Fiber friction is the key to comfortable knits

A good sweater is like a cozy hug made of yarn. For that, you can thank friction.

New research reveals how woven fabrics can take on versatile shapes that allow them to conform to the contours of the head or body. The effect is the result of friction between adjacent loops of fiber that make up a woven fabric, physicist Jérôme Crassous and colleagues report Dec. 13. Physical review papers.

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

In a series of experiments, researchers stretched a square of knitted fabric, created with a basic knit known as a sock, on a rectangular frame. Then they released the force and measured the ratio of the swatch’s length to its width. This ratio varied depending on how far the fabric was stretched and in which direction, indicating that the fabric could assume different metastable states.

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

The phenomenon helps explain the process that knitters often go through after knitting a garment, known as “blocking,” which involves wetting the fabric, shaping it, and laying it out to dry. This process closes the fabric in the right configuration to cover the body with warmth.