Why does dry spaghetti always break into several pieces -- and not just two pieces -- when snapped? This perplexing question has now been answered by two physicists at the University of Paris 6 in France, who say that elastic waves travelling along the pasta cause it to fragment. The result has applications in materials and civil engineering (Phys. Rev. Lett. 95 095505).
When a dry strand of spaghetti is bent beyond its limit of curvature, it does not usually break in half but instead into several pieces, from as few as three to as many as ten. This behaviour has puzzled many scientists over the years, among them Nobel laureate Richard Feynman. To try and understand this behaviour, Basile Audoly and Sébastian Neukirch at the Laboratoire de Modélisation en Mécanique at the University of Paris 6 used the Kirchhoff equations to model how a thin elastic rod reacts to being bent.
According to Audoly and Neukirch, flexural – or elastic — waves travel along the length of the rod just after the initial break. These waves increase the local curvature of the rod and trigger an “avalanche” of new breakages, which in turn initiate more waves, so causing the rod to fragment.
“When a rod breaks, one would expect that the elastic waves triggered by the first crack would help the two halves of the rod to relax to equilibrium,” says Audoly. “Instead, we showed that these waves locally increase the stress, making the rod even more likely to fail again at a different place. Hence the many fragments.”
The French duo confirmed their predictions by taking high-speed images of individual strands of spaghetti as they broke (see movie). They say their model can be applied to other materials, such as fibre glass and the metal rods used in buildings and bridges, and could therefore help improve the safety of structures built using these materials.
