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Surface tension propels tiny boat without sails or moving parts Answered

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?



9 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?

Good observations, and good questions, by the way.

Can you make a small one that acts as a outboard?

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.

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.

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...

It really is a small-scale device. There are reasons why nothing bigger than arthropods use surface tension for support.

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...

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.

I understand that. But if you adjust the height of the standing wave created by the forward motion of the ship by one millimeter per a set length, can it be cumulative? I.E., can the wave height be altered? Say, like a meter for a 400 meter ship... The surface tension wouldn't create the wave, simply shape it... The distance between wave crests (frequency) in a ship's pressure wave is governed entirely by speed (for a given hull.) It's not unreasonable to speculate about what changing the wavelength might do. I realize the total mass of water actually effected by the surface tension is small, so maybe it's cannot be cumulative due to the weight of water. But again, it's just speculation.

Still, it is something to play around with, as it may lead to a totally different eureka moment :-)

Better to have a creative idea that fails, then no idea at all... ;-)

Quite true! And creativity is most productive within some constraints. Ideas which blatantly violate simple physical laws ("over-unity" and "perpetual motion," anti-gravity, etc.) deserve the ridicule they get. Ideas which push the boundaries of our mundane expectation, which make us think, "Is that really possible? Hey, it just might work...", those are the interesting ones.

Yes, I just meant in a brain storming situation, nothing is ridiculous enogh to not get written down. Anything, even if not taken as stated, can lead to other ideas, even the totally nonsensical .

I related this before, but I remember reading about DuPont looking for the next innovation in paint some way to remove it easily, without jeapordizing longevity , in a Brain Storming session, one fellow shouted out: we could mix gunpowder with the paint and when it came time to remove it, it could be blown of the building.
Of all the ones they picked that one idea was what made a special type of long lasting but very easily removed (by a specific chemical process) paint. We have to be careful to stay true to what can be done, but if we are looking for ideas, it is often a good idea ;-) to let them flow freely, and deal with doable later.

Goodhart says: We have to be careful to stay true to what can be done, but if we are looking for ideas, it is often a good idea ;-) to let them flow freely, and deal with doable later. There is much truth in your statement. "Serendipity is looking in a haystack for a needle and discovering a farmer's daughter." Julius H. Roscoe Jr. (1976)

gmoon's question is quite interesting. The propulsion is driven by an external power source, which essentially "reacts" against the surface tension. There's no a prioiri reason why that technique shouldn't work on a larger vessel, provided the L vs. L2 scaling problem could be overcome with sufficient input power.

Certainly, my first response is as I posted (and you as well). However, it's not impossible that there are countervailing issues. As gmoon has pointed out, the efficiency boost of "bulbous bow" ships isn't obvious, but it's true.

Thanks Kelsymh and Kiteman for casting a critical eye on the idea. It's a bit off the wall, but still fun to speculate about...

can you make the bottom of the boat of pieces that together have large edge and small surface ?

Yes, i realized that, I meant a small one

My apologies, that wasn't meant as a reply to you (my bad). It was supposed to be a top-level comment on the posting itself, but I didn't notice. Now that there's a thread under it, I don't want to delete and repost.

sooo, does it keep spinning forever?