— New high-speed footage of the humble rubber band has treated physicists to a rare insight into the properties of elasticity, and discovered the truth about exactly why an elastic band recoils.
Nicolas Vandenberghe at the University of Provence in Aix-Marseille, France, and colleagues filmed the pinging acrobatics in an attempt to study a process known as "dynamic buckling instability".
When a stretched rubber band is released at one end, a front of stress-free elastic material propagates towards the clamped end. When this front rebounds it results in a compression front that propagates backwards. This triggers an elastic instability referred to as dynamic buckling.
Normal buckling occurs when a solid rod is compressed by a load above a critical threshold. The material can no longer support the load and maintain its structure, and so the rod bends.
Dynamic buckling is the version of this instability which occurs when the compressive load is applied suddenly when a rod is smashed into along its axis, for example. It creates a well-defined compression wave that zings along the rod.
Now our work shows that this [dynamic buckling] is responsible for rubber band recoil, Vandenberghe says. Moreover, since rubber can be considerably stretched compared to other materials, we were able to visualise and to document the different stages leading to rubber band recoil using a high speed camera.
The team has now used the new measurements to propose a complex theoretical description of the dynamic buckling phenomenon, which they say can be used in the study of fragmentation of elastic bodies experiencing impacts.
Movie 1 shot at 1930 frames per second shows a rubber band initially stretched and suddenly released. It is an event that usually lasts for no more than a millisecond. It shows the propagation of longitudinal waves along the elastic and its bending, which results from dynamic buckling.
In movie 2 recorded at 14300 frames per second the band is initially stretched and one end is released. The front propagates towards the clamped end and drags the free region. The wave front reaches the clamped end, and a compression front propagates backward and triggers a dynamic buckling instability.
In movie 3 recorded at 38461 frames per second the band is stretched to 120% of its original length and then both ends are released. When the two fronts cross in the middle of the elastic the configuration is equivalent to the impact of two strain free rubber bands. After impact, the two compression fronts propagate away from the impact point and trigger a buckling instability.
Movie 4 recorded at 2000 frames per second shows a band that is initially stretched and then released in a viscous fluid of glycerol. When the front propagates, it drags the elastic and fluid particles. A boundary layer is formed around the band, dragging the elastic and leading to a modification of the wave-front profile.
Journal Reference: Proceedings of the Royal Society A (DOI: 10.1098/rspa.2006.1781)