How do water striders walk on the surface of water?
How can water striders and similar insects stand and skate across the surface of a pond without breaking through, and how do they push themselves forward without solid ground to push against?
History and references
Settled in 2003 by D. L. Hu, B. Chan, J. W. M. Bush (2003).
Their weight is supported by surface tension acting on their non-wetting, hair-covered legs, which dent the water surface without piercing it -- this part was understood well before 2003. The propulsion mechanism was a genuine puzzle known as Denny's paradox: infant water striders row too slowly to generate the capillary waves that adult striders were thought to push against, yet the babies clearly can still move. Hu, Chan, and Bush resolved this in their 2003 Nature paper by showing high-speed video evidence that striders actually transfer momentum to the water mainly via horseshoe-shaped subsurface vortices shed by their rowing legs, not primarily via surface waves, which works at any size and speed.
An insect's tiptoe waltz on a pond turns out to be powered by miniature vortex rings, not the ripples you can actually see.
Explanations that circulated
- Striders are supported and propelled mainly by generating surface (capillary) waves they push against
- They are simply too light for gravity to matter, with no role for surface tension physics
References
- The hydrodynamics of water strider locomotion, D. L. Hu, B. Chan, J. W. M. Bush, Nature, 2003
- Walking on water: biolocomotion at the interface, J. W. M. Bush, D. L. Hu, Annual Review of Fluid Mechanics, 2006
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