Emergent Beating in Colloidal Matter: Stabilization via Symmetry Breaking*

Emergent order is routinely observed in far-from-equilibrium collectives as varied as swarms of robots and active colloidal suspensions. Here, we study the self-organized behaviors of a colloidal system of active Janus particles (~250um) suspended at the air-liquid interface of a hydrogen peroxide droplet. The particles consist of an upward facing SU8 side and a downward facing Pt patch that grows oxygen bubbles by catalyzing the decomposition of the peroxide bath. An individual particle's bubble growth is self-limited by the resulting decrease in exposed surface area of the Pt patch, leading to stable bubble growth that does not tend to burst spontaneously. Once other particles are introduced, however, their respective bubbles can merge into larger shared bubbles capable of bursting. We show that a pair of particles exhibit robustly periodic beating despite the well-known chaotic dynamics of bubble collapse. As additional particles are introduced, the system gradually transitions from periodic beating into uncorrelated random bursts in time. However, we find that breaking permutation symmetry by increasing the Pt patch size of a single particle in the collective allows the system to access configurations where many-body order is possible, and recover the robust periodic beating behavior originally observed solely in the two-particle setting. Using techniques from dynamical systems, stochastic processes, and nonequilibrium thermodynamics, we characterize the different behavioral regimes of the system, the nature of transitions between regimes, and offer hypotheses for the role of symmetry-breaking in the emergent periodicity of the system.