phaseseparate

Phase separation is the process by which a homogeneous mixture becomes energetically favorable to split into two or more distinct phases with differing compositions or structures. It occurs in liquids, polymers, alloys, colloids, and biological systems, driven by thermodynamic instability or incompatibility among components.

In a phase diagram, a miscibility gap or binodal line marks conditions under which the mixture is unstable to demixing. Inside the spinodal region, small fluctuations grow spontaneously (spinodal decomposition). Between the spinodal and binodal curves, phase separation typically proceeds via nucleation and growth, where droplets or domains form and expand.

Two main pathways characterize phase separation. Spinodal decomposition creates interconnected patterns with a characteristic length scale that coarsens over time as domains merge. Nucleation and growth involves overcoming an energy barrier to form discrete droplets that grow, collide, and coalesce. Both pathways are followed by coarsening processes such as Ostwald ripening, where larger domains grow at the expense of smaller ones.

The kinetics and resulting morphology depend on composition, temperature, and molecular mobility. Mathematical descriptions often use free-energy functionals and the Cahn–Hilliard equation to model diffusive phase separation; Flory–Huggins theory is commonly used for polymer blends and copolymers.

Applications of phase separation include tailoring properties in polymer blends and composites, controlling microstructures in alloys, stabilizing emulsions, and understanding biological organization where membranes and condensates form through demixing phenomena.