Spinodaalinen

Spinodaalinen, the Finnish term for spinodal decomposition, describes a mechanism of phase separation in a homogeneous mixture that becomes unstable when quenched into a miscibility gap. In the spinodal region, small composition fluctuations grow spontaneously without a nucleation barrier, producing a bicontinuous structure that coarsens as the two phases separate.

Thermodynamically, instability arises because the second derivative of the free energy with respect to composition is negative, so long-wavelength fluctuations amplify. The process is commonly described by the Cahn–Hilliard theory, which models conserved-order-parameter evolution with a gradient-energy penalty. The fastest-growing mode has a finite wavelength, setting a characteristic domain size that increases during aging.

Morphology begins from random fluctuations and evolves into interconnected networks. In liquids, hydrodynamic flow can accelerate coarsening; in polymers and solids, diffusion dominates, and the growth law is typically diffusive (L ~ t1/3). The late stage may resemble droplets or bicontinuous structures, depending on interfacial tension and composition.

Spinodaalinen occurs in many systems, including polymer blends, metal alloys, oxide glasses, and miscible liquids driven out of equilibrium by quenching. It is observed with light scattering, small-angle X-ray or neutron scattering, TEM, AFM, or confocal microscopy, which reveal an evolving structure with a characteristic length scale.

Distinguishing spinodal decomposition from nucleation-and-growth is essential: spinodal lacks a barrier to formation and yields bicontinuous morphologies. By tuning quench depth, mobility, and interfacial energy, researchers can tailor nanoscale morphologies for functional materials.