Water striders are little insects that spend their existence skating around on the surface of lakes, ponds, and streams, relying on surface tension to keep them dry and happy. Watching them zip around is very cool, and its equally cool to think about the physics going on between the water and their toes to allow them to do what they do. Water striders are also able to jump, which substantially ups the difficulty on the whole not-sinking thing, since they have to somehow exert a substantial amount of force on the surface of the water without breaking through. How do they do it? South Korean researchers built a robotic water strider to find out…
…Real water striders are spectacular jumpers, and from watching slow motion videos of the insects, the researchers were able to understand how they manage it. Interestingly, water striders are able to jump just as high on water as they are on land, suggesting that the technique that they use is unique to their environment, since most other insects that can jump on water are way more efficient jumping on land. The movement of water striders is all based on using surface tension to keep themselves on top of the water’s surface, and their jumping behavior is no different.
To jump, a water strider rises upward while smoothly pushing the water surface downward and closing four of its legs inward [right]. It’s very careful not to rupture the water’s surface (doing so dissipates an enormous amount of energy and renders jumping highly inefficient), and in fact manages to exert a peak force that’s consistently just below the amount of force that would break the surface tension of the water (about 144 mN/m). By closing its legs inward, the insect is also able to pull them across the surface of the water as it begins to jump, allowing it to exert up to 42 percent more force over the surface of the water.
The 68 milligram robot (with a 2-cm body) that the researchers designed to mimic this behavior is based closely on real water strider: the wire legs are coated with a superhydrophobic material, and curve up at the toes, just like the real insect. The “muscle” that does the jumping is a bio-inspired torque reversal catapult that’s based on the leg of a flea; when it’s triggered, it applies a gradually increasing amount of torque to the legs of the robot, driving them smoothly downwards and achieving a maximum vertical acceleration of just under 14 g.
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