Water strider examines Robostrider, a mechanical water strider believed to be the first mechanical water walker. Robostrider was designed and constructed to mimic the motion of a water strider. An important design criterion was that the force-per-unit length along the driving legs not exceed twice the surface tension.
Robostrider's legs, composed of 0.2-millimeter-gauge stainless steel wire, were naturally hydrophobic. Its middle driving legs were powered by an elastic band (310 dynes/centimeters) coupled to a pulley. High-speed video indicated that Robostrider did not break the surface despite leg tip speeds of approximately 18 centimeters. Robostrider, whose body length is 9 centimeters long, traveled half a body length per stroke.
Robostrider was created for use in a National Science Foundation-supported project in which dye studies were performed in order to determine what the propulsion mechanism is of the water strider (Gerris remigis), a common water-walking insect. [Image 4 of 5 related images. See Image 5.]
More about this Image
Water striders (Gerris remigis) are common water-walking insects approximately 1 centimeter long that resides on the surface of ponds, rivers and the open ocean. In the past, it was believed that water striders developed momentum using the tiny waves they generate as they flap their legs across the water's surface. This was because striders move so quickly that all you see is the waves. But baby water striders legs are not big enough to generate waves, and therefore should be incapable of propelling themselves along the surface. So how are they able to move?
Enter John W.M. Bush, a mathematician from the Massachusetts Institute of Technology (MIT), and his team of researchers who, using high-speed video and blue-dyed water, tracked the movement of water striders. Bush's high-speed images and dye studies show that the water strider propels itself by driving its central pair of legs in a sculling motion. In order for it to move, it must transfer momentum to the underlying fluid. Previously it was assumed that this transfer occured exclusively through capillary waves excited by the leg stroke, but Bush and his team found that, conversely, the strider transfers momentum to the fluid principally through dipolar vortices shed by its driving legs. The strider thus generates thrust by rowing, using its legs as oars, and the menisci beneath its driving legs as blades.
Bush received a grant from NSF's Fluid Dynamics and Hydraulics program (CTS 01-30465) for this project. An NSF Graduate Fellowship award supported David Hu, a graduate student who worked on the project. (Year of image: 2003)