Hummingbirds, umbrellas and hopper poppers do it. But why not as quickly as expected?

Many elastic structures have two possible equilibrium states. For example umbrellas that become inverted in a sudden gust of wind, nanoelectromechanical switches, origami patterns and even the hopper popper, which jumps after being turned inside-out. These systems typically move from one state to the other via a rapid ‘snap-through’. Snap-through allows plants to gradually store elastic energy, before releasing it suddenly to generate rapid motions, as in the Venus flytrap . Similarly, the beak of the hummingbird snaps through to catch insects mid-flight, while technological applications are increasingly exploiting snap-through instabilities.

In all of these scenarios, it is the ability to repeatedly generate fast motions that gives snap-through its utility. However, estimates of the speed of snap-through suggest that it should occur more quickly than is usually observed. In their research published n Nature Physics, Oxford Mathematicians Michael GomezDominic Vella and Derek Moulton study the dynamics of snap-through in detail, showing that, even without dissipation, the dynamics slow down close to the snap-through transition. This is reminiscent of the slowing down observed in critical phenomena (for example the time taken for oscillations in the climate to die down is thought to grow larger as a ’tipping point’ is reached). As well as providing a handheld demonstration of such phenomena, the work provides a new tool for tuning dynamic responses in applications of elastic bistability: for example it shows that to obtain faster snap-through in applications such as robotics, the system needs to be pushed well beyond the snap-through transition.