Functional Morphology and Design Constraints of Smooth Adhesive Pads
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6/12/2007
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Functional
Morphology and Design Constraints of Smooth Adhesive Pads W. Jon. P. Barnes
Abstract Smooth adhesive pads are found among the arthropods, amphibians (particularly tree frogs), and in some mammals. They are used for dynamic adhesion when an animal is climbing steep or overhanging smooth surfaces. There is a need for strong attachment to avoid falling and easy detachment to enable the animal to move. This article describes the morphology and physical properties of smooth adhesive pads, stressing how there is little variation in structure, within tree frogs at least, even among pads that have evolved independently. This is clear evidence of an optimum design; best adhesion occurs when there is a continuous, thin film of fluid between the pad and the surface. Smooth adhesive pads adhere by wet adhesion, the main force component being capillarity, produced by the air/liquid interface (meniscus) around the edge of each pad. Smooth adhesive pads also produce substantial friction forces, probably because of actual contact between the pad surface and substrate (tree frogs) or nonNewtonian properties of the secreted fluid (insects). This is possible because the fluid layer beneath the pad has an average thickness of only a few nanometers. The article also discusses the scaling of adhesive force with size and, finally, implications for biomimetics.
Introduction: From Climbing to Dynamic Adhesion As any human climber knows, falling is something to be avoided, because it can lead to serious and sometimes fatal injury. At the very least, falling involves the expenditure of considerable amounts of energy to recover one’s former position. Therefore, climbing animals have evolved mechanisms, both morphological and behavioral, that minimize the risk of falling. Although standing or moving along a horizontal branch may not demand any special adaptations, vertical and inclined surfaces present unique problems, since the force of gravity will cause the animal on such a surface to fall unless it generates a reaction force between itself and the surface that is equal
and opposite to the force generated by the gravitational pull.1 Animals can generate forces that oppose gravity in three different ways. The first of these is by interlocking the surface of the animal with that of the support. This is how claws work, either by catching on preexisting surface irregularities or by pushing into the surface. Second, animals may use friction. Friction involves both microinterlocking and intermolecular forces between materials at points of contact. In general, friction is proportional to load but independent of the area of macroscopic contact. Friction is most effective when both surfaces have a degree of roughness, but, by itself, is only possible at
MRS BULLETIN • VOLUME 32 • JUNE 2007 • www/mrs.org/bulletin
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