A good sweater is like a cozy hug made out of yarn. For that, you can thank friction.
A new study reveals how knit fabrics can take on so many different shapes. It results from the friction between neighboring loops of the fiber knitted into a fabric. And it allows knits to hug close to the curves of a head or body.
Physicist Jérôme Crassous led the research. He works at the University of Rennes in France.
When a knit fabric is stretched and released, it springs back. You might think the fabric always returns to its original size and shape, like a rubber band. But “there is no unique shape,” says Crassous. “There [are] many different possible shapes.” These forms are known as “meta-stable states.”
In a series of experiments, Crassous and his colleagues stretched a square of knit fabric. It was created on a rectangular frame using a basic stitch known as stockinette. After releasing the force on the fabric, the researchers measured the ratio of its length to its width. That ratio varied, depending on how much the fabric was stretched and in which direction. This showed that the fabric could take on various metastable states.
Computer models of fiber loops showed the same effect. And when the scientists decreased or removed the friction in those models, the many metastable states disappeared. Without friction, the fabric always sprang back to the same shape.
Explainer: What is friction?
The researchers shared their findings December 13 in Physical Review Letters.
The effect helps explain a process called “blocking” that knitters often go through after knitting a garment. Blocking involves wetting the fabric, shaping it and then laying it out to dry. That locks the fabric into just the right shape to swaddle the body in warmth.
