Ivars Peterson's MathTrek
The curvature of a liquid's surface at a boundary is a consequence of the liquid's surface tension. The sloped surface marking the border between wet and dry is called the meniscus.
To creatures about a millimeter in length, these slopes appear as frictionless mountains. The insects typically can't climb them using their normal rowing motions or running gaits. If they try to stride up, they slide back down. Instead, these insects have to rely on a novel form of propulsion that doesn't require moving their legs back and forth.
That's what mathematicians David Hu and John Bush of the Massachusetts Institute of Technology discovered when they used high-speed video to capture the meniscus-climbing actions of three types of water-walking insect. The researchers describe their findings in the Sept. 28 Nature.
In effect, the insect creates tiny menisci with its front and rear legs. Because menisci are attracted to other menisci, the net effect is to pull the insect up the slope at the water's edge. These creatures can reach speeds as high as 30 body lengths per second.
In technical terms, the insects take advantage of lateral capillary forces that exist between small floating objects, Hu and Bush say. This effect is responsible for the formation of bubble rafts in champagne and the clumping of breakfast cereal in a bowl of milk.
The force of attraction between body and meniscus "wall" depends on the body's buoyancy and exponentially on its distance from the wall. Because the insect's front legs are closer to the wall than its rear legs are, the net effect is to propel the insect forward and upward.
In the realm of fluid dynamics, few researchers have previously tackled situations that involve surface tension as an important component. The new results and related research may have important applications not only for understanding biolocomotion but also potentially in nanotechnology.
Copyright © 2005 by Ivars Peterson
2005. It's a bug's life: MIT team tells moving tale. Massachusetts Institute of Technology press release. Sept. 28. Available at http://web.mit.edu/newsoffice/2005/insects.html.
Hu, D.L., and J.W.M. Bush. 2005. Meniscus-climbing insects. Nature 437(Sept. 29):733-736. Abstract available at http://dx.doi.org/10.1038/nature03995.
For images, movies, and other materials about meniscus-climbing insects, go to http://www-math.mit.edu/~dhu/Climberweb/climberweb.html.