Walking on water

Water, thanks to so called surface-tension, can sustain a small coin. When a small insects, water strider stands on water surface, it gets deformed, like an elastic membrane.

a, An adult water strider Gerris remigis. b, The static strider on the free surface, distortion of which generates the curvature force per unit leg length 2 sin that supports the strider's weight. c, An adult water strider facing its mechanical counterpart. Robostrider is 9 cm long, weighs 0.35 g, and has proportions consistent with those of its natural counterpart. Its legs, composed of 0.2-mm gauge stainless steel wire, are hydrophobic and its body was fashioned from lightweight aluminium. Robostrider is powered by an elastic thread (spring constant 310 dynes cm^-1) running the length of its body and coupled to its driving legs through a pulley. The resulting force per unit length along the driving legs is 55 dynes cm^-1. Scale bars, 1 cm. Source: Nature (http://www.nature.com/nature/journal/v424/n6949/fig_tab/nature01793_F1.html).

Water striders are also smart enough to walk, or better: to run on water surface.

Striders walking and running on the water

A recent scientific work showed how they managed: creating small hemispherical vortices on water surface. Striders leave a trace on water, like skiers on snow.

Images captured from a side view indicate their hemispherical form. a, A thin layer (2−5 mm) of thymol blue was established on the surface of the water, disturbance of which revealed the vortical footprints of the water strider. b, The ambient texture results from Marangoni convection³⁰ in the suspending fluid prompted by thymol blue on its surface. The starburst pattern results from the chunk of thymol blue evident at its centre reducing the local surface tension, thus driving surface divergence that sweeps away the dyed surface layer. The fluid is illuminated from below; consequently, the light-seeking water strider is drawn to the starbursts. Scale bars, 1 cm. Source: Nature( http://www.nature.com/nature/journal/v424/n6949/fig_tab/nature01793_F4.html).

The hydrodynamics of water strider locomotion
David L. Hu, Brian Chan and John W. M. Bush
Nature 424, 663-666 (7 August 2003)
http://www.nature.com/nature/journal/v424/n6949/full/nature01793.html