Dynamic friction in natural and synthetic gecko setal arrays

Geckos can cling to almost any surface using dense arrays of microscopic, hierarchical setae. The flat, terminal branches of the setae adhere by the van der Waals dispersion force, and the mechanics of the gecko attachment system are a current topic among biologists and researchers of smart materials for adhesion. We studied the interaction between shear velocity (v = 0.0005 mm s(-1) to 158 mm s(-1)) and materials properties on dynamic friction of isolated natural gecko setal arrays. We varied the materials properties (complex modulus) of the setal beta-keratin by adjusting atmospheric humidity (RH). Alongside the natural material, we performed similar experiments on synthetic arrays of polyurethane micropillars. Our experiments demonstrate the presence of two regimes in the friction force (F) vs. velocity behavior of the natural adhesives: a materials/RH-dependent domain exists at low v (< 1 mm s(-1)) and a materials/RH-independent domain at higher v. At intermediate velocities, F(v) curves at different RH converge to an RH-independent value. From the dynamic experiments on natural arrays, we calculated a high-v activation volume (V*) of (90.1 +/- 0.3) nm(3). V* gives an indication of the strength of coupling between sliding elements. Velocity strengthening occurred in synthetic arrays. However, in contrast to the natural material, strengthening of adhesion and friction of synthetic gecko setae occurred at low v and weakened at high v. Activation volumes calculated for the synthetic arrays indicate weaker coupling. These results indicate (i) that the theory of state-rate friction (SRF) can adequately describe the behavior of sliding fibrillar adhesives and (ii) that the macroscopic performance of natural and synthetic setal arrays, when interpreted with an SRF model, provides some insight into the microscopic dynamics of frictional sliding.