Variants of the famous Modica-Mortola functional can be used to model phase separation in heterogeneous fluids by supposing a free energy of the fluid mixture to have periodic heterogeneities of the scale δ and the phase separation to happen on the scale of ε. The limiting energies depend on the rate of convergence between δ and ε (and many of the regimes are still open–especially with inhomogeneous fluid phases). We will survey the field and discuss recent results in the regime δ<< ε and ε<< δ.