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Ultrahydrophobicity indicates a non-adhesive default state in gecko setae

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Abstract

Geckos may represent the world’s most demanding adhesives application. The adhesive setae on the toes of climbing geckos must adhere strongly yet avoid fouling or attachment at inappropriate times. We tested the hypothesis that gecko setae are non-adhesive in their unloaded default state by comparing the water droplet contact angle (θ) of isolated setal arrays to the smooth surface of eye spectacle scales of tokay geckos (Gekko gecko). At equilibrium, θ was 98.3 ± 3.4° in spectacle scales of live geckos and 93.3 ± 3.5° in isolated spectacles. Isolated setal arrays were ultrahydrophobic, with θ of 160.6 ± 1.3° (means ± SD). The difference in θ of setal arrays and smooth spectacles indicates a very low contact fraction. Using Cassie’s law of surface wettability, we infer that less than 6.6% of the surface of unloaded setae is solid and at least 93.4% is air space. We calculated that the contact fraction must increase from 6.6% in the unloaded state to 46% in the loaded state to account for previously measured values of adhesion. Thus gecko setae may be non-sticky by default because only a very small contact fraction is possible without mechanically deforming the setal array.

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Abbreviations

E :

Young’s modulus

E eff :

Effective modulus

f solid :

Fraction of surface area occupied by solid; contact fraction

f air :

Fraction of surface area occupied by air

γ :

Surface energy

PSA:

Pressure sensitive adhesive

PTFE:

Polytetrafluoroethylene

θ :

Water droplet contact angle

θ eq :

Equilibrium water droplet contact angle

θ ad :

Advancing water droplet contact angle

θ rec :

Receding water droplet contact angle

W :

Adhesion energy

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Acknowledgments

We thank Jon Barnes, Andrew Dittmore, Valeurie Friedman, Bill Geisler, Stas Gorb, Jacob Israelachvili, Allen Liu, Carmel Majidi, and an anonymous reviewer. Supported by DARPA N66001-03-C-8045, DARPA N660001-00-C-8047, NSF-NIRT 0304730, and Johnson & Johnson/DePuy Mitek Corporation. This study complies with “Principles of animal care,” publication no. 86-23, revised 1985 of the National Institute of Health, and also with the current laws of the USA.

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  1. Wendy Hansen

    Present address: Biophysics Graduate Group, University of California, Berkeley, CA, USA

Authors and Affiliations

  1. Department of Biology, Lewis and Clark College, 0615 SW Palatine Hill Road, Portland, OR, 97219-7899, USA

    Kellar Autumn & Wendy Hansen

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  1. Kellar Autumn
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Correspondence to Kellar Autumn.

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Autumn, K., Hansen, W. Ultrahydrophobicity indicates a non-adhesive default state in gecko setae. J Comp Physiol A 192, 1205–1212 (2006). https://doi.org/10.1007/s00359-006-0149-y

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