spinodal

Curve defining the region of composition and temperature for a binary mixture across which a transition occurs from conditions where single-phase mixtures are metastable to conditions where single-phase mixtures are unstable and undergo phase separation by spinodal decomposition.
Notes:
  1. The spinodal curve for a binary mixture is defined as the geometrical locus of all states with
    ( ∂ 2 Δ mix G ∂ ϕ 2 ) T , p = 0.
  2. In the unstable region bounded by the spinodal curve, phase domain separation is spontaneous, i.e., no nucleation step is required to initiate the separation process.
Source:
PAC, 2004, 76, 1985 (Definition of terms related to polymer blends, composites, and multiphase polymeric materials (IUPAC Recommendations 2004)) on page 1998