Out-of-equilibrium processes in suspensions of oppositely charged colloids: liquid-to-crystal nucleation and gel formation

TL;DR

This study investigates the kinetics of liquid-to-crystal and gel formation in oppositely charged colloids through simulations and experiments, revealing different pathways depending on temperature and packing fraction, and highlighting the role of interaction range.

Contribution

It provides new insights into the pathways and conditions for crystallization and gelation in charged colloids, combining experimental and simulation data.

Findings

Fluid-to-crystal pathway varies with temperature and packing fraction.

Gel formation occurs via arrested spinodal decomposition at low temperatures.

Increasing interaction range favors crystallization over vitrification.

Abstract

We study the kinetics of the liquid-to-crystal transformation and of gel formation in colloidal suspensions of oppositely charged particles. We analyse, by means of both computer simulations and experiments, the evolution of a fluid quenched to a state point of the phase diagram where the most stable state is either a homogeneous crystalline solid or a solid phase in contact with a dilute gas. On the one hand, at high temperatures and high packing fractions, close to an ordered-solid/disordered-solid coexistence line, we find that the fluid-to-crystal pathway does not follow the minimum free energy route. On the other hand, a quench to a state point far from the ordered-crystal/disordered-crystal coexistence border is followed by a fluid-to-solid transition through the minimum free energy pathway. At low temperatures and packing fractions we observe that the system undergoes a gas-liquid spinodal decomposition that, at some point, arrests giving rise to a gel-like structure. Both our simulations and experiments suggest that increasing the interaction range favors crystallization over vitrification in gel-like structures.