last mentioned here in my Aug. 10, 2012 posting, doctoral candidate Alyssa Stark  and her colleagues were then testing the geckos by placing them on wetted glass plate surfaces and also by immersing them on water-filled tubs with glass bottom,

Next, the trio sprayed the glass plate with a mist of water and retested the lizards, but this time the animals had problems holding tight: the attachment force varied each time they took a step. The droplets were interfering with the lizards’ attachment mechanism, but it wasn’t clear how. And when the team immersed the geckos in a bath of room temperature water with a smooth glass bottom, the animals were completely unable to anchor themselves to the smooth surface. ‘The toes are superhydrophobic [water repellent]’, explains Stark, who could see a silvery bubble of air around their toes, but they were unable to displace the water surrounding their feet to make the tight van der Waals contacts that usually keep the geckos in place.

Then, the team tested the lizard’s adhesive forces on the dry surface when their feet had been soaking for 90 min and found that the lizards could barely hold on, detaching when they were pulled with a force roughly equalling their own weight. ‘That might be the sliding behaviour that we see when the geckos climb vertically up misted glass’, says Stark. So, geckos climbing on wet surfaces with damp feet are constantly on the verge of slipping and Stark adds that when the soggy lizards were faced with the misted and immersed horizontal surfaces, they slipped as soon as the rig started pulling.

In this latest research, from the Ap. 1, 2013 news release issued by the University of Akron on EurekAlert, Stark and her colleagues announce they’ve discovered the conditions under which geckos can adhere to wet surfaces,

Principal investigator Stark and her fellow UA researchers Ila Badge, Nicholas Wucinich, Timothy Sullivan, Peter Niewiarowski and Ali Dhinojwala study the adhesive qualities of gecko pads, which have tiny, clingy hairs that stick like Velcro to dry surfaces. In a 2012 study, the team discovered that geckos lose their grip on wet glass. This finding led the scientists to explore how the lizards function in their natural environments.

The scientists studied the clinging power of six geckos, which they outfitted with harnesses and tugged upon gently as the lizards clung to surfaces in wet and dry conditions. The researchers found that the effect of water on adhesive strength correlates with wettability, or the ability of a liquid to maintain contact with a solid surface. On glass, which has high wettability, a film of water forms between the surface and the gecko’s foot, decreasing adhesion. Conversely, on surfaces with low wettability, such as waxy leaves on tropical plants, the areas in contact with the gecko’s toes remain dry and adhesion, firm. [emphasis mine]

“The geckos stuck just as well under water as they did on a dry surface, as long as the surface was hydrophobic,” Stark explains. “We believe this is how geckos stick to wet leaves and tree trunks in their natural environment.”

For interested parties, this is where the paper can be found,

The discovery, “Surface Wettability Plays a Significant Role in Gecko Adhesion Underwater,” was published April 1, 2013 by the Proceedings of the National Academy of Sciences. The study has implications for the design of a synthetic gecko-inspired adhesive.

Here’s an image of a gecko (from the University of Akron’s webpage with their Ap. 1, 2013 news release),

Courtesy University of Akron [downloaded from http://www.uakron.edu/im/online-newsroom/news_details.dot?newsId=ec9fd559-e4af-487f-a9cc-2ea5f5c9612d&pageTitle=Top%20Story%20Headline&crumbTitle=Geckos%20keep%20firm%20grip%20in%20wet%20natural%20habitat]

Courtesy University of Akron [downloaded from http://www.uakron.edu/im/online-newsroom/news_details.dot?newsId=ec9fd559-e4af-487f-a9cc-2ea5f5c9612d&pageTitle=Top%20Story%20Headline&crumbTitle=Geckos%20keep%20firm%20grip%20in%20wet%20natural%20habitat]


Not mentioned in this news release, one of the relevant applications for this work would be getting bandages and dressings  to adhere to wet surfaces.

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