Why does a towel get stiff when it hangs to dry but soft when it gets in the dryer?
In: Physics
Electrical forces between water molecules make them kinda sticky to each other, in a way which causes things like surface tension, meniscus, and capillary action.
When cloth fibers are wet and the water is evaporating, the capillary action of the water remaining can produce a kind of sticky sucking-force between neighbouring fibers, which pulls them together and gets their rough edges tangled and clumped up with each other. And when they’re closely tangled and clumped with their neighbours like this, they’re more rigid. When the fabric air dries, there’s a lot of time for this to happen, and there’s no movement to disrupt the process. In a tumble dryer, the fabric is constantly bending and flexing, and this breaks up those tangles and clumps in the microscopic fiber.
The stiffness after line drying is mostly due to the water that doesn’t evaporate quickly “gluing” the fibers of the towel together.
It has to do with the ability of the cellulose in the cotton fibers to retain water.
This website breaks it down really simply and even has diagrams! https://www.insidescience.org/news/why-towels-get-so-stiff-when-you-dry-them-line
Take that stiff towel and roll it back and forth a few times. Notice how it’s no longer as stiff? Now imagine rolling it continuously for 20-40 minutes. Any tightness caused from the drying process and the material shrinking is broken up.
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When fabric dries, chemical bonds (hydrogen bonds) form between fibers. This makes it stiff. If you shake it out, the stiffness goes away. When fabrics tumble dry, the stiffness gets shaken out right away.
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