Conventional phase segregation is controlled by a positive interfacial tension, which implies that the system relaxes towards a state in which the interfacial area (or length) is minimized, typically manifesting as a single droplet that grows with the system size. Intriguingly, the extension of the underlying Model B paradigm by two nonpotential terms (Active Model $\mathrm{B}+$) is able to describe the stable coexistence of many finite droplets. Here we numerically study Active Model $\mathrm{B}+$ in the vicinity of the transition between a single droplet (macrophase segregation) and multiple droplets (microphase segregation). Our results show that, although noise shifts transitions, the overall agreement with the mean-field theoretical predictions is very good. We find a strong correlation of droplet properties with a single parameter that determines the number, density, and fractal dimension of droplets. Deeper inside the domain of microphase segregation we observe another transition to a hexagonal lattice of regular droplets.

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• Liquid-liquid phase transition
• Mean field theory
• Numerical techniques
• Theories of collective dynamics & active matter