Why cotton holds on to water
Short answer: Cotton fibre is hollow and draws water into itself. It can hold up to a quarter of its weight as moisture and gives the water back slowly, which is the whole explanation for why cotton feels heavy and dries late.
The fibre is a small tubular cell
Cotton is a plant cell with a hollow space inside, almost like a thin tube. When the fibre is dry, the hollow is empty and flattened. That hollow is what sets cotton apart from fibres that are solid all the way through, and everything that follows builds on it.
Water is drawn in and bound
When cotton gets damp, water is pulled into the hollow and bound to the cellulose in the fibre wall. That is why the fibre swells when wet. It is not the surface that takes in the water but the inside of every single fibre, which is why the fabric feels so clearly wet.
Why cotton gets heavy
Once the hollow is full, the garment weighs far more than when dry. The weight is not on the surface but inside each fibre, so you cannot shake it off. That is one reason a soaked cotton shirt drags at the shoulders. See bamboo sleepwear for women.
And why it dries slowly
The water inside the fibre first has to reach the surface before it can evaporate. That journey takes time and makes cotton a poor fit for hard activity in the cold, where a wet, heavy shirt quickly turns unpleasant. See bamboo sports bra and bamboo underwear for women.
Cotton is not a bad material
It is worth saying that none of this is criticism. For everyday wear and bedding the properties are pleasant, and the ability to take up moisture can feel good against the skin. It is only when a garment has to handle large amounts of sweat for a long time that the slow drying becomes a problem.
How viscose compares
Viscose is also made of cellulose, but the structure of the fibre is more open. It takes up more water inside the fibre than cotton, yet dries about as slowly. The difference between the two is mostly how much water is absorbed, not how quickly it lets go again.
What dries fastest
Polyester and other synthetic fibres dry fastest because they take up almost no water. The water stays on the surface and evaporates from there. The price is that a synthetic garment does not absorb moisture in the same way, so sweat stays on the skin. Every choice has a cost.
What it means day to day
In practice, cotton suits calm days and the night, when you are not sweating heavily. If you head out in the cold or raise your pulse, cotton is the weaker choice, because the moisture you produce stays put. Think about the activity before you put the garment on, and change if you get wet.
How to handle it
If a cotton garment has got wet, change it as soon as you can rather than waiting for it to dry on your body. Hang it out flat to dry, and turn it inside out so the seams get air too. When choosing something new, read the fibre content and pick for the need.
Frequently asked questions
Is cotton a poor material then
No, not at all. For everyday wear and bedding the properties are pleasant, and the problem only arises when the garment has to cope with a lot of sweat or hard activity for long.
How does viscose compare
Viscose takes up more water inside the fibre than cotton, yet dries about as slowly. The big difference lies in the amount of water taken up, not in the drying time.
What dries fastest
Polyester and other synthetic fibres, because they take up almost no water. The water stays on the surface and evaporates quickly, but sweat also sits longer on the skin.
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