Interfacial vortex recapture enhances thrust in tiny water skaters

Vortex recapture underpins the exceptional mobility of nature's finest fliers and swimmers. Utilized by agile fruit flies and efficient jellyfish, this phenomenon is well-documented in bulk fluids. Despite extensive studies on organismal locomotion at the water's surface, a vital fluidic interface where diverse life forms interact, hydrodynamics of interfacial vortex recapture remains unexplored. We investigate interfacial (on water) vortical hydrodynamics in Microvelia americana, one of the smallest and fastest water striders, skating at 50 body lengths per second (BL/s) or 15 cm/s. Their middle legs shed counter-rotating vortices, re-energized by their hind legs, demonstrating interfacial vortex recapture. High-speed imaging, particle imaging velocimetry, physical models, and CFD simulations show re-energization increases thrust by creating positive pressure at the hind tarsi, acting as a virtual wall. This vortex capture is facilitated by the tripod gait, leg morphology, and precise spatio-temporal placement of the hind tarsi during the power stroke. Our study extends vortex recapture principles from bulk fluids to the interface, offering insights into efficient interfacial locomotion, where surface tension and capillary waves challenge movement. Understanding interfacial vortex hydrodynamics can guide the development of energy-efficient microrobots to explore the planet's water surface niches, critical frontlines of climate change and pollution.