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GERRIDS ARE a family of insects including water bugs, pond skaters, water striders and jesus bugs, all of which can travel atop the water. Water bugs use their mass, musculature and balance to make use of the liquid’s surface tension. Recently, researchers have devised a robot that mimics the gerrid’s aquatic talents.
Science, the weekly magazine of the American Association for the Advancement of Science, July 31, 2015, has two articles on this robotic advance. “Jumping on Water: Surface-tension-dominated Jumping of Water Striders and Robotic Insects” reports on research performed by Je-Sung Koh and colleagues at Seoul National University, Harvard and the Polish Academy of Science. “Two Leaps Forward for Robot Locomotion,” by Dominic Vella, is a summary of their research. Abstracts are available at Jumping on Water and Two Leaps, respectively.
Water molecules attract each other, causing a thin film at its surface. This surface tension can support something, provided it distributes its miniscule weight over a large surface area.
Gerrids have low body mass and six long slender legs evolved to exploit this. The front two are shortest and used to capture prey. The middle ones, of intermediate length, have locomotion duties. The rear two legs steer and distribute the insect’s weight in an optimal manner.
Koh and his colleagues identified that water striders propel themselves by rotating the curved tips of their legs inward and downward—with a force just below that of piercing the water’s surface film. Amazingly enough, the legs’ action produces minute waves off of which they push, generating forward thrust. A water strider can move faster than a meter/second.
The insect’s body and legs are hydrophobic, water-repelling, by means of microscopic hairs, thousands of them per sq. in. Should the insect become submerged, these tiny hairs trap air. They also increase the surface area of its leg tips.
Like a real water strider, the robotic one can jump as well as stride. Researchers modeled its triggered-hinge mechanism after a flea’s musculature. They used a MEMS (MicroElectroMechanicalSystem) process to fabricate a “full-size” robotic water strider just 2 cm. (0.8 in.) in length and weighing 68 milligrams, about 0.002 oz.
By studying varied triggering forces for the robotic legs, Koh and his colleagues were able to quantify the physics of water strider locomotion. They conclude, “The experimental results improve our understanding of the dynamic interaction between an unconstrained free body and a liquid surface, as observed in semi-aquatic arthropods in nature.” ds
© Dennis Simanaitis, SimanaitisSays.com, 2015