Weakly polydisperse systems: Perturbative phase diagrams that include the critical region

The phase behavior of a weakly polydisperse system, such as a colloid with a small spread of particle sizes, can be related perturbatively to that of its monodisperse counterpart. I show how this approach can be generalized to remain well behaved near critical points, avoiding the divergences of existing methods and giving access to some of the key qualitative features of polydisperse phase equilibria. The analysis explains also why in purely size-polydisperse systems the critical point is, unusually, located very near the maximum of the cloud and shadow curves.