# Aluminum ever floats?March 2, 2010 1:39 PM   Subscribe

If I take an aluminum ball and drop it in water it'll sink as it is denser than water. However, if I make it super duper flat, is there any chance it might float?

Ideally, you would have a link to an article or something which goes into this type of problem in more detail. I am not a science guy.

Also, if we broke the water's surface tension, would this have a different result?
posted by fantasticninety to Science & Nature (24 answers total)

I am literally about to leave to make aluminum foil boats with elementary school students. of course it can float :).
posted by brainmouse at 1:41 PM on March 2, 2010 [1 favorite]

Um... I just went into my kitchen, filled a bowl with water, and put a little piece of aluminum foil in it. It floated on the surface but after I broke the tension it sank. Bam. Science.
posted by showbiz_liz at 1:42 PM on March 2, 2010 [3 favorites]

Can you shape the aluminum so that it's buoyant, like a boat? I'd imagine it would float then.
posted by nitsuj at 1:44 PM on March 2, 2010

The reason why aluminum foil boats float is because of the air contained within the boat, which makes the density of the boat as a whole less than that of water. If the foil is perfectly flat, you don't get that buoyancy, so only the surface tension of the water holds it up.
posted by Ery at 1:44 PM on March 2, 2010 [1 favorite]

http://www.ehow.com/how-does_4579716_a-canoe-float.html

Archimedan Principle, y'all. If the ball was just a flat sheet of aluminum, it would still sink because aluminum is denser than water. But if you make it into a boat shape, you're essentially making a skin of aluminum with air inside of it, so it will be seaworthy :D

This is why one common metric for a boat (which are often made of metal) is its displacement.
posted by kataclysm at 1:44 PM on March 2, 2010 [1 favorite]

You can try this for yourself, by attempting to float pieces of aluminium foil. I expect you'd find that as long as the edge of the foil doesn't pierce the surface, it will float quite well. Of course, it might not be the metal itself that's floating so much as a very small, shallow and badly designed tinnie.
posted by flabdablet at 1:45 PM on March 2, 2010

Response by poster: Not aluminum foil. Real aluminum.
posted by fantasticninety at 1:51 PM on March 2, 2010

Thin enough anything will float because it is not heavy enough to counteract the surface tension of water. Thin enough lead would float. Once it's thick enough to break the surface tension, though, it's all over.
posted by musofire at 1:55 PM on March 2, 2010

Not aluminum foil. Real aluminum.

a) Aluminum foil is aluminum. It's just very thin.

b) The material is completely irrelevant. Warships are made out of steel. Given a large enough displacement, you can make lead float. What matters is the density, as described above.
posted by chrisamiller at 1:56 PM on March 2, 2010 [7 favorites]

Having paddled around in an aluminum canoe or two in my time, I say yes, aluminum can be shaped such that it floats atop water.
posted by usonian at 1:57 PM on March 2, 2010 [1 favorite]

Objects float if their average density is less than that of the fluid they are in. Unless you are trapping air inside (i.e. making a boat), the density of aluminum is the density of aluminum, regardless of its shape. It will sink.

Now, there's one way out. Even for materials denser than water, you can place objects on _top_ of water. In fact, in this video someone does just that with a flat piece of aluminum. It's not floating, surface tension is such that force of the water pulling on itself keeps the object from breaching the surface of the water. If you poke such an object, it'll sink just as it ought.
posted by Schismatic at 2:01 PM on March 2, 2010 [3 favorites]

Wait, are you interested only in perfectly flat materials and interested only in the effects of surface tension?

If so, then this page states that it takes 0.074 dynes of force/cm to break the surface tension of water at 20 degrees C. I'll leave the conversion to weight supported as an exercise for the reader.
posted by chrisamiller at 2:04 PM on March 2, 2010

I'm not sure how the talk of boats and canoes is relevant, since they aren't flat. Their bottoms may be flat but that wasn't the question.
posted by smackfu at 2:04 PM on March 2, 2010

b) The material is completely irrelevant. Warships are made out of steel. Given a large enough displacement, you can make lead float. What matters is the density, as described above.

