Spiraling crystallization creates layered biomorphs

Abstract

Biomorphs are micrometer-sized polycrystalline structures that, despite their inorganic nature, display smoothly curved, life-like shapes. The edges of substrate-bound biomorph sheets are known to trace logarithmic spirals and can be reproduced by reaction-diffusion fronts in subexcitable media where disrupted fronts shrink tangentially. While freely rotating Archimedean spirals do not exist in subexcitable systems, our numerical simulations show that pinning sites can stabilize the rotating front. This finding is confirmed experimentally for biomorph systems that, around pH 11, produce both logarithmic and Archimedean spirals. The latter are pinned to globular ${\mathrm{BaCO}}_{3}$ crystals and have a constant pitch in the range of 20 to $40\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Akin to screw dislocations, the Archimedean spirals consist of multiple layers of constant height. Their front speeds on glass and pre-existing biomorph substrate are essentially identical. We also report the detachment of intermittently pinned fronts at highly curved features of the pinning site.