Why the thinnest sticky hairs rule

Many creatures can walk on walls and ceilings thanks to slender hairs on their feet. Intermolecular attractions between a surface and the tips of those plentiful hairs, or setules, on geckos, spiders, and other creatures add up to a strong adhesive force (SN: 6/7/03, p. 356: Caught on Tape: Gecko-inspired adhesive is superstrong).

TOEHOLDS. This spider is shown positioned above a micrograph of the fine hairs, viewed from below, on the bottoms of its feet. S. Gorb/ Max Planck Institute for Metals Research

Now, a pair of theorists in Germany has calculated an optimal shape for the tips of those hairs. Working with an idealized cylindrical model of a setule, Huajian Gao and Haimin Yao of the Max Planck Institute for Metals Research in Stuttgart, Germany, determined that the tip of such a hair should modestly dimple inward to achieve the greatest adhesion. Further analysis showed, however, that any divergence from that optimal shape in their model could imbalance adhesive forces enough to crack the tip.

That susceptibility to damage would seem to pose a problem for living creatures, which rarely grow identical microstructures. Gao and Yao subsequently calculated that both the superiority of the optimal shape and its vulnerability to damage don’t hold for the thinnest of hairs, such as the gecko’s setules which have girths about a thousandth of those of human hairs. As Gao puts it, “The smaller the size, the less important the shape.”

The scientists report their results in the May 25 Proceedings of the National Academy of Sciences.

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