Sticking like sticky tape: tree frogs use friction forces to enhance attachment on overhanging surfaces

Abstract

To live and clamber about in an arboreal habitat, tree frogs have evolved adhesive pads on their toes. In addition, they often have long and slender legs to facilitate not only long jumps, but also to bridge gaps between leaves when climbing. Both adhesive pads and long limbs are used in conjunction, as we will show in this study. Previous research has shown that tree frogs change from a crouched posture (where the limbs are close to the body) to a sprawled posture with extended limbs when clinging on to steeper inclines such as vertical or overhanging slopes. We investigated this change in posture in White's tree frogs (Litoria caerulea) by challenging the frogs to cling onto a tiltable platform. The platform consisted of an array of 24 three-dimensional force transducers, which allowed us to measure the ground reaction forces of the frogs during a tilt. Starting from a crouched resting position, the normal forces on the forelimbs changed sign and became increasingly negative with increasing slope angle of the platform. At about 1068 + 128, tilt of the platform the frogs reacted by extending one or two of their limbs outwards. At a steeper angle (1318 + 118), the frogs spread out all their limbs sideways, with the hindlimbs stretched out to their maximum reach. Although the extension was strongest in the lateral direction, limbs were significantly extended in the fore-aft direction as well. With the extension of the limbs, the lateral forces increased relative to the normal forces. The large contribution of the in-plane forces helped to keep the angle between the force vector and the platform small. The Kendall theory for the peeling of adhesive tape predicts that smaller peel angles lead to higher attachment forces. We compare our data with the predictions of the Kendall model and discuss possible implications of the sliding of the pads on the surface. The forces were indeed much larger for smaller angles and thus can be explained by peeling theory.

Whispering Gallery Mode Biosensor

“…The term frictional adhesion has been coined to describe adhesion in geckos. It is unclear whether this term should also be applied to tree frog adhesion, but there is certainly A recent study by Endlein et al [26] has explained the frog's spreading behavior by studying the underlying ground reaction forces during this behavior. By using an array of multiple force transducers embedded in a walkway, the authors were able to record the ground reaction forces of a frog clinging on to the platform at varying slopes.…”

Recent advances in gecko adhesion and friction mechanisms and development of gecko-inspired dry adhesive surfaces

“…Using a finite element model, Peng et al [51] analyzed the change in the peel zone length and the peel force at different peel angles, Young's moduli, and spatula thicknesses. Endlein et al found that the adhesion forces of tree frogs can also be explained by the peeling theory [57].…”

Wet Chemical Processing

“…It is unclear whether this term should also be applied to tree frog adhesion, but there is certainly interplay between friction and adhesion forces in frog toe pads too. A recent study by Endlein et al [26] has explained the frog's spreading behavior by studying the underlying ground reaction forces during this behavior. By using an array of multiple force transducers embedded in a walkway, the authors were able to record the ground reaction forces of a frog clinging on to the platform at varying slopes.…”