Materials Science

How to Walk on Water

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Science  11 Jan 2008:
Vol. 319, Issue 5860, pp. 137
DOI: 10.1126/science.319.5860.137c

Water striders can skate and jump on water without drowning. The legs of these insects have a hierarchical structure of hydrophobic hairs; the resulting highly water-repellent surface is thought to help the insects move on water, because small hydrophobic objects can float, or sink with a delay, even when their density is higher than that of water. Floating is one thing though—jumping onto and off of a liquid water surface quite another. How do the impact conditions affect the response of the water surface? To investigate this question, Lee and Kim studied the impact behavior of small superhydrophobic spheres. They found that at low impact velocity, the spheres oscillate on the surface while afloat. As the impact velocity increases, they bounce off the water surface. At even higher velocity, the spheres penetrate the water surface and sink. Whether the spheres bounce off depends on the viscosity of the liquid and the hydrophobicity of the sphere. Thus, the superhydrophobic legs are crucial to the water striders' ability to jump on water and avoid drowning. Future robots may mimic such behavior. — JFU

Langmuir 24, 142 (2008).

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