Evidence for self-cleaning in fluid-based smooth and hairy adhesive systems of insects.

Insects possess adhesive organs that allow attachment to diverse surfaces. Efficient adhesion must be retained throughout their lifetime even when pads are exposed to contamination. Many insects groom their adhesive structures, but it is possible that self-cleaning properties also play an important role. We measured attachment forces of insect pads on glass after contamination with microspheres and found that both smooth pads (stick insects: Carausius morosus) and hairy pads (dock beetles: Gastrophysa viridula) exhibit self-cleaning. Contaminated pads recovered high levels of adhesion after only eight simulated steps; this was accompanied by the deposition of spheres. Self-cleaning was strongly enhanced by shear movements, and only beetle pads showed the ability to self-clean during purely perpendicular pull-offs. Hairy pads also self-cleaned more efficiently than smooth pads for both large (45 microm) and small (1 microm) particle sizes. However, the beetles' self-cleaning was not superior to smooth pads when contaminated with 10-microm beads. This limitation of self-cleaning is explained by the coincidence of bead diameter and inter-seta distance, which caused beads to remain trapped in between setae.