Two-step crystallization and solid-solid transitions in binary colloidal mixtures*

The pathways to colloidal self-assembly could be more complicated than we might naively expect. While the classical picture predicts that a crystal assembles by a fluctuation that kicks the system over a single free-energy barrier, a growing body of evidence shows that this process can proceed via multiple transformations of metastable structures towards the thermodynamic minimum. In this talk, I will present results of an experimental and computational study of the crystallization of a binary mixture of DNA-coated colloids. We observe both classical one-step pathways, and non-classical two-step pathways that proceed via a solid-solid transformation. We use enhanced sampling to compute free energy landscapes corresponding to our experiments, and find that both one- and two-step pathways are driven by thermodynamics alone. Specifically, the two-step transition is governed by a competition between different crystal phases with free energies that depend on the crystal size. These results extend our understanding of available pathways to crystallization and may provide new approaches to programming the self-assembly of materials made from colloids.