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I regularly drink tea at work and I often reuse the tea bags (yes I know I'm a cheapskate). Yesterday afternoon I used a tea bag once and kept it in the cup in case I wanted another cup before I left. As it turned out, I didn't and the tea bag was still there when I came into the office this morning with a very curious pattern of discoloration.

I believe I understand most of the process; over night, the bag was drying out with the upper portion drying most quickly and the liquid from the leaves wicking up the bag and into the string. As the process continued, progressively more suspended solids were carried up the system and deposited near the top where the rate of drying was most rapid. As a result, there is a dark line of tea deposits around the upper edge of the bag and on the string. Oddly, though, the deposits very abruptly stop at the edge of the cup where the string drops to the dangling tag. Close examination reveals that this discoloration stops not exactly at the edge of the cup (as might be expected if contact with the cup were the cause) but slightly beyond it where the string descends. The only reason I can think of as to why this should be so is that the wicking action responsible for the discoloration of part of the string, stopped at that point. But why should this be? After all, a siphon ONLY works when the receiving end is below the apex. Am I wrong in my assumptions? Or does anyone know:

Why does a string not wick down?

A photo of the situation.

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It would be nice if you could add a picture for my understanding :) –  Bernhard Feb 21 '13 at 13:49
    
I would do if it were not blocked from my work PC. Perhaps I will when I get home. –  AdamRedwine Feb 21 '13 at 13:51
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I thought this was going to be about string theory and Wick rotation! –  Mitchell Porter Feb 21 '13 at 14:38
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@MitchellPorter Me too! I just attended a public lecture by Brian Schmidt where an audience member asked him "What is the relevance of this to humans?" So I guess this is the answer: the string theory of tea bags. (His actual answer was much better.) –  Michael Brown Feb 22 '13 at 9:59
    
I left another tea bag in the cup over the weekend with the wetted portion of the string hanging farther down. That should provide some good evidence of what's happening. –  AdamRedwine Feb 22 '13 at 23:57
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4 Answers

The wicking is not occuring because of a siphon action. Rather, capillary action is responsible. The abrupt stop at the edge of the cup has two likely explanations; one -- all your solvent evaporated before capillary action wicked solute that far, or two -- The cup preserves a higher relative humidity within, which drops abruptly outside the cup, thus preventing fluid from wicking any significant distance once it gets outside the cup.

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The humidity effect is an excellent suggestion. It is complicated by another aspect I did not mention, that it isn't really in a "cup" but rather in a perforated porcelain filter basket that would, presumably, reduce such a discrepency. –  AdamRedwine Feb 21 '13 at 15:58
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@Adam: Also, where the string bends over the edge of the cup, I would assume the string is somewhat laterally compressed at that point, squeezing shut the voids that the capillary flow goes through. –  Mike Dunlavey Feb 21 '13 at 18:32
    
@MikeDunlavey this squeezing, as long as the voids are not completely closed, would actually increase the capillary flow because a smaller 'channel' diameter increases the capillary pressure. ---- What would be possible though, is that the squeezed channels become too small to allow any suspended particles to pass and therefore the string does not change color after the edge –  Michiel Feb 22 '13 at 8:10
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Perhaps because oils from your fingers have interfered with the wicking, and that is where you held the string.

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Oooh, I didn't think of that. –  KDN Feb 21 '13 at 15:32
    
It is a worthwhile hypothesis. What I did not mention is that this isn't the first time this has happened (it's just the first time I thought to ask about it here) and the coincidence of locations seems a bit of a stretch. Nonetheless, it would be a good thing to control for if I were to do experimentation. –  AdamRedwine Feb 21 '13 at 15:55
    
One hypothesis I had was that the apparantly wicking section was that portion of the string that was wetted when dipped in the cup since the coincidence between that length and the way the bag lies in the cup seems to be more likely to coincide. This, also, could be easily controlled for in experimentation. –  AdamRedwine Feb 21 '13 at 16:00
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Okay, so it turns out that the stopped wicking was not from the downward portion of the string, but from the portion of string that was already wetted. I guess the question, then, becomes why does string only wick where wetted?

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I am guessing a bit here, but this could be due to an effect called contact angle hysteresis which means that there is a certain threshold force needed before a fluid-fluid interface can move when in contact with a surface. Because the surface of the previously wetted part remains wet there is no hysteresis here, but as soon as the wetted part ends the hysteresis will keep the interface in place and thus stop the wicking –  Michiel Mar 3 '13 at 9:22
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It will wick down, if the wicking action is strong enough. To demonstrate this with a stronger wicking action, get a one-inch wide strip of paper towel. Fold it over so one end goes to the bottom of the cup or glass, and the other end goes down to the desktop. Now pour in water (or tea) to fill the cup, and come back in an hour to see the result.

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