Climbing without a pump
Why Does Water Climb Up a Paper Towel?
Water climbs narrow gaps because it sticks to the walls harder than it sticks to itself.
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Step 1 of 5
Water sticks to things as well as to itself.
Two different grips are at work: water gripping water, and water gripping whatever it touches. Which one wins decides the behaviour.
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Step 2 of 5
Against a wall it climbs a little way.
Look closely at water in a glass and the edges curve upward, because water grips glass more strongly than it grips itself.
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Step 3 of 5
In a narrow gap the climb becomes dramatic.
A thin tube is nearly all edge, so the upward grip acts on almost all the water in it and can lift a column a long way.
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Step 4 of 5
Paper is a maze of very fine gaps.
A towel is a tangle of fibres with countless tiny channels between them, so water races up through the whole sheet at once.
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Step 5 of 5
It stops when weight balances the grip.
The column keeps rising until the weight of the water lifted equals the pull holding it, which is why the climb always stops somewhere.
Capillary action helps water reach the top of tall trees, though it cannot do the whole job on its own.
The short version
Water climbs narrow gaps because it grips the walls harder than it grips itself, rising until the weight it has lifted balances that pull.
Try it yourself
Stand a strip of kitchen roll in a shallow dish of coloured water. The colour climbs steadily upward with nothing pushing it.
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