There are two methods for mixing. Species where the materials are in the same phase and can diffuse (as well as convect) and multiphase where the materials may mix in a cell but do not mix at a molecular level. The main difference is in species the cell density is taken from the mixture so mixed fluids won't then stratify. With multiphase each phase (material) retains it's own material properties so separation can occur. We'd typically use species in most situations, but there are instances where phases may be required.