Meeting Abstract

20.2  Tuesday, Jan. 4  Attachment in smooth insect pads: the influence of surface energy LABONTE, D.*; BULLOCK, J. M. R.; FEDERLE, W.; University Of Applied Sciences, Bremen; Dept. of Zoology, University of Cambridge; Dept. of Zoology, University of Cambridge labontedavid@web.de

All insects studied to date adhere to surfaces via small amounts of fluid secreted into the adhesive contact zone. In insects with smooth adhesive pads, this fluid is an emulsion consisting of watery droplets dispersed in an oily continuous phase. To understand its effect on attachment, we measured friction, adhesion and contact area in single adhesive pads of stick insects (Carausius morosus) on six surfaces designed to cover a wide range of surface energies. The surfaces were produced by depositing self-assembled monolayers of Octadecyltrichlorosilane (OTS) on glass substrates. Surface energy was varied by exposing the OTS-layers to plasma oxygen for different times. The resulting surface energies ranged from 22.9 to 67.8 mN m^-1 with little variation in its dispersive component, but strong variation in polar surface energy. In stick insects, shear force was low not only on the low-energy hydrophobic OTS substrates but also on the most polar, high-energy surface. Forces were up to four times higher on substrates with intermediate surface energy. The effect of surface energy suggests that attachment is either mediated by direct contacts of the cuticle with the surface or by specific effects of wettability on the structure of the adhesive emulsion. Interference reflection microscopy of the contact zone revealed a tendency of the aqueous droplets to coagulate and cover large areas on the highly polar surfaces, explaining the reduced shear forces on these substrates. We will compare our results to the performance of the hairy adhesive system of a leaf beetle (Gastrophysa viridula).