For millennia, we have known how to make threads, but no one had really understood why the assembly of fragile fibers constitutes a solid thread. Thanks to a study published on February 18 in Physical Review Letters by two Frenchmen, Antoine Seguin (University of Paris-Saclay) and Jérôme Crassous (University of Rennes), the answer is finally known.
Before revealing this secret, let’s remember that the threads in question are those obtained by twisting several parallel fibers, which, from a certain number of turns, become very robust and no longer unravel. Of course, if you twist too much, the individual fibers break and the whole braid comes apart and breaks.
“In 2018, an English team led by Raymond Goldstein of the University of Cambridge had described this sudden increase in resistance by a phase transition phenomenon, like water suddenly becoming solid with temperature, recalls Jérôme Crassous. But that did not convince me and we decided to study this phenomenon in more detail. » In particular by doing experiments, which everyone can repeat at home. Take, for example, mops made of several large threads. Put them head to tail by tangling the wires, then turn: it becomes impossible to separate the two brooms!
The researchers did not use these brooms, but wire brushes, also put head to tail, then twisted together. Then they measured the pulling force as a function of the stretch and the number of turns. Surprisingly, the force amplification is smooth and exponential, but it is not a phase transition.
Friction in intermingled systems
The main reason is to be found in an old law of mechanics that we owe to Coulomb in the XVIIIe century, for whom the force required to pull a mass horizontally is proportional to its weight. This is how a few turns of the rope around a mooring bollard, without a knot, are enough for the rope to stop slipping. This is also what explains another spectacular phenomenon: two large books with pages interspersed into each other are very difficult to separate, as shown in 2016 by the French team of Frédéric Restagno, CNRS researcher at the Paris-Saclay University.
In the case of twisted yarns, the friction takes place between individual fibers which wind around each other. This generates a very strong frictional force, which increases with the number of revolutions, as the researchers also modeled. “This work is very elegant. It presents a natural extension of ours on intercalated books, since it looks at the amplification of friction in intertwined systems. My admiration is mixed only with a little jealousy for not having done it before them! »appreciates Frédéric Restagno.
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