Water repellency of hierarchically structured legs of water-walking striders and fire ants

Some insects have the ability to walk on the surface of water due to their hierarchical leg structure and wax coating. In this study, resistance forces of water strider and fire ant legs to submersion in water, under their various orientations, were quantified using a high-sensitivity microbalance. Legs oriented parallel to the water surface could support up to ten times as much force before immersion compared to legs in a perpendicular orientation. Water pressure affected the seta structure differently at parallel and perpendicular approaches, and complete wetting was more difficult in the structure observed during parallel immersion. The wax coating on water strider legs was found to decrease adhesion force with little effect on immersion force. Overall, wax-coated strider legs, having conical setae with nanogrooves to facilitate removal of water, are natural models for legs of a biomimetic aquatic robot when oriented parallel to the water surface.