[Reading Science] The Secret of Water-Expanding 'Coin Tissue'

What is the principle behind the 'coin tissue' that swells up when water is poured on it?
[Photo by YouTube screen capture]

[Asia Economy Reporter Kim Jonghwa] There are tissues that swell up when water is poured on them. They are called 'coin tissues' because they are compressed into a coin shape, or 'aqua tissues' because water is poured on them.

Why does this coin tissue, which has a very hard and solid surface, rapidly increase in volume when it meets water? It is generally understood that this is because it absorbs water, but not all materials with good absorbency swell like coin tissues just by absorbing water.

If you cut a coin tissue in half and look at the cross-section, you can see layers stacked on top of each other. This suggests that it is compressed with considerable force. Under a microscope, very fine fibers are intertwined. These tangled fibers are the core of the coin tissue.

Water seeps through these fibers, causing the volume to increase. The water penetrating between the fibers is due to the 'capillary phenomenon.' The capillary phenomenon refers to the situation where, when a fine tube (capillary) is placed in a beaker containing liquid, the liquid level inside the capillary is higher or lower than the liquid level outside.

If the liquid in the beaker is water, the liquid level inside the capillary rises above the liquid level in the beaker, but if it is mercury, the liquid level inside the capillary is lower than that in the beaker.

The simplest example of the capillary phenomenon is when you pour water halfway into a cup; the water level at the part touching the inner surface of the cup is higher than at the center. If you cut it in cross-section, it forms a concave shape. In the case of water, the adhesion between the cup and the water is stronger than the cohesion between water molecules, so the water sticks to the cup and the level rises.

The thinner the capillary, the higher the liquid level inside the tube compared to the liquid level in the beaker. The coin tissue utilizes this property of water. The tangled fibers create empty spaces where water can easily penetrate.

To enhance this property further, the tissue material uses highly absorbent and soft 'viscose rayon (nonwoven fabric).' It is also manufactured using the 'spun-lace' method, which entangles fibers with a high-pressure water flow but causes almost no damage to the fibers.

Highly absorbent fibers are rolled like a cylinder and compressed into a coin-shaped tissue by applying strong pressure from top and bottom. When water is poured on it, the highly absorbent fibers instantly soak up the water, causing the volume to expand.