Polydispersity Effects in Colloid-Polymer Mixtures

TL;DR

This study investigates how polydispersity influences phase separation and gelation in colloid-polymer mixtures, revealing unique coexistence behaviors and kinetic effects not observed in less polydisperse systems.

Contribution

It provides new insights into the phase behavior and kinetics of highly polydisperse colloid-polymer mixtures, including re-entrant coexistence and metastable binodal effects.

Findings

Polydispersity leads to a re-entrant fluid-solid coexistence shape.

A metastable gas-liquid binodal causes two-step crystallization.

High colloid volume fractions kinetically suppress multiple solid phase separation.

Abstract

We study phase separation and transient gelation in a mixture consisting of polydisperse colloids and non-adsorbing polymers, where the ratio of the average size of the polymer to that of the colloid is approximately 0.063. Unlike what has been reported previously for mixtures with somewhat lower colloid polydispersity, the addition of polymers does not expand the fluid-solid coexistence region. Instead, we find a region of fluid-solid coexistence which has an approximately constant width but an unexpected re-entrant shape. We detect the presence of a metastable gas-liquid binodal, which gives rise to two-stepped crystallization kinetics that can be rationalized as the effect of fractionation. Finally, we find that the separation into multiple coexisting solid phases at high colloid volume fractions predicted by equilibrium statistical mechanics is kinetically suppressed before the system reaches dynamical arrest.