Geckos feet stickier in the heat
15 Nov 2010 by Evoluted New Media
Geckos are amazing creatures – put them on a sheet of glass turn them upside down and they cling to the surface, but what do they do in humid environments?
Geckos are amazing creatures – put them on a sheet of glass turn them upside down and they cling to the surface, but what do they do in humid environments?
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Softer feet enable to geckos to hold on tight. Credit Matt Reinhold/Flickr |
It is known that geckos cling to smooth surfaces via van der Waals forces with no need for additional adhesives – the soles of their feet are covered in millions of self-cleaning microscopic spatula-shaped hairs or setae. In humid environments it is thought they need to hold on even tighter but scientists weren’t sure how.
It was thought that the gecko might benefit from additional adhesion in humid conditions through capillary action provided by microscopic droplets of water sandwiched between the setae and the surface. However, a team of scientists found that no such ‘water bridge’ existed – instead a change in the elastic modulus of the setae helped to strengthen the contact between setae and the surface, and made it easier for the reptile to peel its foot off.
Kellar Autumn, Jonathan Puthoff and Matt Wilkinson collected patches of setae that the gecko had moulted and attached them to a mechanical device known as Robotoe. The device reproduces the way the gecko drags its foot as it contacts a surfaces. They dragged the setae across two surfaces – one hydrophobic and one hydrophilic – at different velocities and in environments ranging from 10% to 80% humidity.
Their results – published in the Journal of Experimental Biology – showed that the setae’s adhesion and friction was the same on both surfaces and that there was no supplementary force keeping the setae on the surface.
The trio knew that setae are composed of keratin which is softened by high humidity, and wondered if softer setae led to better interaction with the surface. They repeatedly stretched and released a strip of setae at three different rates – 0.5. 5 and 10Hz – in environments ranging from 10% to 80% humidity measuring the force transmitted through the strip to calculate the strip’s elastic modulus. As humidity rose, the elastic modulus decreased by 75% and the strip became softer and enabled the setae to stick more effectively to the surface.
