Phase behaviour of very asymmetric binary mixtures

Abstract. The phase behaviour of very asymmetric binary mixtures can be understood in terms of the depletion interaction. For hard particles this yields a narrow deep attractive well surrounding the hard core. Colloids with similar interaction potentials are known to destabilize the liquid, causing it to show a wide fluid–solid coexistence, and in extreme cases they exhibit an exotic solid–solid condensation. For a mixture this means that phase separation is not fluid–fluid, as previously thought, but normally fluid–solid, and if the asymmetry is very large, even solid–solid. We present in this work the result of devising a density functional theory for an infinitely asymmetric mixture of parallel hard cubes. This model is singular and undergoes a collapse in a close-packed solid (an extreme fluid–solid demixing). We avoid this collapse by introducing a small amount of polydispersity in the large particles; the resulting phase diagram shows the fluid–solid and solid–solid demixing scenarios described above. When we have a system with more than one microscopic component (a mixture), the system can exist as a single, uniform phase, or it can undergo a phase transition and separate into two or more phases, each with a different composition. This phase transition is commonly referred to as demixing. One can think of a simple microscopic mechanism by which demixing ca