Spontaneous Fusion of Aqueous Two-Phase Droplets: Effect of Coexisting Phospholipids on Stability

The stability of aqueous two-phase system (ATPS) droplets is a key requirement for extending their utility beyond conventional applications, particularly as minimal models of biological cells and reaction compartments. However, the mechanisms by which their dynamic process in droplet size growth remain poorly understood. In this study, lecithin is demonstrated to effectively stabilize ATPS droplets by modulating two fundamental processes that determine their size evolution: 1) approaching migration driven by thermal motion and inter-droplet attraction and 2) the coalescence initiation time, defined as the delay between first contact and the onset of merging. Using dextran-rich droplets dispersed in a polyethylene glycol phase, significant suppression of both processes is shown that occurs only when lecithin concentrations exceed its critical micelle concentration. The findings indicate that lecithin coats droplet surfaces spontaneously to form a membrane-like layer, thereby reducing inter-droplet attraction and strongly inhibiting coalescence. This stabilization strategy offers a simple yet powerful means to control droplet dynamics, thereby opening up new possibilities for engineering ATPS droplets as versatile platforms for biochemical reactions and synthetic cell research.