Merging-limited coarsening of nanoscale condensates

Droplet coarsening is a common phenomenon in which smaller droplets naturally grow into larger ones to minimize their interfacial free energy and achieve global thermodynamic equilibrium. Here, we observe that coacervate droplets of small sizes (ranging from tens to hundreds of nanometers) remain stable over hours with significantly slower coarsening rates than predicted by classic theories. Using scaling analysis, Monte Carlo simulations, and analytical theory, we demonstrate that the anomalously stable coacervates can be explained by a merging-limited coarsening (MLC) mechanism, in which merging probability becomes markedly low among coacervates of sizes smaller than a critical value, which is controlled by the internal viscosity and interfacial tension of the droplets. We find that biological condensates typically exhibit large critical sizes, making them prone to undergo slow coarsening through the MLC mechanism. Such merging-limited coarsening may represent a universal mechanism underlying condensate size control in synthetic systems and living cells.