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Why water levels even out in walking water experiment

Posted: Mon Feb 13, 2017 6:16 pm
by dewunderlich
My daughter is doing the 'walking water experiment' where you put paper towels between glasses of colored water and watch the absorption process. We can find all kinds of information explaining why the paper towel absorbs the water, but we can't find any explanation on why the water levels end up even. Can somebody point us in the right direction?

Thank you

Re: Why water levels even out in walking water experiment

Posted: Fri Feb 17, 2017 8:22 am
by tdaly
Hi dewunderlich,

Once the three cups have equal levels of water in them the driving force for moving water from one cup to another cup has equalized. The system of cups, water, and paper towels wants to evolve to an equilibrium state, which is a state where the forces that drive the water to move from one cup to another cup are all equal. The difference in the height of the water in the cups creates a driving force that moves water from one cup to another cup via capillary action. But, once the water levels are equal, the driving forces are equal. So, the system looks like it is no longer changing. In actuality, individual water molecules continue to move between the cups via capillary action; however, there is no longer net movement of water from one cup to another.

Re: Why water levels even out in walking water experiment

Posted: Sun Feb 19, 2017 11:06 am
by dewunderlich
Thank you for that great reply to our post. That was a nice answer that my daughter could understand. It also answered another question we had. In this experiment there are 2 different colors of water draining into the cup that's between them so we ended up with orange water in the cup between the red and yellow ones and purple between the blue and red ones. After the water levels in all of the cups were evened out and the absorption process appeared to stop, the paper towels that were in the orange and purple cups started turning that color; so they actually were absorbing water the opposite direction.

Thanks again.