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When I touch a paper towel to a drop of water I can see the water move upwards into the paper towel. What force is responsible for this gravity defying phenomenon?

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Capillarity, or capillary action, occurs when intermolecular bonding between water molecules and molecules of another material is greater than the intermolecular bonding among water molecules themselves. This is called adhesion, and when it happens water will tend to spread over the surfaces of the other material, even climbing against gravity to do so. Cohesion among the water molecules will drag along those water molecules not in direct contact with the material, and surface tension will hold the water together as it adheres to the surface. Adhesion may be strong enough to raise the water against gravity. If a material is porous enough, the water can continue to climb for quite a distance. But on a smooth non-porous surface, the water will stop climbing when the weight of the cohered water is greater than the adhesion can support.

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  • $\begingroup$ one could add that that is the way plant matter lives, drawing water up against gravity. $\endgroup$ – anna v Apr 12 '17 at 4:15
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Capillary action. In simple terms, water really likes to wet certain surfaces (e.g. your paper towel fibers), and is willing to do so against a small amount of gravity. If you have lots of wettable surface area per volume of water, then the water can be pulled up quite a distance.

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