Linear composition bias leaves coexistence compositions unchanged (source code)

= Linear composition bias leaves coexistence compositions unchanged

Adding $d\phi$ to a <free-energy density> adds a constant at fixed total <compositional order parameter>. In the <common-tangent construction for phase coexistence>, it shifts the tangent slope by $d$ while preserving its points of contact. Thus coexistence compositions are unchanged, whereas their <chemical potential> shifts. This does not assert unchanged equilibrium composition at a fixed externally imposed <chemical potential>.