Why Can a Paper Towel Draw Water Upwards?

Dip one corner of a paper towel into water and the wet region climbs above the level in the glass. The towel is not producing suction. Its fibres contain a network of tiny, connected gaps that act as irregular capillary passages.

Paper is largely cellulose, whose hydroxyl groups can form hydrogen bonds with water, so water readily wets the fibres. Where water meets the walls of a narrow pore, the curved liquid surface and surface tension create a pressure difference that drives water into drier spaces. Cohesion between water molecules helps the liquid behind it follow. What looks like the towel pulling water upwards is capillary action produced by wetting, molecular attraction and surface curvature together.

Narrower pores can support a greater equilibrium rise in an ideal capillary, but flow through them is slower because viscous resistance is stronger. A real paper towel has uneven pore sizes, and its fibres swell as they become wet, so the advancing edge is rarely neat. The water cannot climb without limit: the growing weight of the water column eventually balances the capillary pressure, while evaporation and the complicated pore network impose further limits. Absorbency therefore depends not only on how readily a material is wetted, but also on how its pores connect and how easily water can continue moving through them.

https://openstax.org/books/chemistry-2e/pages/10-2-properties-of-liquids