Energy Storage, Release, And Dissipation In The Gecko Adhesion System
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ENERGY STORAGE, RELEASE, AND DISSIPATION IN THE GECKO ADHESION SYSTEM Jonathan B. Puthoff1, Michael Prowse2, Matt J. Wilkinson1, and Kellar Autumn1,2 1
Department of Biology, Lewis & Clark College, Portland, OR 97219, USA
2
Materials Science & Eng. Dept., University of Washington, Seattle, WA 98195, USA
ABSTRACT Different types of biological adhesion can be categorized according to the length scales, structures, and materials involved. The setal adhesion system of the gekkonid lizards occupies a hierarchy of scales from the toes (~ 1 cm) to the terminal spatular pads on the setal branches (~ 100 nm). This unique combination of scale and foot-hair morphology allow the animal robust, controllable, and near-universal adhesion via van der Waals attraction, but it is also apparent that the mechanical behavior of the β-keratin plays an important role in an animal's climbing ability. Experimental results show a fourfold increase in the viscoelastic loss tangent of β-keratin, alongside a substantial increase in adhesion of setal arrays, over a range of relative humidity from 10 to 80%. A model of single-spatular deformation predicts that the elastic energy stored in the setal branches, energy which is not completely recovered on detachment, is strongly influenced by these properties changes. The enhanced dissipation characteristics of the system explain the effects of environmental humidity on the clinging ability of geckos. INTRODUCTION Evolution produced in geckos a superlative climbing system which affords them tremendous mobility in environments with mixed terrain. This system is metabolically efficient [1], can be engaged/released quickly and reliably [2, 3], and possesses selfcleaning properties [4]. It is also the inspiration for a large number of synthetic analogs built from structured polymers [5-7]. These analogs have the potential to become versatile and reusable adhesives for a wide range of domestic or industrial uses. The common principle at work in these natural and synthetic fibrillar adhesion systems is the attraction of individual fibers to the substrate by weak intermolecular (van der Waals) forces [5]. The multiplicity of these individually minute attractions can produce considerable net adhesion; this is the “contact splitting effect” [8]. (Conventional pressure sensitive adhesives exploit van der Waals forces over broad interface regions accommodated by deformation in a layer of soft material.) In most mechanical approaches to the adhesion between bodies, the van der Waals interactions are represented by the work of adhesion Wad. Wad is the energy required to generate a unit area of unadhered surface between two contacting bodies. The energy required to overcome Wad is typically supplied by the applied loads or stored elastic energy in the material [9, 10]. There are, however, dissipative mechanisms at work which can reduce the energy available for detachment, effectively strengthening the interface. The influence of these mechanisms on fibrillar adhesive systems is mostly unexplored. We performed
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