FEDERLE, W; FULL, RJ: Thin Liquid Films and Ant Adhesion

Some insects can produce adhesive forces equivalent to more than 100 times their own body weight on smooth surfaces. Attachment is mediated by specialized smooth or hairy cuticle pads on the (pre-)tarsus. Both the mechanism of adhesion/friction and of how insects can control their adhesive force when they walk with ‘sticky feet’ are poorly understood. Surface adhesion in Asian Weaver ants, Oecophylla smaragdina, is mediated by a thin liquid film. We studied this ant’s arolia in contact with glass and examined the footprint droplets deposited using interference reflection microscopy (IRM). Comparison of droplet images taken at two different wavelengths enabled us to calculate the fluid’s refractive index, to measure contact angles and estimate the amount of liquid deposited in a single step. Analysis of IRM images of ant pads in contact with glass surfaces yields an estimate of the fluid layer thickness, which is a critical parameter for the generation of adhesive forces by surface tension and viscosity. To investigate what role this liquid film plays for in-plane forces, we measured attachment forces of ants with a centrifuge. On a smooth turntable, the insects did not detach immediately, but gradually slid outwards. Sliding friction was linearly related to sliding velocity, but there was also a ‘static’ friction component. Velocity-dependent frictional forces were much greater at lower temperatures. No temperature dependence was found for slow, vertical detachment forces. These results suggest that viscous forces (viscosity of the adhesive liquid and/or ‘rubber friction’ of the cuticle pad) dominate in horizontal pulls, whereas other forces (capillary or intermolecular forces) are more significant in the vertical direction. Future direct measurements of the fluid’s surface tension and viscosity will provide a quantitative assessment of its contribution to observed adhesive forces.