Q: What is the science squee mwaist cincher behind cotton T-shirts shrinking in the dryer?
A: You may have heard that if you measure your height when you wake up and again when you go to bed, the results would differ by about a half an inch. Just the stress of daily gravity compresses the discs in our spine, forcing fluid out. As we sleep, the discs reabsorb the fluid, plumping up for another day.
Similarly, it’s a stress-relaxation cycle that can shrink your T-shirts, only the process occurs in reverse and generally can’t be undone. Sai Janani Gansean at the University of California-San Francisco explains:
Your T-shirt is cloth woven from threads that themselves have been made from cotton fibers. These fibers are long molecular chains linked end-to-end by hydrogen bonds.
As these fibers are spun into thread and the thread into cloth, the fibers get stretched. So when we buy the shirt, it’s at its most stretched-out state. The bonds holding everything together are, in human terms, stressed out.
Just as you jump in a sauna to relax, putting the T-shirt into hot, agitated water and then a hot dryer relieves the stress on the hydrogen bonds. The molecular chains return to their original size, and your T-shirt shrinks.