Meeting Abstract

23.5  Monday, Jan. 5  Smart foot secretion – insects dont slip! DIRKS, JH*; CLEMENTE, CJ; FEDERLE, W; University of Cambridge; University of Cambridge; University of Cambridge jhd32@cam.ac.uk

Many insects are able to cling to surfaces using adhesive organs on their feet. Adhesion is mediated by thin films of fluid secretion, which help to compensate surface roughness and give rise to capillary and viscous forces.
Even though a continuous fluid film between the pad and the surface is expected to lubricate and cause sliding, the shear forces of insect pads strongly exceed adhesive forces, and pads can even produce considerable static friction on smooth substrates. This suggests that the fluid secretion itself plays a role in the resistance against shear.
Interference microscopy revealed that the secretion is not a simple fluid but a water-in-oil emulsion. It is known that many emulsions show non-Newtonian properties and only begin to flow above a certain yield point. We therefore hypothesized that the insects two-phasic adhesive secretion is a mechanism to prevent sliding.
To test this idea, we used water-absorbing polymeric substrates that selectively reduce the volume of the hydrophilic component in the emulsion, thus changing the emulsion’s phase ratio. Single-leg force measurements of stick insects (C. morosus) on more or less absorbing substrates made of the same material demonstrated that pads produced significantly higher friction and shear stress when more droplets of the watery component were present in the adhesive fluid.
Our results suggest that the two-phasic nature of insect pad secretion is an adaptation for combining capillary adhesion with resistance against sliding.
An emulsion-based artificial adhesive may also provide an interesting alternative to many conventional adhesives.