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Comparative Study
. 2006 Dec 19;103(51):19320-5.
doi: 10.1073/pnas.0608841103. Epub 2006 Dec 5.

Adhesion and Friction in Gecko Toe Attachment and Detachment

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Free PMC article
Comparative Study

Adhesion and Friction in Gecko Toe Attachment and Detachment

Yu Tian et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

Geckos can run rapidly on walls and ceilings, requiring high friction forces (on walls) and adhesion forces (on ceilings), with typical step intervals of approximately 20 ms. The rapid switching between gecko foot attachment and detachment is analyzed theoretically based on a tape model that incorporates the adhesion and friction forces originating from the van der Waals forces between the submicron-sized spatulae and the substrate, which are controlled by the (macroscopic) actions of the gecko toes. The pulling force of a spatula along its shaft with an angle between theta 0 and 90 degrees to the substrate, has a "normal adhesion force" contribution, produced at the spatula-substrate bifurcation zone, and a "lateral friction force" contribution from the part of spatula still in contact with the substrate. High net friction and adhesion forces on the whole gecko are obtained by rolling down and gripping the toes inward to realize small pulling angles between the large number of spatulae in contact with the substrate. To detach, the high adhesion/friction is rapidly reduced to a very low value by rolling the toes upward and backward, which, mediated by the lever function of the setal shaft, peels the spatulae off perpendicularly from the substrates. By these mechanisms, both the adhesion and friction forces of geckos can be changed over three orders of magnitude, allowing for the swift attachment and detachment during gecko motion. The results have obvious implications for the fabrication of dry adhesives and robotic systems inspired by the gecko's locomotion mechanism.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
The hierarchical structures of geckos. (af) Structures shown in the order of decreasing size. (gi) The various forces and other parameters used in the equations of this paper. The angles θ are often small (<45°) (7, 8), which makes the friction force contributions nonnegligible in the forces associated with gecko biomechanics.
Fig. 2.
Fig. 2.
The interaction between two surfaces. (a) The origin of the lateral friction force Ff or FL from a consideration of the periodic surface interaction potential along the x direction. The period may be a lattice dimension or mean distance between asperities (31), and the forces themselves may be due to van der Waals or some other interactions. (b) The surface–surface potential in the normal (z direction) to the substrate, which determines the normal attractive or adhesion force, Fn or FvdW. In each case, there is a maximum or critical force which, when exceeded, the surfaces move, either laterally (frictional sliding when Ff = Ffmax) or normally (detachment when FvdW = FvdWmax).
Fig. 3.
Fig. 3.
Geometric parameters of a spatula when pulled at different angles.
Fig. 4.
Fig. 4.
The normalized adhesion force at different pulling angles, θ, as determined by Eqs. 15 and 16.
Fig. 5.
Fig. 5.
The absolute values of the normal (adhesion) force component FvdW, the lateral (friction) force component Ff, the net pulling force F(θ), and the maximum friction force Ffmax that can be obtained from a spatula pad in contact with a substrate (shaded band).
Fig. 6.
Fig. 6.
Total normal adhesion force Fn(θ) (a) and total lateral force FL(θ) (b) of a single spatula, and the contributions from FvdW and Ff to the them as given by Eqs. 19 and 20. Note that, according to Eqs. 9 and 10, Ff = FvdW/tan θ.
Fig. 7.
Fig. 7.
Sketches of attachment and detachment of a single seta by (a) approaching the substrate rolling (or gripping) in (b) and rolling (or peeling) out (c) the toes. The image in c Left is from the left back foot in Fig. 1a.

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