(352h) Origin of Fast Liquid–Liquid Phase Separation Dynamics in Polyelectrolyte Complex Coacervation

Polyelectrolyte complex coacervation (PEC) is an important class of liquid-liquid phase separation (LLPS) process driven by electrostatic associations between oppositely charged polymers, with broad implications in biology and materials science. While PEC thermodynamics is well studied, its dynamics—crucial for cellular organization and rapid biological adaptations—remain poorly understood.

Conventional understanding of LLPS dynamics is based on a droplet coarsening picture, which yields a slow domain growth (R ~ t^1/3). However, experiments and biology reveal much faster phase separation (seconds to minutes) from dense, semi-dilute mixtures. The mechanisms behind this acceleration are unknown.

Using molecular dynamics, we show how initial mixing conditions dictate non-equilibrium pathways that dramatically accelerate LLPS. When starting from well-mixed solutions, coarsening results in early-stage network formation with R ~ t^1/2, which can further accelerate to R ~ t due to hydrodynamic pumping upon network breakup. When starting from separated polycation and polyanion domains, mimicking marine organism adhesives, LLPS exhibits ultrafast dynamics, R ~ t^2/3, driven by electrochemical potential gradients.

Our work reveals the kinetic origins of rapid LLPS, bridging theory with biological and experimental observations. These insights extend beyond PEC to biomolecular condensates, and represent a significant advance in non-equilibrium phase separation dynamics.