Water strider walking on water (Denny's paradox)
How do baby water striders generate enough thrust to skate across water when their legs move too slowly to create the surface waves that classical theory said were needed?
History and references
Settled in 2003 by David Hu, Brian Chan, John Bush (MIT).
Biomechanist Mark Denny formalized a puzzle in 1993: classical hydrodynamic theory implied that water striders must generate capillary waves to propel themselves via surface tension, yet a strider's legs move too slowly, especially as juveniles, to reach the minimum wave-generating speed, seemingly making their propulsion impossible. In 2003, David Hu, Brian Chan and John Bush at MIT used high-speed video and dye-based particle tracking to show striders actually propel themselves by shedding hemispherical vortices beneath the surface with their rowing legs, transferring momentum to the underlying fluid without ever needing to launch a surface wave, resolving the paradox and explaining locomotion at all life stages.
An insect's most famous water trick turns out to work by rowing invisible underwater vortices, not the visible ripples everyone assumed mattered.
Explanations that circulated
- The classical assumption (behind Denny's paradox) that a water strider must generate capillary surface waves to push off the water, which requires exceeding a minimum wave speed (~23 cm/s) that juvenile striders' legs cannot reach; Hu, Chan & Bush showed the real thrust comes from underwater vortices shed by the sculling legs, not surface waves at all.
References
- The hydrodynamics of water strider locomotion, D. L. Hu, B. Chan, J. W. M. Bush, Nature, 2003
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