Why Does Paper Absorb Water?

by Yuanzhang Mao

When you hold a paper towel vertically and gently dip it into water, water climbs up the paper towel. This is a very useful phenomenon when we want to clean up the table as paper towels can absorb so much water/liquid. We almost take this phenomenon for granted. However, thinking about this again, I have realised that objects tend to fall due to their weight. Then, how could water, since it definitely has weight, move against the downward pulling of its own weight, up the paper towel?

Such phenomenon where water climbs a paper towel is called capillary action. Capillary action can also be observed when a very narrow glass tube is dipped into water. (Figure 1)

Figure 1 Capillary action is observed with a thin glass tube dipped into water. There is colouring added into water so that the water can be seen more clearly. Water has climbed up the tube:

To understand the capillary action, we need to understand first what water is under a microscopic scale. Pure water contains water molecules, that is, extremely tiny particles that are each about 27 nanometres (0.000000027 metre!) across. [1] You can analogise water comprising water molecules to a sand dune comprising sands. In addition, these water molecules, due to their special composition, are like tiny magnets, attracting each other. In other words, the water molecules tend to stick together. This is called cohesion. In a similar way, water molecules also tend to stick to the wall of the glass tube, which is called adhesion.

Therefore, when dipped into water, the wall of the glass tube attracts the water molecules that are in direct contact with it, pulling them upwards. Then, due to cohesion, the water molecules in the middle of the tube are pulled upwards too by those raised at the wall of the tube. [2] Hence a column of water above the surface forms. In the example of the paper towel, it is the paper towel that water molecules have adhesion to. And in the same way as described above, water can be absorbed by the paper towel.

But water molecules don’t have adhesion to everything. For example, if a narrow plastic tube is used instead of a narrow glass tube, capillary action will not be observed. This is because water molecules are not attracted to plastic, therefore, the water is not pulled up by the plastic tube. [3] It is also suggested that capillary action is used by trees to transport water from their roots to all other parts of the them. [4]

References

[1] Cornell NanoScale Science and Technology Facility (2012), MOLECULES: Lots of SHAPES and SIZES [online] Last accessed 7 February 2023: https://nnci.net/sites/default/files/inline-files/nanooze_edition_11-1ljz4uy.pdf

[2] Wikipedia (2023), Capillary action [online] Last accessed 8 February 2023: https://en.wikipedia.org/wiki/Capillary_action

[3] Kurihara K, Hokari R, Takada N. Capillary Effect Enhancement in a Plastic Capillary Tube by Nanostructured Surface. Polymers (Basel). 2021 Feb 19;13(4):628. doi: 10.3390/polym13040628. PMID: 33669695; PMCID: PMC7922975.

[4] Wikipedia (2023), Xylem [online] Last accessed 8 February 2023: https://en.wikipedia.org/wiki/Xylem