Human adhesives are famed for their fallibility. Gooey glues soon lose their grip, are easily contaminated and leave residues behind. But not gecko feet. Geckos can cling on repeatedly to the smoothest surfaces thanks to the self-cleaning microscopic spatula-shaped hairs (setae) that coat the soles of their feet.
Back in 2002, Kellar Autumn found that these dry hairs are in such intimate contact with surfaces that the reptiles 'glue' themselves on by van der Waals forces with no need for fluid adhesives. More recent studies had suggested that geckos might benefit from additional adhesion in humid environments through capillary action provided by microscopic droplets of water sandwiched between setae and the surface.
But Autumn wasn't so sure, so he and his lab at Lewis and Clark College and the University of Washington, USA, began testing gecko grip to find out how increasing humidity helps them hold tight Autumn publishes his team's discovery that humidity helps geckos grip tighter by softening the surface of their feet on 15 October 2010 in The Journal of Experimental Biology at http://jeb.biologists.org.
Knowing that geckos replace lost setae when they moult, Autumn, his postdoc Jonathan Puthoff, and Matt Wilkinson collected patches of the 'sticky' hairs from gecko feet and attached them to a mechanical testing device, known as 'Robotoe', that reproduces the way the reptile drags its foot as it contacts a surface. Dragging the setae across two surfaces (one that repelled water and another that attracted water) at different velocities and in environments ranging from 10% to 80% humidity, the team tested whether microscopic water bridges formed in high humidity were helping the geckos hang on.
They reasoned that if the reptiles were using microscopic water bridges then the setae would bond more tightly to the surface that attracted water than the surface that repelled water. But when they measured the setae's adhesion and friction it was essentially the same on the two surfaces. And when the team compared the adhesion of setae that were moving too fast to form water bridges with that of slowly moving feet that could possibly form water bridges, there was no difference. The geckos were not supplementing their van der Waals attachment forces with capillary forces from water bridges. So how were they holding on tighter?
Graduate student Michael Prowse decided to take a closer look at the material properties of the reptile's feet. Knowing that setae are composed of keratin and keratin is softened by high humidity, Autumn wondered whether having softer setae could improve the reptiles' contact with surfaces and increase their van der Waals adhesion. The team decided to measure the setae's softness and how it changed as the humidity rose.
Repeatedly stretching and releasing a strip of setae at three different rates (0.5, 5 and 10 Hz) in environments ranging from 10% to 80% humidity, Autumn's team measured the force transmitted through the strip to calculate the strip's elastic modulus – how much elastic energy is stored – to see how it changed. As the humidity rose, the elastic modulus decreased by 75% and the strip of setae became softer. So the strip of setae became more deformable as the humidity rose, but could the increased softness explain the gecko's improved attachment under damp conditions?
Puthoff built a mathematical model to see if softer, more deformable, setae could explain the gecko's improved attachment at high humidity and found that it did. Not only did increased softness strengthen the contact between the setae and the surface but also it made it easier for the reptile to peel its foot off. So instead of improving gecko's attachment through microscopic bridges, higher humidity softens the setae that coat the reptile's feet to help them hold fast and peel free with ease.
IF REPORTING ON THIS STORY, PLEASE MENTION THE JOURNAL OF EXPERIMENTAL BIOLOGY AS THE SOURCE AND, IF REPORTING ONLINE, PLEASE CARRY A LINK TO: http://jeb.biologists.org
REFERENCE: Puthoff, J. B., Prowse, M. S., Wilkinson, M. and Autumn, K. (2010). Changes in materials properties explain the effects of humidity on gecko adhesion. J. Exp. Biol. 213, 3699-3704.
This article is posted on this site to give advance access to other authorised media who may wish to report on this story. Full attribution is required, and if reporting online a link to jeb.biologists.com is also required. The story posted here is COPYRIGHTED. Therefore advance permission is required before any and every reproduction of each article in full. PLEASE CONTACT firstname.lastname@example.org
Ion treatments for cardiac arrhythmia — Non-invasive alternative to catheter-based surgery
20.01.2017 | GSI Helmholtzzentrum für Schwerionenforschung GmbH
Seeking structure with metagenome sequences
20.01.2017 | DOE/Joint Genome Institute
An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...
Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
19.01.2017 | Event News
10.01.2017 | Event News
09.01.2017 | Event News
20.01.2017 | Awards Funding
20.01.2017 | Materials Sciences
20.01.2017 | Life Sciences