AUTUMN, K.; Lewis & Clark College: Anti-adhesive properties of gecko setae

Gecko toe pads operate under perhaps the most severe conditions of any adhesive application. The adhesive setae on the toes of climbing geckos must adhere strongly yet detach rapidly and avoid fouling or attachment at inappropriate times. Gecko setae do not adhere spontaneously to surfaces, but instead require a mechanical program for attachment. Following preload and drag steps, a single seta on a hydrophobic Si MEMs cantilever can generate a very strong adhesive force of up to 40µN over approx. 43 �² of area, or approx. 917 kPa (9 atm) of adhesive stress. Paradoxical as it may seem given that setae are so strongly adhesive, there is growing evidence that gecko setae are also strongly anti-adhesive. Unlike adhesive tapes, gecko setae do not self-adhere. Pushing the setal surfaces of a gecko�s feet together does not result in strong adhesion. Unlike conventional adhesives, gecko setae do not stay dirty after particulate contamination. New results comparing water droplet contact angle (θ) of isolated setal arrays to the smooth surface of eye spectacle scales of tokays (Gekko gecko) suggest that gecko setae are strongly anti-adhesive in their unloaded default state. 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). By Cassie�s law of surface wettability, the difference in θ of setal arrays and smooth spectacles indicates a very low contact fraction. Less than 6.6% of the surface of unloaded setae is solid and at least 93.4% is air space. The contact fraction must increase from 6% to 46%, or by approx. 7.5x, following preload and drag to yield 917 kPa of adhesive stress. Thus gecko setae may be nonsticky by default because only a very small contact fraction is possible without first mechanically deforming the setal array through preload and drag.