title: When the going gets rough - studying the effect of surface roughness on the adhesive abilities of tree frogs.
abstract: Tree frogs need to adhere to surfaces of various roughnesses in their natural habitats; these include bark, leaves and rocks. Rough surfaces can alter the effectiveness of their toe pads, due to factors such as a change of real contact area and abrasion of the pad epithelium. Here, we tested the effect of surface roughness on the attachment abilities of the tree frogLitoria caerulea. This was done by testing shear and adhesive forces on artificial surfaces with controlled roughness, both on single toe pads and whole animal scales. It was shown that frogs can stick 2-3 times better on small scale roughnesses (3-6 µm asperities), producing higher adhesive and frictional forces, but relatively poorly on the larger scale roughnesses tested (58.5-562.5 µm asperities). Our experiments suggested that, on such surfaces, the pads secrete insufficient fluid to fill the space under the pad, leaving air pockets that would significantly reduce the Laplace pressure component of capillarity. Therefore, we measured how well the adhesive toe pad would conform to spherical asperities of known sizes using interference reflection microscopy. Based on experiments where the conformation of the pad to individual asperities was examined microscopically, our calculations indicate that the pad epithelium has a low elastic modulus, making it highly deformable.
authors.affiliation: Centre for Cell Engineering, Institute of Molecular Cell and Systems Biology, University of Glasgow, Glasgow, Scotland, UK.
authors.forename: Niall
authors.initials: N
authors.lastname: Crawford
authors.affiliation: Max Planck Institute for Intelligent Systems, Stuttgart, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Endlein
authors.affiliation: Max Planck Institute for Polymer Research, Mainz, Germany.
authors.forename: Jonathan T
authors.initials: JT
authors.lastname: Pham
authors.affiliation: Centre for Cell Engineering, Institute of Molecular Cell and Systems Biology, University of Glasgow, Glasgow, Scotland, UK.
authors.forename: Mathis
authors.initials: M
authors.lastname: Riehle
authors.affiliation: Centre for Cell Engineering, Institute of Molecular Cell and Systems Biology, University of Glasgow, Glasgow, Scotland, UK.
authors.forename: W Jon P
authors.initials: WJ
authors.lastname: Barnes
keywords: Litoria caerulea; adhesion; friction; roughness; tree frog