Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 344 / 2 / f / Solution

Both coexisting phases transport the conserved order parameter. The calculation above included only the harmonic field and normal current in the upper half-space. The lower half-space contributes an equal flux into the interface for equal mobilities and symmetric phases. The total interface speed is therefore twice the one-sided result, so the correct coefficient is