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Surface tension propels tiny boat without sails or moving parts

From the New Scientist:

WATCHING Sung Kwon Cho's model boat glide silently across the still water with no propellers or sails, you'd be forgiven for thinking a phantom hand was drawing the vessel forward. The boat is actually being driven by water surface tension, the same force that allows some insects to skate across the surface of a pond.

The design is inspired by Pyrrhalta beetle larvae, which also use surface tension to propel themselves. Since it requires no moving parts, the method should be more robust than those involving propellers and may use just a hundredth of the power.

Another story on the NetworkWorld site (with a few skeptical comments.)

Doesn't look too difficult to replicate, does it?

Picture of Surface tension propels tiny boat without sails or moving parts
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Jaycub8 years ago
I've noticed that small things floating in liquids (like cereal and bits of styrofoam) will automaticaly "suck" together. Is this being propelled by the cereal-sucking force or is it something different?
kelseymh Jaycub8 years ago
Good observations, and good questions, by the way.
ANDY! kelseymh7 years ago
Can you make a small one that acts as a outboard?
kelseymh Jaycub8 years ago
Yes. That's a direct effect of surface tension, combined with the liquid (water or milk, in this case) being able to wet (adhere to) the objects floating in it.

Surface tension refers to the fact that a liquid surface "wants" to be as small as possible (why droplets are spherical). "Wetting" refers to a solid surface tending to attract a liquid and spread it out. If you think about it, those two effects act in opposition to one another. A visible consequence of this is the crescent-shaped surface of water in a narrow tube: wetting pulls the water up the tube, surface tension pulls it back down, and the result is a curved surface.

With a bunch of wettable objects floating in liquid, the wetting will ensure that the liquid sticks to those objects, while surface tension pulls the liquid together, carrying the objects along with it.
LinuxH4x0r8 years ago
Interesting. Someone here should build one
It doesn't scale. Surface tension only acts along the line of contact around the edge of the boat. Drag acts over the whole surface area which is underwater. As you make a boat like this larger and larger, at some point the drag exceeds the tension, and the boat can't move. any more. This sort of technique, however, would be really excellent for something like a fleet of autonomous "mini robots" to do water quality sampling, accessing large pipe systems, and so forth.
gmoon (author)  kelseymh8 years ago
Interesting point. When I first read this two things came to mind: -- The slower a single-prop boat runs, the more difficult it is to maneuver. With all the hull surface area on the sides, maybe this technology could be adapted to improve control in harbors, etc. -- The efficiency and speed of a hull has a great deal to do with it's length, hull shape and the length of the pressure wave it creates in the water. If there is some way to vary the frequency of that wave relative to boat length, it's conceivable that overall efficiency could be influenced. Whether it would be practical to cover a large hull with enough discrete conductive plates or strips to work is another issue...
Kiteman gmoon8 years ago
It really is a small-scale device. There are reasons why nothing bigger than arthropods use surface tension for support.
gmoon (author)  Kiteman8 years ago
Yeah, I'm still curious. For one thing, it's not actually doing the "floating" here. No one's arguing getting something for nothing. It's amazing how much more efficient a ship with a "bulbous" bow becomes...something definitely counter-intuitive, since they were once considered a source of drag. And what's more small-scale than dimples on a golf ball, or the speculated effect of shark skin...
Kiteman gmoon8 years ago
As kelsemh said, it's a matter of scale - the propulsive force is due to a difference in water-height of only a millimetre or two.

Build a full-size boat, and the height-difference is still only a millimetre or two.

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