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Robostrider Meets Water Strider

Caption:

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 mm gauge stainless steel wire, were naturally hydrophobic. Its middle driving legs were powered by an elastic band (310 dynes/cm) coupled to a pulley. High speed video indicated that the Robostrider did not break the surface despite leg tip speeds of approximately 18 cm/s. Robostrider--whose body length is 9 cm 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. [See related images: Hydrodynamic Propulsion of Water Striders, Spider Vortices, Dipolar Vortices of a Water Strider, and Water Walkers.]

More about this Image
Water striders (gerris remigis) are common water walking insects approximately 1 cm long that resides on the surface of ponds, rivers, and the open ocean. In the past, it was believed that water striders develop 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 Dr. 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--track 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. It was previously assumed that this transfer occurs 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.

Dr. Bush received a grant from NSF's Fluid Dynamics and Hydraulics program (grant CTS 01-30465) for this project. An NSF graduate fellowship award supports David Hu, a graduate student working on the project.

Robostrider Meets Water Strider
(Preview Only)

Credit: Courtesy John Bush, MIT
Year of Image: 2003

Categories:

ENGINEERING / Chemical
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No additional restrictions--beyond NSF's general restrictions--have been placed on this image. For a list of general restrictions that apply to this and all images in the NSF Image Library, see the section "Conditions".

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Last Modified: Mar 29, 2001