Toy story: tiny rods self-organise, try to escape

Physicists have found a way to turn brainless toys into an organised army. The secret? Put a fence around them.

Hamid Kellay and his colleagues at the University of Bordeaux in France were playing with some off-the-shelf toys – vibrating, finger-size rods – and found that when corralled together they sometimes acted with apparent common purpose.

“The rods can’t think, can’t see each other, can’t communicate with each other, but when we put them inside something – a flexible arena – that something became alive,” Kellay says.

The team videoed the collection of four-centimetre-long rods pushing the arena fence around objects and through tiny gaps. 

Kellay suggests the behaviour could be adapted to drive systems such as a cleaning robot.

“This entity could access tight spaces or complicated structures that would be awkward for a single larger robot,” he says.

“It could depollute land much more effectively and cheaply.”

The University of Bordeaux team described their experiments in the journal Physical Review Letters. The rods have short, angled legs which, when combined with the battery-driven vibration, propel them at speeds of up to 40 centimetres a second.

The physicists initially imagined that multiple rods let loose in an enclosure would shoot around bouncing off each other in a way reminiscent of how molecules in gases behave. And indeed they did – at first. 

However, as more rods were added, they began to collect around the walls. And when the wall of their corral was made movable, the collective push from the rods started deforming the wall in apparently organised fashion, which the scientists did not expect.

“We were very surprised – why is this happening?” Kellay asks.

As they analysed the movement the researchers realised the way the rods pushed on each other could form a mechanically stable system when the conditions were right.

Their new understanding of the conditions for cooperative behaviour opens the way to designing machines for tasks such as exploration and cleaning of awkward spaces.

Brainless and sightless, the fingerlike rods spontaneously organise to guide their enclosure through a narrow opening.
A. Deblais et al., Phys. Rev. Lett. (2018)

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