The cobra wave is a cousin of the dominos race in which popsicle sticks are interlaced in a diamond lattice. When one ending stick is released, the sticks are ejected one by one, and the lattice goes up, like the head of a cobra. This spectacular phenomenon gave rise to lots of viral videos .
Initially proposed as a problem in the International Physicist Tournament, this phenomenon has recently been explained by a collaboration between Frédéric Chevy (ENS) and Christophe Clanet, David Quéré and Jean-Philippe Boucher (École Polytechnique) in an article publish in Physical Review Letters.
They showed that the cobra wave resulted from a reaction propulsion in which the fuel is the elastic energy stored in the deformations of the sticks, and the propulsion is ensured by their ejection.
In particular, they studied the dynamics of such a lattice during the release of the sticks :
From this analysis, they deduced that the cobra wave could only exist when the sticks obey strict constraints. If the elasticity is too weak, the elastic energy stored is not sufficient to overcome gravity and the cobra remains on the floor. On the contrary, if the elasticity is too strong, the sticks break due to big deformations.