Fiber Friction: The Secret to Cozy Knits

Friction Between Yarn Loops Gives Knit Fabrics Their Shape-Shifting Ability

A well-knit sweater feels like a warm embrace, and the reason for that lies in friction.

A recent study uncovers how knit fabrics adapt to different shapes, allowing them to mold comfortably to a body or head. This adaptability results from the friction between interlocking yarn loops, according to physicist Jérôme Crassous and colleagues, who published their findings in the Dec. 13 issue of Physical Review Letters.

When a knit fabric is stretched and then released, it regains its form. One might assume it always returns to a singular shape, much like a rubber band. However, Crassous, from the University of Rennes in France, explains that this is not the case. “There is no unique shape,” he says. “There are many different possible shapes.” These forms are referred to as “metastable states.”

To investigate, researchers stretched a square of stockinette-stitched knit fabric on a rectangular frame, then released the tension and measured the length-to-width ratio. The results showed variation in the fabric’s final shape, depending on the stretching direction and intensity—evidence of multiple metastable states.

Computer simulations of fiber loops confirmed this effect. When friction between the loops was reduced or eliminated in the simulation, the fabric no longer displayed a variety of metastable states. Instead, it always returned to a single shape.

This discovery sheds light on the knitting practice of “blocking,” where knitters wet, shape, and dry a garment to fix its final form. Thanks to friction, this process helps ensure a perfect, cozy fit.