The Cahn-Hilliard equation ∂φ/∂t = M∇²(φ³ − φ − ε²∇²φ) describes phase separation.
Starting from a nearly uniform state, spinodal decomposition creates interpenetrating domains
that coarsen over time as t^(1/3) (Lifshitz-Slyozov law). Adjust ε to control interface width.