Frictional and elastic energy in gecko adhesive detachment

Abstract

Geckos use millions of adhesive setae on their toes to climb vertical surfaces at speeds of over 1 m s K1 . Climbing presents a significant challenge for an adhesive since it requires both strong attachment and easy, rapid removal. Conventional pressure-sensitive adhesives are either strong and difficult to remove (e.g. duct tape) or weak and easy to remove (e.g. sticky notes). We discovered that the energy required to detach adhering tokay gecko setae (W d ) is modulated by the angle (q) of a linear path of detachment. Gecko setae resist detachment when dragged towards the animal during detachment (qZ308) requiring W d Z5.0G 0.86 (s.e.) J m K2 to detach, largely due to frictional losses. This external frictional loss is analogous to viscous internal frictional losses during detachment of pressure-sensitive adhesives. We found that, remarkably, setae possess a built-in release mechanism. Setae acted as springs when loaded in tension during attachment and returned elastic energy when detached along the optimal path (qZ1308), resulting in W d ZK0.8G0.12 J m K2 . The release of elastic energy from the setal shaft probably causes spontaneous release, suggesting that curved shafts may enable easy detachment in natural, and synthetic, gecko adhesives.

Dynamic self-cleaning in gecko setae via digital hyperextension

“…For an isolated setal array, detaching strength maintains relatively constant, when the detaching angle is varied from 308 to 908 under a load-drag preloading condition [26]. Such a high adhesion force at the microscale will lead to setal jumping off during toe pad scrolling.…”

“…an angle smaller than 908). Thus, the elastic energy pre-stored in the setal shafts could not return to the interface but is instead frictionally dissipated [26] or contributes to the jump-off through higher seta adhesion force. Second, the crowded region is located behind the jump-off direction.…”

“…an angle larger than 908). On the basis of detachment measurements of isolated setal arrays in various linear directions [26], the setal jump-off will be significantly damped or prohibited. Therefore, the proximal-todistal peeling would lead only to a minimum-to-zero dynamic effect, and a maximal static self-cleaning.…”

Sliding-induced adhesion of stiff polymer microfibre arrays. II. Microscale behaviour

“…Frictional adhesion allows geckos to control the magnitude of normal adhesion through the application of shear force. This lets them easily engage and disengage their adhesive pads (Gravish et al 2008).…”

“…Adhesion properties for gecko -inspired synthetic adhesives can be understood by modelling the setae as angled cantilever beams (Sitti & Fearing 2003;Gao et al 2005;Spolenak et al 2005;Autumn et al 2006a;Gravish et al 2008). Naturally angled cantilever beams show greater compliance in the normal direction than vertical fibres, which must undergo buckling in order to conform to a surface (Autumn et al 2006a;Majidi et al 2006).…”