Divergent Interfacial Lifetimes without Bulk Criticality in Fully Miscible Fluids

A fully miscible interface must disappear at equilibrium, but its lifetime need not be governed by a bulk instability. We construct a spatially resolved, dissipative two-field model in which a prepared correlation-excited interface undergoes a localized saddle-node while its homogeneous excited surroundings remain metastable and the relaxed mixture is strictly one phase. The interfacial soft mode produces a deterministic first-passage time that grows as the inverse square root of the distance from the fold and couples directly to the evolving concentration-profile width. The quartic structural potential admits an exact reduction from competing positive harmonic energies on a constrained configuration manifold; sixth- and eighth-order examples provide additional numerical robustness tests. The result is an interface-specific kinetic singularity without equilibrium bulk criticality, subject to explicitly stated microscopic and continuum limits.