
Light, soft, resistant, deformable and sometimes ugly, knitwear is not just an everyday object; it’s also a metamaterial whose extraordinary properties are of great interest to physicists.
While physics research is often associated with such gigantic technical infrastructures as the LHC, it also sometimes takes an interest in everyday objects. For a very long time now, I’ve had a keen interest in everyday materials with extraordinary properties," explains Audrey Steinberger, CNRS research fellow at the Physics Laboratory of the ENS Lyon 1 . My original motivation for studying physics dates back to secondary school, when I learned that a solid is normally denser than its liquid phase, whereas ice floats on water. "
From textile to metamaterial
An interest that led her to her thesis and beyond. In fact, it was during her post-doctorate that Audrey Steinberger became interested in the astonishing properties of an a priori rather exotic material: knitting. One of my colleagues was a knitting fanatic and taught me how to knit," recalls the scientist. Knitting is light, strong and extremely deformable, and it’s unusual to combine these properties. So I started asking myself a lot of questions from the perspective of materials physics. "
When I compare myself with researchers in textile mechanics, I think we have very different approaches," explains Audrey Steinberger. Whereas they’re 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 fiber assemblies. "
The deformability of knitwear comes from its structure, where a yarn forms interlocking loops, also known as stitches. When a sweater or sock is deformed, it’s the loops that deform, 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 high-tech objects made in laboratories, but knitting belongs to 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 theUniversity of Aoyama Gakuin (Japan), Audrey Steinberger presented a study carried out on Jersey knitwear, the most common manufacturing method and the easiest to produce with industrial machines. This work is based on numerical modeling of friction fiber systems carried out by Jérôme Crassous, and on an experimental system developed by Audrey Steinberger to verify and confirm the simulated results.
First observation: knitwear (or socks) has multiple forms of balance. Knitwear is able to conform to the shape of the body, and can then be folded and stored 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 knitwear 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 wires. If you put a notepad on 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 notion is found in avalanches and granular media. We’ve 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 point
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 rectangle shape corresponds to a particular point of equilibrium of the knitted fabric at rest. What’s more, as friction increases, so do the number of possible balance points.
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 terminal point can be used as a reference for all kinds of mechanical experiments on knitting. It allows us to carry out reproducible work and compare results more easily. This is what knitting research has lacked until now. "
The work 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 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- and vibration-absorbing materials.