Kaihua Ji1,Mingwang Zhong1,Kaiyang Yin2,Louise Littles3,Rohit Trivedi4,Ulrike G. K. Wegst1,Alain Karma1
Northeastern University1,University of Freiburg2,NASA Marshall Space Flight Center3,Iowa State University4
Kaihua Ji1,Mingwang Zhong1,Kaiyang Yin2,Louise Littles3,Rohit Trivedi4,Ulrike G. K. Wegst1,Alain Karma1
Northeastern University1,University of Freiburg2,NASA Marshall Space Flight Center3,Iowa State University4
Directional solidification of aqueous solutions and slurries in a temperature gradient has emerged as a promising method to produce cellular materials through a phase separation of solutes or suspended particles between growing ice lamellae. Those ice-templated cellular materials are hierarchically organized into a honeycomb-like porous structure on the largest scale, lamellar cell walls at an intermediate scale, and different unilateral surface features including ridges and other more complex shapes reminiscent of living forms that decorate the cell walls on a yet smaller scale. 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. We report the results of a detailed phase-field simulation study of ice templating of binary mixtures that reproduces the salient features of those cellular materials and that sheds light on the mechanism by which anisotropic ice crystal growth shapes their hierarchical structure. The simulation results reveal 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. The simulation results are compared with experimentally obtained results by directional freezing of binary water mixtures containing small solutes. Good quantitative agreement is found for the lamellar spacing that is shown to obey the scaling law λ ∼ (VG)<sup>-1/2</sup>, where V and G are the local growth rate and temperature gradient, respectively. Preliminary simulation results are presented that address the basic question of how other solvents such as DMSO, which exhibit different anisotropic crystal growth properties than ice, can produce a rich variety of hierarchical structures.