Question

Blotting paper quickly soaks up ink when pressed against a wet page. Which physical phenomenon is responsible for this absorption?

Hint:

Blotting paper has narrow fibre gaps that act like capillary tubes.

Answer 1

Blotting paper looks fairly plain, but under a microscope it is really a mesh of fine fibres with countless narrow spaces running between them, similar to a bundle of extremely thin tubes packed side by side.

When wet ink touches this surface, two forces come into play. First, the ink molecules attract each other, called cohesion, and second, the ink molecules are attracted to the paper fibres, called adhesion. Because adhesion between ink and paper fibre is strong, the liquid clings to the fibre surfaces and gets pulled deeper into the narrow gaps, working against gravity.

The narrower the gap between fibres, the more strongly the liquid is drawn in, this is the essence of capillarity, seen also when water climbs up a thin straw dipped in a glass or moves upward through soil and plant roots.

So it is capillary action, driven by surface tension and adhesion in the narrow fibre gaps, that makes blotting paper absorb ink so effectively.