December 6, 2006 | David F. Coppedge

The Physics of Gecko Toes

Why would anyone want to know the details of physical forces when gecko feet walk on glass?  Here’s why: “The results have obvious implications for the fabrication of dry adhesives and robotic systems inspired by the gecko’s locomotion mechanism.”  A team of scientists from Santa Barbara and China watched gecko toes peel off glass and wrote up their results in PNAS.1  “The extraordinary climbing ability of geckos is considered a remarkable design of nature that is attributed to the fine structure of its toes,” the paper began.  As the abstract describes the physics involved, it’s more complex than one would think:

Geckos can run rapidly on walls and ceilings, requiring high friction forces (on walls) and adhesion forces (on ceilings), with typical step intervals of {approx} 20 ms.  The rapid switching between gecko foot attachment and detachment is analyzed theoretically based on a tape model that incorporates the adhesion and friction forces originating from the van der Waals forces between the submicron-sized spatulae and the substrate, which are controlled by the (macroscopic) actions of the gecko toes.  The pulling force of a spatula along its shaft with an angle {theta} between 0 and 90° to the substrate, has a “normal adhesion force” contribution, produced at the spatula-substrate bifurcation zone, and a “lateral friction force” contribution from the part of spatula still in contact with the substrate.  High net friction and adhesion forces on the whole gecko are obtained by rolling down and gripping the toes inward to realize small pulling angles {theta} between the large number of spatulae in contact with the substrate.  To detach, the high adhesion/friction is rapidly reduced to a very low value by rolling the toes upward and backward, which, mediated by the lever function of the setal shaft, peels the spatulae off perpendicularly from the substrates.  By these mechanisms, both the adhesion and friction forces of geckos can be changed over three orders of magnitude, allowing for the swift attachment and detachment during gecko motion.

Will this be on the test?

1Tian et al, “Adhesion and friction in gecko toe attachment and detachment,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0608841103, published online before print December 5, 2006.

You probably didn’t realize that geckos were such good physicists.  What’s really cool is thinking about the day we will have gecko boots and can play like Spiderman.

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