Probing Friction Forces Using Gecko Materials
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PROBING FRICTION FORCES USING GECKO MATERIALS J. B. Puthoff1, M. Holbrook2, M. J. Wilkinson1, and K. Autumn1 1 2
Department of Biology, Lewis & Clark College, Portland, OR 97219, USA Department of Physics, Lewis & Clark College, Portland, OR 97219, USA
ABSTRACT Geckos can cling to almost any surface using dense arrays of microscopic hierarchical hairs called setae. The flat, regular, terminal branches of the setae adhere by the van der Waals dispersion force, and the mechanics of the gecko attachment scheme are a current topic among biologists and researchers in smart materials for adhesion. We studied the friction behavior of natural gecko arrays. Our experiments demonstrate the presence of velocity strengthening dynamic friction over the range of velocities from 5×10–4 to 158 mm/s and a range of specimen elastic moduli from 1.1 to 3.6 GPa. From these dynamic experiments, we calculate low-v activation volumes between 1500 and 3000 nm3. Since these volumes are 3 orders of magnitude larger then are typical for bulk materials, we conclude that there is weak coupling between individual sliding contacts in the gecko system. INTRODUCTION The gecko climbing system is based around arrays of branched β-keratin hairs (setae) on the animals’ toes [1]. These hairs, 3-4 µm in diameter at the base, bifurcate to a depth of n = 5-6, and the terminal branches are tipped with flat, triangular pads (spatulae) 150 nm wide. These nanoscopic pads produce nearly ideal contacts with a substrate and adhere by van der Waals dispersion forces [2]. Important capabilities of this system, such as controllability [3, 4], self-cleaning [5], and surface roughness tolerance [6, 7], emerge from the mesoscopic mechanics that operate between the spatula and toe level. These capabilities are the inspiration for a large number of synthetic adhesives fabricated from structured polymers [2, 8, 9]. Both the natural and synthetic fibrillar adhesives interact with a substrate through a large number of individually uniform contacts. The multiplicity of these contacts produces considerable net adhesion through the contact-splitting effect [10]. Compared to the multi-contact interfaces [11] between different bulk materials with surface roughness, the interface between a fibrillar adhesive and a flat substrate is well-defined and possesses some regularity. The latter system might be likened to a large number of semiindependent AFM tips engaged with the underlying surface. When a dense fibrillar material slides across a substrate, there will be a fraction of the total population of contacts that are attached to the substrate; the rest will be in the process of slipping. This fraction N/N0, where N is the number of elements that are stuck out of a total number of contacts N0, determines the instantaneous array-level frictional force Farray. N/N0 depends on the imposed velocity v according to [12] N 1 . (1) (v ) = N0 1+ v / v * In Equation 1, v* = λ/τ0 is a characteristic velocity related to the characteristic slip distance for an individual contact λ and the dur
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