Navigating the Interface

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Walking on water may be the stuff of legend at human scales, but it’s a fact of everyday life for many smaller species. Waxy, hydrophobic coatings typically make such insects’ points of contact (feet, legs, etc.) water-repellent, and their light weight can be supported by surface tension. Navigating the interface between air and water is more complicated, though, and these creatures have evolved several mechanisms to help. Some, like water striders, use appendages they insert below the surface for propulsion. At 0:49 in the montage above, you can see flow visualization of the vortices generated by a stroke. Other insects release a chemical in their wake that lowers the local surface tension and drives them away via the Marangoni effect. For more, see here and especially this Physics Today article. (Video credit: D. Hu and J. Bush)

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