Ultrahydrophobicity indicates a non-adhesive default state in gecko setae

Original Paper

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.

Keywords

Adhesion Contact mechanics Locomotion Reptilia Nanotechnology 

Abbreviations

E

Young’s modulus

Eeff

Effective modulus

fsolid

Fraction of surface area occupied by solid; contact fraction

fair

Fraction of surface area occupied by air

γ

Surface energy

PSA

Pressure sensitive adhesive

PTFE

Polytetrafluoroethylene

θ

Water droplet contact angle

θeq

Equilibrium water droplet contact angle

θad

Advancing water droplet contact angle

θrec

Receding water droplet contact angle

W

Adhesion energy

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  1. 1.Department of BiologyLewis and Clark College, 0615 SW Palatine Hill RoadPortlandUSA
  2. 2.Biophysics Graduate GroupUniversity of CaliforniaBerkeleyUSA

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