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Author Topic: What would happen to a boat on a liquid lacking surface tension?  (Read 5803 times)

Offline jackhammer

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Some years ago I read a book, about strange, but true science facts, (I seem to recall it being by the late Isaac Asimov, perhaps not), which I can no longer trace - I hope one of your readers can give me the title.

In it, the writer mentioned a planet covered in a particular liquid (sorry, can't remember what it was) which has the peculiar quality of having no surface tension.
He went on to explain that if you launched a boat from one shore, because there was no surface tension, hence no drag, your boat would not stop until it reached the other shore. He then went on to explain you would never reach the other shore because the liquid would just creep up and over the sides of the boat and sink it. But another reason you would not reach the other shore would be because there would not be one, and you would not have been able to push off from a shore, because the liquid would just creep up and cover all the land on the planet.

Who wrote it ? What is it called ?
« Last Edit: 09/03/2009 22:11:50 by chris »


 

Offline LeeE

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I don't remember it being any of the Azimov books I've read, but it does remind me of one of Arthur C. Clarke's short stories concerning Helium II, although I don't recall it had anything to do with having no surface tension though.

http://en.wikipedia.org/wiki/Helium#Helium_II_state

I would have though that a liquid with no surface tension would act like a molecular dust and would have to be completely inert for it not to react with just about everything.
 

Offline John Chapman

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I donít understand this at all.

Iím not a physicist but my understanding of surface tension is that it is caused by the ability of each molecule in a liquid to attract itís neighbouring molecules. On the surface, this results in a barrier which is a bit like that formed by a line of people all with their arms linked. It resists allowing things to pass through, which is what enables insects to walk across waterís surface. This is also what causes water to form beads on a waxy surface. At the edge of a contained pool, the liquid will meet another surface and the molecules are likewise attracted to that, the attraction being strong enough that it may even pull the liquid slightly up that surface of the container. This is what causes the meniscus, or the very small curling up the sides of a glass of water.

If you destroy the surface tension of water by adding a surfactant such as soap it will not cause the water to creep up the sides of the container. If it did you could not leave dishes soaking in the washing up overnight. By the morning the bowl would be emptied of water. But small quantities of water would not bead and would creep along a horizontal surface until it was just one molecule thick. This creeping action is powered by gravity pulling each molecule in the water to itís lowest level. I can see no reason why water would want to creep uphill.

Also I cannot see why a liquid with no surface tension would produce no friction, or drag, on an object passing through it.

I'm sorry but I don't think your book was called True Science Facts. But it might have been called Science Bo**ocks!
« Last Edit: 09/03/2009 22:13:41 by John Chapman »
 

Offline chris

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The surface tension of petrol is very low, but it tends to stay in the tank quite well!
 

lyner

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Hang on a minute. It's not surface tension which causes significant drag. It's turbulence which even the smoothest hull / airframe will produce. As anything goes through a fluid, the fluid has to part and rejoin. This will produce some changes in velocity of the the fluid which will cause pressure differences. Only at very low speeds can you ensure laminar flow so you will get vortices, producing low pressure regions behind the moving  object.
For surface (displacement) craft, there is the added problem of the bow and stern waves which provide you with an 'uphill' climb when you are approaching your so called design speed; drag increases rapidly then.

Edit - of course, there's viscosity, too, but its effect is small once an object is traveling fast in water of air - turbulence dominates.
« Last Edit: 10/03/2009 12:51:59 by sophiecentaur »
 

Offline jackhammer

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Whoops - oh dear, my 'facts' are obviously wrong, and my memory of the particular chapter of the book is seriously deficient. Thanks to LeeE, John Chapman, Chris and Sophiecentau for your input. Please ignore the mention of surface tension. LeeE pointed me to an article about Helium which in the state of Helium II, (liquid helium below its lambda point), seems to exhibit some of the behaviour/characteristics mentioned. So perhaps my original statement should have mentioned zero viscosity rather than the incorrect surface tension.
 

Offline daveshorts

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yes I think you are talking about a superfluid which has no viscosity - it is the physical analogy of a superconductor. If you cool liquid helium-4 below about 2.1K it looses viscosity and can only rotate in quantised units of angular momentum.

It will also form a thin layer over objects and siphon over an open container. The low viscosity means that the boat would have to be very watertight. I think a boat would still have friction as you woul still get water waves although a submarine would have very little resistance.
« Last Edit: 10/03/2009 11:50:56 by daveshorts »
 

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