If you put a wool sweater through the laundry, it shrinks. So why don’t sheep, which are, you know, basically made of wool, why don’t they shrink in the rain? The answer comes down to friction. Like all mammal hair, wool fibers are covered in overlapping scales that run the length of each fiber. These scales make it easier for the fiber to slide in one direction than the other, which is why, if you grab a strand of your hair and pull it through your fingers, you’ll find it moves more smoothly going toward the tip than toward the root end. When a wool – or human hair – sweater gets thrown into the washing machine and tossed around, this one-way resistance becomes a problem, because, as each fiber rubs against its neighbors, its scales act like little ratchets, only allowing it/the fiber to move in one direction.
The water makes things worse, both by causing the fibers to swell, which brings them into closer contact, and by softening the scales, which makes them floppy enough to get caught on neighboring fibers more easily, but not so floppy that they can give way and allow the fibers to move past each other. Heat also exacerbates the ratchet effect, probably by making the hairs more bendy, which brings them into greater contact, the same way cooked spaghetti noodles touch in more places than hard, uncooked ones. Throughout the course of the wash and dry cycle, the millions of tiny ratchets on the thousands of individual fibers in the wool sweater draw the fabric into a tighter and and tighter configuration, shrinking the overall size of the sweater. When sheep get caught in the rain, the fibers in their thick coats swell and their scales soften too, but their wool doesn’t get tossed around enough for the ratchet effect to cause it to tighten…unless those sheep get really RAMbunctious. Thanks for watching! The idea for this video actually came from a viewer – thanks Rebecca! We love hearing from you, and one of the best ways to weigh in on what we do is to join the MinuteEarth Patreon community, where you can let us know what you’re curious about, and ensure that we’re able to keep making videos.
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