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Wool is a fibrous material, but other fibrous materials do not suffer the same problem.

Let us set the scene; a woollen jumper shrinking when put in the washing machine, then the dryer. This involves wetting and heating the jumper, then allowing it to cool and dry. It also tumbles as it is washed, but I do not think this is important. What about this process causes the jumper to shrink?

Do the fibres themselves shrink, or do the become more tightly bound?

Having googled the matter, it seems the process going on is called "felting" and has to do with heat or water "shocking" the fibres - whatever that means - allowing them to settle into a tighter configuration, but I am interested in the exact physical process we are experiencing. What causes heat or water to "shock" the fibres, and why does it happen to wool much more than other fibrous materials? What is this shocking process; it sounds like the heat of the water overcoming the electrostatic attraction of the fibres, but these website are vague about the physics.

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It's kind of like "material memory." Wool is pretty kinky originally (as sheared), and the production processes pull the strands straight. Hot washing allows the material to revert to a tightly-wound config, which reduces the external dimensions, aka "shrinking."

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