Indeed, you could even make a balloon out of lead.
posted by borkencode at 2:05 PM on March 2, 2010

Or a ship out of concrete.
posted by Knappster at 2:38 PM on March 2, 2010 [1 favorite]

How thin does aluminium need to be before it's not "real aluminum" in your view? Give a thickness figure in millimetres, and we'll give you your answer.
posted by flabdablet at 2:49 PM on March 2, 2010 [1 favorite]

A sheet of tinfoil will float.
posted by pseudostrabismus at 2:51 PM on March 2, 2010

completely flat and very thin should work but it would be practically difficult. You are trying to balance the mass of the object (proportional to volume) to surface forces (proportional to perimeter). If the plate gets thin enough the surface term will dominate. The challenge would be keeping the plate flat so force doesn't become unbalanced.
posted by Fiery Jack at 5:25 PM on March 2, 2010

The wikipedia page on the Willhelmy plate gives some relevant theory.
posted by Fiery Jack at 5:38 PM on March 2, 2010

Here is a photograph of an aluminum coin suspended at the surface of a glass of water by surface tension.
posted by XMLicious at 6:27 PM on March 2, 2010

The way it was explained to me by a young naval architect student way, way backe when (WAAAAAAAY back when), is that it's not about bouyancy, it's about displacement. That's why naval architect students get to have cement canoe contests and aircraft carriers don't sink like 1000 + tons of steel when you put them in the water.

My fuzzy & possibly incorrect recollection of what he spent the next half hour explaining goes like this: If the volume of water displaced by the object can compensate for the mass the object itself, it will float. So wide & thin is good when attemptiing to float materials denser than water.

Y'all feel free to call me out for bad science if my memory has failed me.
posted by Ys at 6:36 PM on March 2, 2010

Best answer: If the volume of water displaced by the object can compensate for the mass the object itself, it will float. So wide & thin is good when attemptiing to float materials denser than water.

Any floating object displaces its own mass of water; any fully submerged object displaces its own volume of water.

Density is a measure of mass per unit volume. The density of water is approximately one kilogram per litre = one tonne per cubic metre = one gram per cubic centimetre.

If you work out the consequences of those two facts, it follows that any object whose overall density is less than that of water will float.

The density of solid aluminium is about 2.7 kg/l. So a solid lump of aluminium won't float. But form it into a hollow shell, and you can make the overall density of the resulting boat way lower than that of water, because the density of the air contained inside the shell is near enough to zero.

The only way to float objects denser than water is to make use of surface tension. Surface tension means that a water-to-air surface behaves as if it were covered with a flexible membrane. Things can be supported on top of that membrane - in effect, they can float in a little boat whose shell is made of surface tension - provided that at no point do they exert enough pressure on any part of the membrane to rupture it.
posted by flabdablet at 7:16 PM on March 2, 2010 [1 favorite]

I suppose a thin enough piece of aluminum (or other denser-than-water material) might also float because of tiny air bubbles attached to its surface. As an example, drop a seed or other small object into a glass of clear soda— the co2 bubbles will attach and hoist it to the surface. Plain water won't be fizzing, but if some air comes out of solution, or if you don't fully wet the foil when you put it in the water, you could get a similar effect there. Tap the foil a few times, though, and the bubbles will come off and it'll sink.

Other than that, and the surface tension (which can support surprisingly hefty objects; I rest small fiberglass circuit boards on etchant all the time), I can't think of any other effect that would counter the straightforward Archimidean behavior of aluminum: it's denser than water, so it sinks, regardless of its shape.
posted by hattifattener at 7:39 PM on March 2, 2010

It will float, yes (think canoe). It will not, however, be buoyant (rise from below the water's surface) unless it contains a material that is less dense than water. Dimensions are irrelevant when the surface tension is overcome.
posted by Oddly at 7:55 PM on March 2, 2010

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