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December 8, 2020

Dipolar vortices generated in wake of an adult water strider

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

More about this Image
In this image, when the chemical thymol blue is sprinkled on the water surface, Marangoni convection in the suspended fluid produces a 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 water strider mixes the fluid in its wake.

For this National Science Foundation-supported project, dye studies were performed in order to determine what the propulsion mechanism is of a water strider (Gerris remigis), a common water-walking insect.

Water striders 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 an NSF grant (CTS 0130465) for this project. An NSF Graduate Fellowship award supported David Hu, a graduate student who worked on the project. (Year of image: 2003)

Credit: Courtesy John Bush, MIT

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