The absorption of ink into blotting paper is mainly due to which physical phenomenon?

Introduction / Context:
Blotting paper is commonly used to soak up excess ink from writing. The way ink quickly spreads into the paper is a visible example of a microscopic physical process. This question asks which phenomenon is primarily responsible for ink absorption into blotting paper, which connects the idea of capillarity with everyday observations.

Given Data / Assumptions:

• Blotting paper is a porous material with many tiny pores or capillaries.
• Ink is a liquid that can flow into these pores.
• We are asked to identify the dominant physical phenomenon that draws ink into the paper.

Concept / Approach:
Capillary action occurs when liquid rises or moves in narrow tubes or pores due to the balance between cohesive forces within the liquid and adhesive forces between the liquid and the solid surface. Blotting paper has a network of microscopic capillaries that draw liquid ink into them. While viscosity, diffusion, and other effects are present, the main cause of the rapid, upward and lateral movement of ink in the paper is capillary action, not siphon action or simple diffusion alone.

Step-by-Step Solution:
Step 1: Observe that ink does not just slowly spread by random motion; it quickly rises and spreads through the pores, even against gravity to some extent. Step 2: Blotting paper is designed to be highly porous, providing many narrow pathways similar to capillary tubes. Step 3: Capillary action in these tiny pores pulls the ink into the paper due to adhesive attraction between ink and fibre and cohesive forces within the liquid. Step 4: Therefore, the dominant phenomenon is capillary action, which explains both the speed and direction of the ink movement.

Verification / Alternative check:
Experiments in school often show that when the lower edge of blotting paper is dipped into coloured water, the liquid climbs upward through the paper. This is similar to the capillary rise of liquids in narrow tubes. The same behaviour is visible when a paper towel soaks up water from a surface. These examples are always explained using capillary action, confirming that this is the relevant phenomenon here as well.

Why Other Options Are Wrong:
Viscosity of ink: Viscosity describes resistance to flow; high viscosity would actually slow down the movement rather than cause it. Diffusion of ink through the paper: Diffusion is random motion of molecules and is a much slower process, not responsible for the rapid rise of ink against gravity. Siphon action: A siphon involves a continuous tube and pressure difference between two reservoirs; this does not match the situation inside blotting paper.

Common Pitfalls:
Students sometimes confuse any spreading of liquid with diffusion and any movement through a tube with siphon action. The key recognition is that capillary action can pull liquid upward against gravity in very narrow spaces due to surface tension and adhesion. Whenever you see liquid rising in narrow pores or thin tubes, think of capillary action first.

Final Answer:
Ink is absorbed into blotting paper mainly due to the capillary action phenomenon.