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Dipolar vortices generated in the wake of an adult water
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Dipolar vortices generated in the wake of an adult water strider.

June 8, 2010
Dipolar vortices generated in the wake of an adult water strider. When the chemical Thymol Blue is sprinkled on the water surface, Marangoni convection in the suspended fluid produces the swirling texture. Motions of the surface of a liquid are coupled with those of the subsurface fluid or fluids, so that movements of the liquid normally produce stresses in the surface and vice versa. The movement of the surface and of the entrained fluid(s) caused by surface tension gradients is called the Marangoni effect. The pH-sensitive dye changes color as the strider mixes the fluid in its wake.

In this National Science Foundation-supported project, 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 3 of 5 related images. See Image 4.]

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 (CTS 01-30465) for this project. An NSF graduate fellowship award supports David Hu, a graduate student working on the project. (Year of image: 2003)


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