Rate-dependent frictional adhesion in natural and synthetic gecko setae

Abstract

Geckos owe their remarkable stickiness to millions of dry, hard setae on their toes. In this study, we discovered that gecko setae stick more strongly the faster they slide, and do not wear out after 30 000 cycles. This is surprising because friction between dry, hard, macroscopic materials typically decreases at the onset of sliding, and as velocity increases, friction continues to decrease because of a reduction in the number of interfacial contacts, due in part to wear. Gecko setae did not exhibit the decrease in adhesion or friction characteristic of a transition from static to kinetic contact mechanics. Instead, friction and adhesion forces increased at the onset of sliding and continued to increase with shear speed from 500 nm s 21 to 158 mm s 21. To explain how apparently fluid-like, wear-free dynamic friction and adhesion occur macroscopically in a dry, hard solid, we proposed a model based on a population of nanoscopic stick-slip events. In the model, contact elements are either in static contact or in the process of slipping to a new static contact. If stick -slip events are uncorrelated, the model further predicted that contact forces should increase to a critical velocity (V *) and then decrease at velocities greater than V*. We hypothesized that, like natural gecko setae, but unlike any conventional adhesive, gecko-like synthetic adhesives (GSAs) could adhere while sliding. To test the generality of our results and the validity of our model, we fabricated a GSA using a hard silicone polymer. While sliding, the GSA exhibited steady-state adhesion and velocity dependence similar to that of gecko setae. Observations at the interface indicated that macroscopically smooth sliding of the GSA emerged from randomly occurring stick -slip events in the population of flexible fibrils, confirming our model predictions.

Improving controllable adhesion on both rough and smooth surfaces with a hybrid electrostatic/gecko-like adhesive

“…These adhesives showed reusability out to 30 000 trials, which matches favourably with the performance of geckos [39], demonstrated a strong directional preference and were successfully deployed on a climbing robot [40].…”

Locomotion and attachment of leaf beetle larvaeGastrophysa viridula(Coleoptera, Chrysomelidae)

“…Adult G. viridula slid continuously when centrifuged, suggesting either an aquaplaning-like effect [64] or stick-and-slip reduction owing to the presence of multiple minute contacts instead of one or few larger ones [65,66]. Owing to multiple minute contacts, stickand-slip events happen at each single contact, but at different times.…”

“…Similar to sawfly larvae [58] and adult codling moths [7], beetle larvae did not slide continuously: they rather stayed at the same position until they suddenly lost contact at certain critical centrifugal force. While hairy pads [66] or sheet-like aroliae [64] start slipping when lateral forces are applied, round larval pads are more likely to tilt and roll. This would lead to sudden detachment, when the muscle strength, required for keeping the leg in position, is surpassed by centrifugal force.…”

Buckling of sheared and compressed microfibrils

“…More recently, the friction behaviour of these microfibril arrays has been investigated by different research groups (e.g. Majidi et al 2006;Bhushan & Sayer 2007;Kim et al 2007;Varenberg & Gorb 2007;Yao et al 2007;Shen et al 2008Shen et al , 2009Gravish et al 2009;Vajpayee et al 2009). In this work, we focus on synthetic bioinspired surfaces instead of fibrillar structures in small animals such as gecko.…”