Active-matter systems are composed of elements that consume energy. Living things are the most obvious members of this category, but the definition is broad enough to include a number of artificial systems as well.

Credit: Macmillan Publishers Ltd

As is the case in many other fields of physics, the appeal of studying synthetic active-matter systems comes from the possibility of breaking down complex phenomena into their components, and investigating them in easily tunable environments. Testing hypotheses on an ensemble of bespoke robots sounds a lot simpler than messing around with a flock of birds.

A characteristic of many active-matter systems is their tendency to exhibit collective motion or self-organization, leading to non-equilibrium steady-state structures. But what exactly drives that tendency? In other words, what degrees of freedom need to be excited in order to elicit collective behaviour? As real systems are so complex, this sounds like the sort of question that a synthetic analogue might help us unravel.

This is precisely what drove Christian Scholz and co-workers to study the dynamics of rotors — 3D printed cog-shaped robots. Each rotor comprises a cross-shaped cap atop a cylinder that rests on seven tilted legs, angled at 18 degrees. Put in contact with a vibrating surface, the legs transform the vibration into a rotation, clockwise or counter-clockwise depending on their tilt.

What Scholz and colleagues have now shown (Nat. Commun. 9, 931; 2018) is that an ensemble of 420 rotors — 210 rotating in one direction and 210 in the other — exhibit collective motion and phase separate into domains of co-rotating rotors (pictured, with black and white rotors spinning in opposite directions). This might sound counter-intuitive at first: two adjacent rotors spinning in the same direction tend to get in each other’s way and can’t freely rotate — so why would they clump together? In fact, the authors argue, it’s precisely the long contact time generated by collisions that leads to the spontaneous separation of the ensemble.

The authors established a quantitative analogy between their ensemble of discrete and colliding rotors with real active-matter systems, typically driven by completely different forces, by showing that the dynamics they observed can be fitted to a system of coupled Langevin equations.

Although it’s too early to say whether this system of synthetic active spinners may help us understand living systems, it’s a welcome addition to our toolbox for studying active matter.