 Spinodal

The spinodal is the boundary where a metastable phase loses local stability. In a mean-field free-energy description the relevant curvature vanishes there, separating local minima from unstable configurations.

The term "spinodal" is primarily used in the context of thermodynamics, specifically in phase separation processes in materials science and physical chemistry. It refers to a specific type of phase transition that occurs when a mixture is unstable and can separate into two or more distinct phases. In a broader sense, spinodal decomposition is a mechanism that describes how a homogeneous mixture becomes unstable. The spinodal curve defines the limits within which phase separation can occur spontaneously, even without nucleation.