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I recently asked a question on a canvas water bag and I realized that there was something really interesting that I didn’t understand. When a dry water bag gets filled, it leaks a lot of water; however, after sometime the water stops leaking and some of the water seeps and is cooled by evaporative cooling.

My question here is that in order for the water to stop leaking, the canvas fibers must “expand” to stop the water. It can’t be a linear or thermal expansion since this is occurring at the same temperature. Is it the unfolding of canvas fibers or something else? That is, what type of expansion occurs in a water bag to stop it from leaking?

Furthermore, is this the same process that occurs when doors swell during the rainy part of the year? That is, why do doors swell when they get wet?

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2 Answers 2

If the woven bag expanded like a camera zoom lens picturing it, it would leak faster, for the holes would expand as well. The fibers plump radially faster than they elongate longitudinally. Look closely...are the fibers twisted? Ah! When they each get longer the collection gets fatter.

Water plasticizes cellulose, swelling it. This is fine even if wetted to saturation. However, drying is not symmetric. Mensicus forces on the hydrophilic fiber crush porosity when the fiber dries. If we had a long molecule with a small cellulose-loving head and a greasy tail, the pore surfaces would coat and become hydrophobic. Reverse meniscus forces would keep them open and fluffy during drying. Fabric softener is greasy.

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Furthermore, is this the same process that occurs when doors swell during the rainy part of the year? That is, why do doors swell when they get wet?

The answer to this is that the door absorbs water. Water has volume, so if some flows into the door it will take up space in-between the wood fibers and so cause the door to swell.

EDIT: As Brandon Enright pointed out, my explanation isn't quite complete. If you had a porous rigid material there is no reason that water couldn't flow into the air spaces without increasing the volume. The difference in this situation is because the fibers in the door are not rigid, they are free to move/expand. Moreover there is some interaction (possibly due to capillary action and osmotic pressure from the wood having a higher solute concentration) which causes water to flow in and saturate the fibers, causing them to absorb so much water that they swell.

Interestingly this effect is reversed in salt water. From the 'Rime of the Ancient Mariner':

Water, water, every where,

And all the boards did shrink;

Water, water, every where,

Nor any drop to drink.

Here the reason is to do with osmotic pressure. As seawater has a higher dissolved solute content it will tend to draw water out of the wood, causing it to shrink.

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"When a dry water bag gets filled, it leaks a lot of water; however, after sometime the water stops leaking and some of the water seeps." - I have to confess I have no experience with canvas bags so haven't seen the phenomenon you are describing. It could be that the fibers absorb water and so expand through the same mechanism, but that is just a guess. How long does it take for the rate of leaking to slow? –  Andrew Ledesma Feb 5 at 23:45
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I think this is misleading. There is no reason why you couldn't put water into empty spaces and thereby increasing the density of the object without expanding it. There is definitely some interaction with the water and the fibers that isn't just "water has volume". –  Brandon Enright Feb 5 at 23:57
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@BrandonEnright Thankyou, that is a good point. I have updated my explanation. –  Andrew Ledesma Feb 6 at 0:24

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