Stable non-compact prenucleation clusters of attractive colloidal crystals observed directly†

Prenucleation clustering is key to understanding elementary processes of nucleation more precisely. Although in situ, real-time observation of the clustering processes of atomic or molecular crystals is generally difficult due to their very small sizes and extremely high mobilities, we directly observed prenucleation clusters of two-dimensional (2D) colloidal crystals of polystyrene particles in aqueous sodium polyacrylate solution. Cluster dynamics were tracked seamlessly prior to critical nuclei formation. Contrary to the expectation that compact clusters are more stable, we found that non-compact clusters were more prevalent among trimers. This suggests that non-compact clusters possess higher configurational entropy and lower Gibbs energy of cluster formation than compact clusters.