Taking cues from the insect kingdom, scientists have built a tiny boat that uses surface tension, but no moving parts to navigate through water.

Miniature electrodes at the rear of the thumbnail-size vessel neutralize the surface tension of the water there, thereby allowing tension at the front to tug it forward. An electrode on the side of the boat enables it to steer clockwise. The device moves at a speed of 0.16 inch (four millimeters) per second, or about a foot (30.5 centimeters) every 1.5 minutes. That might seem slow, experts say, but it is in the same league as previously developed devices that rely on surface tension for forward progress.

Researchers say the new technology could easily be modified to hold chemical sensors and other detectors to perform water and air quality testing. It could also be rigged with a camera to conduct surveillance missions. Although the prototype boat ran on plug-in power, future versions could operate on batteries, radio-frequency waves or even miniature solar panels, says Sung Kwon Cho, a mechanical engineer at the University of Pittsburgh (Pitt), who led the project.

Surface tension tugs evenly at an object resting in a meniscus (the top, slightly curved layer of water), as if suspending the object in a web of taut rubber bands. Severing some of those bands—either chemically, with substances called surfactants; mechanically with paddles; or, in the latest research, with electricity—allows the object to move in the direction opposite from the break. See the boat in action here and here. (video files)

"With no action, surface tension is symmetric, but if you change the direction of the force, the propulsion is directly related" to the interference, Cho says.

Engineers have long understood the potential of surface tension as a means of propulsion through water. Nature is rich with examples of such locomotion, including insects such as water striders.

Cho, whose group is presenting its invention today at a conference in Sorrento, Italy, sponsored by the IEEE Institute of Electrical and Electronics Engineers, says he was inspired by video images of leaf beetle larvae using surface tension to travel through water. To do so, the insect bends itself into a shallow "U," disrupting the symmetry of the tension of the meniscus surrounding them and generating forward thrust.

Because surface tension is a relatively feeble force on large objects, Cho says his boat could be at most about four inches (10 centimeters) long. It could be shrunk, however, from its current size of 0.8 inch (two centimeters) in length to as small as 100 microns or so. (A micron is one millionth of a meter, or about four one hundred-thousandths of an inch.)

David Hu, a mechanical engineer at the Georgia Institute of Technology in Atlanta, calls the Pitt invention "a very elegant device that could actually have practical application."

As a researcher at the Massachusetts Institute of Technology, Hu was part of a team that invented the Robostrider, the first mechanical water walker that, like the water striders, uses surface tension to sit on and walk across water's surface. But, he says, the Robostrider and related devices have drawbacks, notably numerous delicate parts, that don’t apply to the electric boat.