Fiber friction is the key to cozy knits

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 versatile shapes that allow them to conform to the contours of a head or a body. The effect is the result of friction between the adjacent loops of fiber that make up a knit fabric, physicist Jérôme Crassous and colleagues report in the Dec. 13 Physical Review Letters.

When a knit fabric is stretched and released, it springs back. One might imagine that the fabric always returns to the size and shape it previously had, akin to a rubber band. But “there is no unique shape,” says Crassous, of the University of Rennes in France. “There [are] many different possible shapes.” These forms are known as “metastable states.”

In a series of experiments, the researchers stretched a square of knit fabric, created with a basic stitch known as stockinette, on a rectangular frame. Then they released the force and measured the ratio of the swatch’s length to its width. That ratio varied depending on how much the fabric was stretched and in which direction, indicating the fabric could take on various metastable states.

Computer simulations of simplified fiber loops showed the same effect. And when the scientists decreased or removed the friction in the simulation, the multitude of metastable states disappeared. Without friction, the fabric would always spring back to the same shape.

The phenomenon helps explain the process knitters often go through after knitting a garment, known as “blocking,” which involves wetting the fabric, shaping it and laying it out to dry. That process locks the fabric into just the right configuration to swaddle the body in warmth.