Phase separation dynamics in colloid-polymer mixtures: the effect of interaction range

TL;DR

This study investigates how the range of attraction influences phase separation and gelation in colloid-polymer mixtures, revealing a transition from fluid-fluid separation to gelation with deeper quenches and identifying distinct dynamic regimes.

Contribution

It provides a real-space analysis of phase separation dynamics across different attraction ranges, highlighting the crossover from classical to viscoelastic phase separation.

Findings

Fluid-fluid phase separation persists over a wide range of attraction lengths.

Deeper quenches lead to gel formation through a continuous crossover.

Surface particles exhibit higher mobility, aiding coarsening.

Abstract

Colloid-polymer mixtures may undergo either fluid-fluid phase separation or gelation. This depends on the depth of the quench (polymer concentration) and polymer-colloid size ratio. We present a real-space study of dynamics in phase separating colloid-polymer mixtures with medium- to long-range attractions (polymer-colloid size ratio q_R=0.45-0.89, with the aim of understanding the mechanism of gelation as the range of the attraction is changed. In contrast to previous studies of short-range attractive systems, where gelation occurs shortly after crossing the equilibrium phase boundary, we find a substantial region of fluid-fluid phase separation. On deeper quenches the system undergoes a continuous crossover to gel formation. We identify two regimes, `classical' phase separation, where single particle relaxation is faster than the dynamics of phase separation, and `viscoelastic' phase separation, where demixing is slowed down appreciably due to slow dynamics in the colloid-rich phase. Particles at the surface of the strands of the network exhibit significantly greater mobility than those buried inside the gel strand which presents a method for coarsening.