Metastable Self-Assembly of Theta-Shaped Colloids and Twinning of Their Crystal Phases

Anisotropic colloids self-assemble into different crystal structures compared to spherical colloids. Exploring and understanding their self-assembly behavior could lead to creation of new materials with hierarchical structures through a bottom-up process. Herein, we report metastable self-assembly of theta-shaped SiO₂ colloids interacting with a depletion force in a quasi-two-dimensional space and we demonstrate that both a metastable "prone" crystal phase and a stable "standing" crystal phase can be formed, depending on the self-assembly path. Path selection stems from an interplay between particle-particle interactions and particle-wall interactions. In particular, a twinning of the metastable crystals was observed and two twinning mirror axes were found. A variety of complex twinned crystals were formed by each individual mirror axis or their combinations.