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Take two empty cups (cup A and cup B) and fill cup A with water. Take a length of wet cloth and run it from the bottom of cup A to the bottom of cup B, while the cups are standing next to each other on a table. Wait a long time. The cloth will wick water from cup A to cup B until the water levels are equal in the two cups, just like a siphon would. My question is how does the information of the water level in cup A get transmitted through the cloth to the water in cup B?

In the case of a siphon, this information is transmitted as a pressure gradient through the tube that is acting as the siphon. This requires that the tube walls sustain compressive (and sometimes tensile) forces, so that water pressure varies with height throughout the tube length. But in the case of a cloth wick it is capillary forces that pull the water along. The water all along the cloth has free surfaces exposed directly to air, and so presumably at or close to atmospheric pressure. So if there are no semi-rigid external walls to contain the water flowing along the cloth, how doe sit transmit pressure information?

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Water flows by capillarity both ways. But the flow from the glass with higher level will be greater because it needs a smaller hill to climb. When levels are equal, flow from both directions equalize.

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  • $\begingroup$ Thanks, this makes sense. I am still unclear as to how the micro-scale forces balance out though. For example, your explanation can be understood from the observation that a vertical piece of cloth will only suck water up to some maximum height via capillary action. At that maximum height, there is a water-solid-air contact line where the water is trying to further wet the cloth. But the force up from the surface tension matches the force down from all the water below it. But in this case the cloth is fully wetted so there is no contact line. So where are the force balances coming from? $\endgroup$
    – Sean49
    Jan 4, 2020 at 23:25
  • $\begingroup$ When we try to go deeper it is really not easy to understand. For example: one glass full of water and another empty. The bottom of the empty one is set above the level of water of the full glass. If we put a dry toilet paper joining them, it gets wet after some time, even inside the empty glass. But no drop of water is released on it... $\endgroup$
    – Claudio Saspinski
    Jan 6, 2020 at 1:16

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