Insights into the Adhesive Mechanisms of Tree Frogs using Artificial Mimics

Abstract

Elastic, microstructured surfaces (hydrophobic and hydrophilic) mimicking the surface structure of tree-frog toe-pads are fabricated. Their adhesion and friction behaviour in the presence of a liquid layer is evaluated and compared to fl at controls. Tree-frog-like patterns are benefi cial for wet adhesion only if the liquid does not wet the surface. The situation is different in friction, where the surface structure lead to signifi cantly higher friction forces only if the liquid does wet the surface. Taking into account that tree-frog attachment pads are hydrophilic and that their secretion wets all kind of surfaces, our results indicate that the surface structure in tree-frog toe-pads has been developed for climbing, when shear (friction) forces are involved. These results evidence the benefi ts and limitations of the surface design (microstructure and hydrophilicity) for adhesion and friction under wet conditions.

Morphological studies of the toe pads of the rock frog,Staurois parvus(family: Ranidae) and their relevance to the development of new biomimetically inspired reversible adhesives

“…Also, as noted previously in a tree frog [23], the tops of the nanopillars are clearly concave (figure 4b, a cryo-SEM image and figure 11c, the AFM height profile), with a dimple depth in the range 6-8 nm, similar to those measured in tree frogs [22,23]. The reason for this concavity is unclear, but it is noteworthy that the shape (flat, T-shaped, concave) of the tops of fibrillar microstructures can significantly affect both adhesion and friction [11,46]. Varenberg and Gorb propose that, under appropriate circumstances, each element can act as a passive suction device, but this is disputed by Drotlef et al, who favour a combination of capillary and direct contact forces.…”

“…Although the presence of a deep circumferal groove (figure 2b) is not unique to rock frogs (WJP Barnes 2012, unpublished data on rsfs.royalsocietypublishing.org Interface Focus 5: 20140036 rhacophorid tree frogs), this groove, which surrounds the distal and lateral parts of the pads, is ideally placed to channel fluid around rather than under the pad. Studies on toe pad mimics have also investigated the importance of aspect ratio [11]. Microstructures with low aspect ratio features are more resistant to bend and collapse during shear and allow more effective establishment of direct contact; i.e.…”

“…They are absent from dorsal toe epithelium, where nanopillars are also absent ( figure 7). The role of the channels in reversible 'wet' adhesion has been recently investigated in artificial tree frog toe pad replicas rsfs.royalsocietypublishing.org Interface Focus 5: 20140036 [11]. The channel structure allows draining of a wetting fluid out of the contact surface and establishment of direct (dry) contact in the presence of shear forces (i.e.…”

“…Many of these biomimetic studies have been aimed at replicating the dry adhesive mechanisms of geckos (e.g. [4][5][6][7]), but tree frog adhesion has also been highlighted as having potential for the development of new smart adhesives able to adhere under wet conditions [8][9][10][11].…”

Controllable load sharing for soft adhesive interfaces on three-dimensional surfaces

“…Soft PDMS molds containing cylindrical cavities were obtained by replicating SU-8 lithographic templates as previously reported (55,56). Sylgard 184 siloxane base and curing agent were mixed in a 10:1 ratio, degassed, and casted on the PDMS mold shaped in the cylindrical cavities.…”