Meeting Abstract
20.1 Tuesday, Jan. 4 Surface dependent attachment in fluid-based smooth adhesive pads of insects DIRKS, JH*; FEDERLE, W; Trinity College Dublin; University of Cambridge jan-henning.dirks@tcd.ie
Smooth adhesive pads of many insects adhere via nanometre-thin films of a two-phasic fluid. This fluid fills in gaps between the pad and the surface, thereby maximises contact area and adhesion. Although previous studies have measured forces on a variety of substrates, the detailed effects of surface energy and wettability on insect attachment have remained unclear. To test the effects of the dispersive and polar components of surface energy on insect adhesion and friction forces, single-pad force measurements were performed on smooth, transparent substrates with standardised surface energies. While contact angles of footprint droplets showed only little variation between different surfaces, friction and adhesion forces were much larger on hydrophobic polymer substrates than on hydrophilic surfaces. Attachment significantly increased with the dispersive component of surface energy. Forces were negatively correlated with the polar component. The strong effect of surface energy and wettability speaks against the presence of a continuous mediating fluid layer. Instead, attachment might be mediated by a direct solid-like interaction of the cuticle with the surface, which is weakened on hydrophilic substrates by lubricating water films. Despite very similar surface energies, forces on water-absorbing polyimide surfaces were much lower than those on PMMA. This finding demonstrates that attachment forces are not only influenced by direct contacts but also by the shear-enhancing two-phasic nature of the adhesive secretion. Further work is needed to establish whether the observed effects are generally valid for insect adhesive pads, including hairy systems. In general, force measurements on substrates with standardised surface energies appear to be a very powerful tool in understanding the principles of biological adhesion.
