When solutions and slurries are directionally solidified, complex dynamics of solvent crystal growth and solvent templating determine the final hierarchical architecture of the freeze-cast material. With continuous X-ray tomoscopy, it is now possible to study in situ intricate and otherwise elusive ice crystal growth and solvent-templating phenomena. Quantifying these phenomena both time-resolved and in three dimensions provides novel insights into the formation of performance-defining features of freeze-cast cellular solids at several length scales: the material’s pore morphology (first hierarchical level), the molecular, fibrillar and particle self-assembly of components in the cell walls (second level) and the cell wall surface structures (third level). The freeze casting process is attractive because the features of the final hierarchical material architecture — which determine the material’s structural, mechanical and physical properties — can be custom designed for a given application. Overall porosity, pore size, geometry, orientation, particle packing in cell walls and cell wall surface features can be tailored for applications in, for example, biomedicine, environmental engineering, catalysis, power conversion, and energy generation and storage.

Directional solidification of aqueous solutions and slurries in a temperature gradient is widely used to produce cellular materials through a phase separation of solutes or suspended particles between growing ice lamellae. While this process has analogies to the directional solidification of metallurgical alloys, it forms very different hierarchical structures. The resulting honeycomb-like porosity of freeze-cast materials consists of regularly spaced, lamellar cell walls which frequently exhibit unilateral surface features of morphological complexity reminiscent of living forms, all of which are unknown in metallurgical structures. While the strong anisotropy of ice-crystal growth has been hypothesized to play a role in shaping those structures, the mechanism by which they form has remained elusive. By directionally freezing binary water mixtures containing small solutes obeying Fickian diffusion, and phase-field modeling of those experiments, we reveal how those structures form. We show that the flat side of lamellae forms because of slow faceted ice-crystal growth along the c-axis, while weakly anisotropic fast growth in other directions, including the basal plane, is responsible for the unilateral features. Diffusion-controlled morphological primary instabilities on the solid-liquid interface form a cellular structure on the atomically rough side of the lamellae, which template regularly spaced “ridges” while secondary instabilities of this structure are responsible for the more complex features. Collating the results, we obtain a scaling law for the lamellar spacing, λ∼(VG)-1/2 , whereVandGare the local growth rate and temperature gradient, respectively.