Stickiness takes on new shapes
Model of how animals stick to walls could aid engineers.
Animals such as insects and lizards employ an impressive range of tools to achieve surface-scaling superpowers. The structures they use include everything from flat attachment pads, used by grasshoppers, to microscopic hairs, which cover the feet of geckos.
But scientists have never had the maths to calculate how well these different shapes perform. Now, new equations enable them to compare different shapes, which could allow us to design artificial surfaces that stick to walls better than anything found in nature, according to researchers.
Whereas some animals, including flies and beetles, produce sticky secretions to glue themselves to vertical surfaces, others, such as lizards, use a dry system. A very sticky glue is harder to release from a surface, so animals sometimes opt for dry systems that make it easier for them to detach when required.
Exactly how such systems worked was a mystery until two years ago, when scientists proved that geckos use intermolecular attractions, known as van der Waals forces, to climb walls2.
The lizards take advantages of these normally weak forces by maximizing the surface area between their feet and the wall. The bottoms of their feet are covered in millions of microscopic hairs, each of which splits off into hundreds of tips just nanometres across. The increased area of contact makes the intermolecular attractions between hair and wall significant enough to support the gecko.
Until now, the mathematical models available have assumed that all the contacts were spherical. "If you look in nature, this is not what you really find," says materials scientist Ralph Spolenak.
So he and his colleagues at the Max Planck Institute for Metals Research in Stuttgart reworked the equations to take account of the range of shapes used by different animals, including hairs, flat pads, rods and suction cups. Tweaking the shape of the contact increased stickiness by more than an order of magnitude, says Spolenak, now at the Federal Institute of Technology in Zurich.
Unsurprisingly, the model found that if you assume a completely even surface, flat contacts perform best because they maximise the amount of surface contact.
Completely smooth surfaces rarely appear in natural environments, but Spolenak and his colleagues found that the grasshopper, which has relatively flat contact pads, gets around this by having slightly flexible pads that can conform to uneven spots.
But the researchers recommend that engineers seeking to design artificial coatings should investigate doughnut-type shapes. To increase the stickiness, you need to split the contact elements into tinier and tinier shapes, and their model showed that toroidal contacts are the most efficient contacts on the smallest scales. The researchers report their results in the Proceedings of the Royal Society A1.
Shape of things to come
"This is an important theoretical advance," says Kellar Autumn, a biologist at Lewis and Clark College in Portland, Oregon, who previously showed how geckos run up smooth surfaces.
He says the findings could help explain how the tiny hair tips, or setae, of lizards detach from surfaces, and how the animals keep dirt from sticking to their feet.
Spolenak believes that his model will help engineers design specialized adhesive materials for spaces both inside and outside. Choosing contacts with the right dimension and shape for different situations could aid robots climbing rocks on distant planets or give us sticky-backed pictures that can be easily moved around a room.
- Spolenak R., et al. Proc. R. Soc. Lond. A, 460. 1 - 15 (2004).
- Autumn K., et al. PNAS, 99. 12252 - 12256 (2002).
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