Knitwear, a metamaterial of every day

A researcher from the Physics Laboratory at ENS de Lyon "unravels knitwear" to turn it into a research subject...

This article made the front page of the latest Pop Sciences newsletter. Written by Martin Koppe for the CNRS, the article honours Audrey Steinberger and her subject, which might be considered "exotic": knitwear. And yes, she studies knitwear, but through it she "studies metamaterials and disordered fibre assemblies".

Light, soft, strong, deformable and sometimes ugly, knitwear is not just an everyday object; it is also a metamaterial whose extraordinary properties are of great interest to physicists.

While physics research is often associated with technical infrastructures as gigantic as the LHC, it also sometimes takes an interest in everyday objects. "For a very long time, I’ve had a keen interest in everyday materials that have extraordinary properties," explains Audrey Steinberger, CNRS scholar at the Physics Laboratory of the ENS de Lyon 1 . "My original motivation for studying physics dates back to secondary school, when I learnt that a solid is normally denser than its liquid phase, whereas ice floats on water."

From textile to metamaterial

It’s an interest that will carry her through to her thesis and beyond. It was during her post-doctorate that Audrey Steinberger became interested in the astonishing properties of a seemingly exotic material: knitwear. "One of my colleagues was a knitting fanatic and taught me how to knit," recalls the scientist. "Knitwear is light, strong and extremely deformable, and it’s not at all common to combine these properties. That’s when I started asking myself a lot of questions about the physics of materials."

These questions have since become a real research project. "When I compare myself to researchers in textile mechanics, I think we have very different approaches," explains Audrey Steinberger. "Whereas they are interested in the details of the material, I’m looking for minimal models where I no longer see knitwear as a textile, but as a frictional metamaterial. I study knitwear, but through it I study metamaterials and disordered assemblies of fibres."
The deformability of knitwear comes from its structure, where a thread forms intertwined loops, also known as stitches. When a jumper or sock is deformed, it is the loops that are deformed, not the yarn itself. "This detail is essential" says Audrey Steinberger. "In fact, an artificial material whose properties are dictated by its structure is called a metamaterial. These are normally cutting-edge objects made in laboratories, but knitting is part of everyday life."

The secret of jersey

In a recently published article 2 with Jérôme Crassous, professor at the University of Rennes and member of the Rennes Institute of Physics 3 , and Samuel Poincloux, assistant professor at Aoyama Gakuin University (Japan), Audrey Steinberger presented a study carried out on Jersey knitwear, the most common manufacturing method and the easiest to produce using industrial machines. This work is based on numerical modelling of friction fibre systems carried out by Jérôme Crassous, and on an experimental system developed by Audrey Steinberger to verify and confirm the simulated results.

First observation: the knitted fabric (or sock) has multiple forms of equilibrium. The knitwear is able to conform to the shape of the body and can then be folded to lay flat, moving easily from one shape to another. However, if a sock is pulled too hard, it will retain some of this deformation. There are therefore several possible states of equilibrium, which the knitted fabric can maintain without the influence of external forces, unlike a latex glove, which will always return to its initial conformation. In fact, the overall shape of the knitted fabric depends on the history of its deformations.

Audrey Steinberger and her colleagues have shown that this memory comes from solid friction at the points of contact between the yarn. "If you put a notepad on top of a copy of CNRS le Journal and tilt it, the notepad will initially be held in place by its solid friction, until the angle is too great and the pad starts to slide," explains Audrey Steinberger. "This concept is found in avalanches and granular environments. We have shown that a similar concept can be found in knitted fabrics, which retain their shape in the absence of external forces. But unlike classical elastic objects, several equilibrium configurations exist for knitted fabrics."

Socks and balance stitch

A network of interwoven loops, the geometry of the knitted fabric is defined by the ratio between the diameter of the yarn and the length of yarn taken up in each stitch. All knitted fabrics with the same ratio will behave in the same way from a physical point of view. The shape of the knitted fabric is thus determined by the dimensions of the rectangle in which a stitch fits, measured in relation to the length of yarn per stitch: each shape of rectangle corresponds to a particular point of equilibrium of the knitted fabric at rest. What’s more, the greater the friction, the more different possible points of equilibrium there are.

"Samuel Poincloux wanted to know if there was a well-defined state of equilibrium, and we discovered that there were several," explains Audrey Steinberger. "But one of them is remarkable: the end point can be used as a reference for all sorts of mechanical experiments on knitting. It allows us to carry out reproducible work and compare results more easily. This is what has been missing from knitting research until now."

The work has also enabled the researchers to explain what makes knitted socks comfortable, tight around the ankle, and less so elsewhere. This configuration is made possible by the existence of several different points of equilibrium, while the whole is held together by the interplay of friction. These results are still too fundamental to be exploited by the textile industry, but they open up a new field of research that could lead to the development of new shock-absorbing and vibration-damping materials.

  • CNRS/University of Rennes Unit.
  • . - Courtesy of CNRS