Meeting Abstract
88.1 Thursday, Jan. 7 Effects of humidity on the mechanical properties of gecko setae PROWSE, M; WILKINSON, M; MAYER, G; AUTUMN, K*; University of Washington, Seattle; Lewis and Clark College; University of Washington, Seattle; Lewis and Clark College autumn@lclark.edu
The mechanism of adhesion in gecko setae has been the topic of scientific scrutiny for over two centuries. Previously, we discovered that van der Waals forces are sufficient for strong adhesion and friction in gecko setae, and that water-based capillary adhesion is not required. A van der Waals mechanism does not rule out a role for water in gecko adhesion under some conditions. Other studies have shown that adhesion increases at high relative humidity (RH), raising the possibility that capillary forces or other surface hydration effects may contribute to adhesion. However, the presence of true capillary forces involving a water meniscus is difficult to reconcile with prior data showing only sub-nanometer gaps between spatula and substrate. How then can a humidity-related increase in adhesion and friction be explained? It is well known that the stiffness and damping of structural proteins such as keratins are affected strongly by humidity. In this study we tested the hypothesis that an increase in RH causes changes in the mechanical properties of setal keratin that promote adhesion and friction. We measured the effect of RH on forces of deformation and dynamic mechanical response of single isolated tokay gecko setae in tension. The mechanical properties of gecko setae were affected strongly by RH. The complex elastic modulus (measured at 5 Hz) of a single seta at 80%RH was 1225 MPa, only 39% of the value when dry, 3095 MPa. The loss tangent increased significantly with humidity, suggesting that viscoelastic losses increase due to water absorption. The changes in mechanical properties were concomitant with increases in contact forces, supporting the hypothesis that an increase in RH softens setal keratin, which increases adhesion and friction. Support: NSF-IOS-0847953 (KA), NSF-NBM-0900723 (KA).
