Evidence for out-of-equilibrium crystal nucleation in suspensions of oppositely charged colloids

TL;DR

This study reveals that in oppositely charged colloid suspensions, the crystal phase that forms is metastable and does not have the lowest nucleation barrier, indicating out-of-equilibrium nucleation behavior.

Contribution

It provides numerical evidence that crystal nucleation in such colloids can favor metastable phases with higher barriers, challenging classical nucleation assumptions.

Findings

Metastable crystal phase nucleates despite higher free-energy barrier.

Nucleation does not always select the lowest free-energy structure.

Competing crystal structures influence nucleation pathways.

Abstract

We report a numerical study of the rate of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for sub-critical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation.