Spinodal fractionation in a polydisperse square well fluid

TL;DR

This study uses Kinetic Monte Carlo simulations to explore how particle sizes separate during gas-liquid spinodal decomposition in a polydisperse colloidal fluid, revealing that fractionation can be reversed by subtle potential changes.

Contribution

It demonstrates that minor potential differences can reverse size fractionation directions, supported by a perturbative equilibrium theory, providing new insights into colloidal phase separation.

Findings

Fractionation begins shortly after phase ordering.

Size fractionation direction can be reversed by potential choice.

Kinetic Monte Carlo effectively models fractionation in colloids.

Abstract

Using Kinetic Monte Carlo simulation, we model gas-liquid spinodal decomposition in a size-polydisperse square well fluid, representing a 'near-monodisperse' colloidal dispersion. We find that fractionation (demixing) of particle sizes between the phases begins asserting itself shortly after the onset of phase ordering. Strikingly, the direction of size fractionation can be reversed by a seemingly trivial choice between two inter-particle potentials which, in the monodisperse case, are identical -- we rationalise this in terms of a perturbative, equilibrium theory of polydispersity. Furthermore, our quantitative results show that Kinetic Monte Carlo simulation can provide detailed insight into the role of fractionation in real colloidal systems.