Clinging of Geckos on Smooth Surfaces

* Motivation

Geckos have the ability to cling on smooth surfaces with their toe pads (fig. 1). They easily move along vertical walls and are even able to hang upside down. As it is widely accepted in the scientific community, the clinging on - even optically flat - surfaces relies on Van der Vaals forces. The necessary large contact area is provided by a huge number of tiny hairs (setae) on the toe pads, as shown in fig. 2.

Our aim is to investigate these forces and come up with a model explaining the clamping mechanism. For the future, one might think of building synthetic clinging pads and for climbing robots.

    Fig. 1: Bottom view of a gecko's rear left leg during clinging on a glass plate (courtesy of Tonya Hsieh)

Fig. 2: SEM-picture of setae bundles (courtesy of Kellar Autumn)

    * Approach

    We measure the adhesion force at different levels of contact area, e.g. on the setae (pad hair) level, on extended setae areas or at the whole toe. Therefore we use compliant structures and visually determine their spatial deformation. Fig. 3 shows an experiment where we brought a single setae into contact with a bonding wire with a diameter of 25 micron and a lenght of 10 mm -> k = 4x10e-6 N/m. Fig. 3, left, shows the undeformed wire, fig. 3, middle, the maximum deformation due to tangential motion and fig. 3, right, the position after the wire detached and flipped back to its original configuration. This experiment yields adhesion forces of 350 nN +/- 100 nN.

More accurate data is provided by using calibrated MEMS force sensors, such as piezoresistive cantilevers from an atomic force microscope (AFM), shown in fig. 4. At the moment, we are able to record 1D force information only and rely on precise geometric models to extract the desired quantities.

Fig. 4: Gecko seta touching the force sensor (AFM cantilever with a silicon block glued on its front section).

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Zesch Wolfgang, last updated June 11, 2000