Visualizing kinetic pathways of homogeneous nucleation in colloidal crystallization

Abstract

When a system undergoes a transition from a liquid to a solid phase, it passes through multiple intermediate structures before reaching the final state. However, our knowledge on the exact pathways of this process is limited, mainly owing to the difficulty of realizing direct observations. Here, we experimentally study the evolution of symmetry and density for various colloidal systems during liquid-to-solid phase transitions, and visualize kinetic pathways with single-particle resolution. We observe the formation of relatively ordered precursor structures with different symmetries, which then convert into metastable solids. During this conversion, two major cross-symmetry pathways always occur, regardless of the final state and the interaction potential. In addition, we find a broad decoupling of density variation and symmetry development, and discover that nucleation rarely starts from the densest regions. These findings hold for all of our samples, suggesting the possibility of finding a unified picture for the complex crystallization kinetics in colloidal systems. Assemblies of colloidal particles provide a micrometre-scale analogue of atomic and molecular liquids and solids. Now, real-time visualization of the liquid-solid transition in systems of spherical colloids reveals complex pathways involving precursors of hexagonal close-packed, body-centred cubic and face-centred cubic symmetry.