Rhythmic crystal growth into hierarchical patterns by polymer-mediated self-assembly.

Controlled, bottom-up self-assembly of ordered and hierarchical structures remains a major challenge and increasingly attracts attention in basic and technology-driven research. A simple process is described for the generation of such structures, which is based on slow solvent evaporation of a polymer solution blended with a crystal-forming species (Krogmann's salt). Upon drying, the viscosity of the polymer-blend solution increases in a progressing solidification zone, which precisely controls crystal growth by limiting the transport of the crystallizing units through this gel-like solidification zone and gives rise to a position- and time-dependent diffusion rate. The progressing solidification zone also leads to a preferential crystallographic orientation on a centimeter scale and introduces an instability that drives spatial pattern formation and hierarchical ordering on five distinct levels, ranging from the atomic positions in crystals to the assembly on a microscale and up to a centimeter length scale. Together with a quantitative description, the presented findings are envisaged to improve the understanding and application of periodic precipitation processes.