Control of collective activity to crystallize an oscillator gas

Motility-induced phase separation occurs in assemblies of self-propelled units when activity is coupled negatively to density. By contrast, the consequences of a positive coupling between density and activity on the collective behaviour of active matter remain unexplored. Here we show that collective activity can emerge from such a positive coupling among non-motile building blocks. We perform experiments with self-sustained oscillators powered by contact-charge electrophoresis. Although the oscillators are non-motile by design, they spontaneously form an active gas when confined together. The super-elastic nature of collisions constitutes a positive density–activity coupling and underlies the active gas properties. Elucidating the origin of binary collisions allows us to precisely control the structure of the active gas and its eventual crystallization. Beyond considering the overlooked positive coupling between density and activity, our work suggests that rich collective properties can emerge not only from the symmetry of interactions between active building blocks but also from their adaptable and responsive behaviour. In active matter systems, increasing density usually reduces activity. Now, in a system where density enhances activity, collective motion is shown to arise from non-motile oscillators when they are confined.