Tiptoe acrobats get it just right

If walking on water takes grace, jumping on it requires exquisite care.

Water striders spend most of their lives on a water surface, typically that of a pond. Microscopic hairs, coated with a waxy substance, make the striders’ long legs extremely water-repellent, enabling the bugs to rest on water as if the surface were a rubber membrane.

As the name implies, water striders are also accomplished at water walking—more like skating. Scientists know that water striders propel themselves by creating vortices under the water’s surface, similar to how birds fly by creating vortices in air. But what has been puzzling is that water striders also manage to jump, says Ho-Young Kim, an applied physicist at Seoul National University in South Korea.

To understand how the striders do it, Kim and his colleague Duck-Gyu Lee dropped highly water-repellent spheres the size of large sand grains onto a water surface. For low-impact speeds, the spheres stuck to the surface like leaves falling on a pond. When moving fast enough, the spheres broke the surface and sank. But for speeds within a narrow window of about 1.3 meters per second, the spheres bounced off.

The findings mean that to leap off the surface, a water strider “pushes the water surface just the right amount,” Kim says. The report appears in the Jan. 1 issue of Langmuir.

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