Life on stormy seas: water striders are impervious to raindrop impacts

Water striders are abundant in areas with high humidity and rainfall. Raindrops can weigh 42 times the adult water strider and some pelagic species spend their entire lives at sea, never contacting ground. In this experimental study, we use high-speed videography to film drop impacts on water striders and dynamically-scaled mimics for Froude number Fr=850. Drops force the insect subsurface upon direct contact. As the ensuing crater collapses, the water strider is shot into the air by a Worthington jet. We show the water strider's locomotive responses, low density, resistance to wetting when briefly submerged, and ability to regain super-surface rest state, render it impervious to impacting water drops. When pulled subsurface during secondary crater formation, water striders face the possibilities of being ejected above surface, or submerged below surface. Water striders trapped within the gas-filled crater at the point of crater retraction are expelled from the liquid bath with the ensuing Worthington jet. Water striders with center of mass below the air-water interface of the secondary crater wall at the point of crater retraction remain submerged due to the rapid collapse of the crater causing separation from the insects. Submersion makes the water strider poised on penetrating the air-water interface from below, which appears impossible without the aid of a plastron, or proper locomotive techniques. Submerged water striders employ a series of power strokes in locomoting the aquatic environment. Drops impacting the liquid bath several body lengths away from water striders elicit escape jumps as striders maneuver surface perturbations. Our results show water striders are robust to adverse super-surface conditions and augur well for the development of biomimetic robots.