phaseseparation

Phase separation is the process by which a homogeneous mixture becomes heterogeneous, forming two or more phases with distinct compositions and properties. It occurs when thermodynamic stability is lost under the given temperature, pressure, or composition, creating a miscibility gap. The result is coexisting phases separated by interfaces.

Two primary mechanisms are recognized: spinodal decomposition and nucleation-and-growth. In spinodal decomposition, fluctuations grow spontaneously within the spinodal region, producing interconnected structures that coarsen by diffusion. In nucleation-and-growth, a barrier to nucleation must be overcome and small droplets appear and expand until coarsening reduces interfacial area. The kinetics are often described by models such as the Cahn-Hilliard equation for conserved order parameters and by Ostwald ripening concepts.

Phase separation appears in many systems. In liquids, immiscible pairs separate into layers (oil and water). In polymer science, blends and block copolymers exhibit macrophase separation or microphase separation, yielding varied morphologies. In metallurgy, phase separation influences alloy microstructures. In biology, liquid-liquid phase separation of proteins and RNA forms membraneless organelles, driven by multivalent interactions.

Phase diagrams in temperature-composition or temperature-chemical potential space summarize equilibria, while experimental methods such as optical or confocal microscopy, light scattering, small-angle X-ray scattering, and rheology probe structure and dynamics. The concept links thermodynamics with kinetics to predict phase boundaries, domain sizes, and material properties.