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

The absorption of ink by blotting paper happens due to capillary action.

Step 1: Structure of blotting paper.
Blotting paper is made of loosely packed fibres that leave behind a network of extremely thin, narrow gaps between them, acting like tiny tubes or capillaries.

Step 2: What capillary action means.
Capillary action is the tendency of a liquid to rise or move through a narrow tube or narrow space, without help from any outside force, because of a combination of the liquid's cohesion, the attraction between its own molecules, and adhesion, the attraction between the liquid molecules and the surface of the tube.

Step 3: Applying this to blotting paper.
Because ink wets the paper fibres well, it is drawn into these narrow gaps by the same adhesive and surface tension effects that pull water up a thin glass tube. The narrower the gap, the stronger the pull, so the fine pores of blotting paper draw the ink in quickly.

Step 4: Answer.
So the phenomenon responsible is capillary action, the same effect that lets water rise through the thin tubes in a plant's stem or through a narrow glass capillary tube.