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If I take a sponge and place it in a shallow dish of water (i.e. water level is lower than height of sponge), it absorbs water until the sponge is wet, including a portion of the sponge above the water level. In other words, it seems the sponge pulls some water from the bath up into itself, doing work, and the water gains some gravitational potential energy.

Where does the energy required to do this work come from? My suspicion is that the answer involves physical and/or chemical bonds between the water and the sponge, or possibly the change in the surface area to volume ratio of the water.

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Capillary action is part of the explanation: en.wikipedia.org/wiki/Capillary_action –  Brandon Enright May 31 '13 at 18:11
    
@BrandonEnright Also I think a type of spring action: when dry it has potential energy released when wet. –  anna v May 31 '13 at 18:48
    
@annav I think it only has spring-like potential energy when it is squeezed. This question seems to be about an unsqueezed sponge soaking up water above the water line. –  Brandon Enright May 31 '13 at 19:00
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@BrandonEnright is right. It's mostly a type of capillary action. Specifically the adhesion between the sponge fibers and the water molecules is high enough that water prefers to be touching the sponge more than other water molecules and so there is a tendency for it to "creep" up the sponge. Going above the water line is the easy part since only gravity is holding it down, but the EM force (much stronger) pulls it up. Once above it, there are pockets in the sponge that can trap excess water (released when you squeeze it) –  Jim May 31 '13 at 19:06
    
I vividly remember my hydrodynamics prof telling us that it is much more efficient to suck up water with a sponge if it is already slightly wet vs completely dry... –  Isopycnal Oscillation Jun 1 '13 at 6:54

2 Answers 2

up vote 6 down vote accepted

This effect is called capillarity and is not that straightforward.

The contact between water and a solid surface is determined by the chemical bonds. It is macroscopically observed in the contact angle that the water/air surface makes with the solid surface. This angle depends on the strength of the bonds between the solid and the water molecules. You can see this when you pour water in a glass: the water at the edge of the glass is a bit higher than in the center; it makes an angle with the glass surface.

Now, if there is a lot of solid around the water, such as water in a tiny tube, there are a lot of contact points. Therefore, the water/air interface will be strongly curved. The curvature of this interface modifies the surface tension, which represents the energy contained in that surface. A good way to interpret the effect of curvature is that you surround a given portion of the interface by more (or less) water molecules as you curve the interface. The pressure on the interface is thus reduced or increased depending on the curvature.

In a small vertical tube, the curvature can be such that the pressure is higher than for a flat interface. Thus, it can counteract the gravity more easily.

In conclusion, the energy comes from the thermal (pressure) energy of the water molecules which push from the bottom.

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That is, soaking up water results in a lower energy state. The amount of energy saved is (roughly) equal to the potential energy of the water raised against gravity above the water line. –  Brandon Enright May 31 '13 at 19:59

Actually, I think that is due to your squeezing of the sponge.

Just read my answer patiently and surely leave me a reply.

The sponge is basically a soft bundle of fibre with a hell lot of holes (or should I say pores? Basically it has a lot of space between it. That's why it is called a semi solid.). When you squeeze it, all the air inside it goes out. Then after that you put it in a bowl of water (no matter whatever the water level is) it will "suck" all the water inside it. If you simply put it in a bowl of water without squeezing it, the water will only enter the pores which are open enough for air to go out and water to come in. Also due to capillary action, which should bring a tiny amount of water inside the sponge above the water level. And ya, about the energy, it is simply a matter of suction. Please tell me i am right now?

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this doesn't address how a non-squeezed sponge soaks up water to above the water line, which is a common enough occurrence to be anecdotal –  Jim May 31 '13 at 19:24
    
you should add the capillary action too discussed in the comments above.Squeezing supplies part of the energy but a natural sponge acts like a wick too. –  anna v May 31 '13 at 19:29
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This doesn't even correctly address a squeezed sponge. The squeezed and sucking action only really applies to a submerged sponge. You haven't said anything about what maintains the water in the sponge or why the water in the sponge will stay above the water line. –  Brandon Enright May 31 '13 at 19:41
    
+1 this answer at least explains something and surely doesn't deserve downvotes. –  user10001 May 31 '13 at 20:59
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Jim, I told above that the space which is empty (which is inside the water) is wet because the pores have enough space to let air out and water come inside. It is just a little (mind you, a tiny amount) above the level due to capillary action (I didn't deny that). So should I edit my answer to finally make it complete? –  Rohinb97 May 31 '13 at 21:57

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