Posted by Mike on January 11th, 2008 in biology, material

Science Volume 319, Number 5860,

Issue of 11 January 2008

©2008 by The American Association for the Advancement of Science.

MATERIALS SCIENCE: How to Walk on Water

Julia Fahrenkamp-Uppenbrink 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).

Here’s another interesting tidbit from nature and an opening for Biomimetics - the use of “nature’s” engineering bounty to solve human problem. Almost everyone has watched a water strider walk Christ-like (of course, Christ did not have multiple appendages) across the surface of a pond. Most of us just assumed that the mechanism for the strider’s success is because the combined pressure of their legs is lighter than the surface tension of the water can support.

As usual, the truth is much more complicated. As the article suggests, simply floating on the surface is one thing, impacting the surface in a jump is something else indeed. The strider’s impact on the surface is helped by the hydrophobic (water repelling) characteristics of it legs. So that even in a jump or a landing from the jump, the legs do not break the surface tension limit of the water. The implications of this go well beyond robotic leaping.

How much could frictional losses be reduced on hull surfaces, torpedoes, cruise ships? How much energy saving could be gained from such surfaces?