Frictional adhesion: a new angle on gecko attachment

Abstract

SUMMARY Directional arrays of branched microscopic setae constitute a dry adhesive on the toes of pad-bearing geckos, nature's supreme climbers. Geckos are easily and rapidly able to detach their toes as they climb. There are two known mechanisms of detachment: (1) on the microscale, the seta detaches when the shaft reaches a critical angle with the substrate, and (2) on the macroscale, geckos hyperextend their toes, apparently peeling like tape. This raises the question of how geckos prevent detachment while inverted on the ceiling, where body weight should cause toes to peel and setal angles to increase. Geckos use opposing feet and toes while inverted, possibly to maintain shear forces that prevent detachment of setae or peeling of toes. If detachment occurs by macroscale peeling of toes, the peel angle should monotonically decrease with applied force. In contrast, if adhesive force is limited by microscale detachment of setae at a critical angle, the toe detachment angle should be independent of applied force. We tested the hypothesis that adhesion is increased by shear force in isolated setal arrays and live gecko toes. We also tested the corollary hypotheses that (1) adhesion in toes and arrays is limited as on the microscale by a critical angle, or (2)on the macroscale by adhesive strength as predicted for adhesive tapes. We found that adhesion depended directly on shear force, and was independent of detachment angle. Therefore we reject the hypothesis that gecko toes peel like tape. The linear relation between adhesion and shear force is consistent with a critical angle of release in live gecko toes and isolated setal arrays, and also with our prior observations of single setae. We introduced a new model,frictional adhesion, for gecko pad attachment and compared it to existing models of adhesive contacts. In an analysis of clinging stability of a gecko on an inclined plane each adhesive model predicted a different force control strategy. The frictional adhesion model provides an explanation for the very low detachment forces observed in climbing geckos that does not depend on toe peeling.

Macroscale adhesion of gecko setae reflects nanoscale differences in subsurface composition

“…figure 1). Reported adhesion and friction values are taken from the force sensor readout during the steady-state portion of each test [31]. We can specify parameters such as the drag velocity v and the approach distance during the experiment, as well as the temperature and humidity in the chamber.…”

Sliding-induced adhesion of stiff polymer microfibre arrays. II. Microscale behaviour

“…II B. Schubert et al 847 In tests on the natural setal arrays (Autumn et al 2006b), frictional adhesion is exhibited during the drag phase, as well as the pull phase. This is because the natural setae pull into tension while dragging.…”

Rectangle-capped and tilted micropillar array for enhanced anisotropic anti-shearing in biomimetic adhesion

“…Geckos are an excellent example of creatures able to achieve controlled adhesion [5]. When loaded in forward direction, gecko's adhesive toe pads stick with a strong force to the surface, however, they can also be easily peeled off from the surface when loaded in the reverse direction [6]. This directional or anisotropic adhesive behaviour that allows smart or controllable surface attachment and detachment has inspired many applications such as climbing robots [7], clean transportation systems [8,9] and micro-transfer printing mechanisms [10].…”