Unifying framework of vortex interactions from air to water and in between*

We explore how organisms from jellyfish to fruit flies harness vortex interactions to boost locomotion. While previous studies focus on bulk fluids, we examine vortex re-energization at fluid interfaces, using water-walking insects as a case study. These insects employ an alternating tripod gait, where hind legs re-energize vortices shed by middle legs, enhancing efficiency. We show how leg positioning either re-energizes or dissipates these vortices. Using dimensionless analysis, we compare this phenomenon across organisms in air, water, and at fluid interfaces, offering a universal framework for understanding energy recapture from vortices.