Excitability and oscillations of active droplets

In living cells, cycles of formation and dissolution of liquid droplets can mediate biological functions such as DNA repair. However, the minimal physicochemical prerequisite for such droplet oscillations remains elusive. Here, we present a simple model composed of only two independent chemical components with their diffusive and chemical fluxes governed by non-equilibrium thermodynamics. There is turnover of fuel that maintains a chemical reaction away from equilibrium, leading to active droplets. We find that a single active droplet undergoes a pitchfork-bifurcation in the droplet volume upon increasing the fueling strength. Strikingly, the active droplet becomes excitable upon adding a further chemical reaction. For sufficient fueling, the system undergoes self-sustained oscillations cycling between droplet formation and dissolution. The minimal nature of our model suggests self-sustained active droplets as functional modules for de novo life.