Asymmetric Morphology of Ice Crystals during Freeze Casting: In Situ Experiments and Phase-Field Simulations

Ice crystals growing along the a-axis ⟨112̅0⟩ direction during freeze casting exhibit an asymmetrical morphology, which is related to the strong kinetic anisotropy of the ice–water interface. However, only symmetric kinetic anisotropy cannot cause an asymmetric morphology, which must originate from asymmetric factors. To study these factors, in situ experiments and phase-field simulations are conducted using water–0.58 wt % NaCl solutions. The results show that the uneven spacing and kinetic anisotropy are important factors contributing to the asymmetric morphology of the ice crystals. To validate the hypothesis, quantitative phase-field simulations are implemented by setting ice crystal seeds with equal spacing. The simulations demonstrate that ice crystals symmetrically grow for a long time, while after the tip splitting, the uneven spacing appears, leading to a nonuniform solute distribution and breaking the symmetry of ice crystals. Other phase-field simulations are adopted by eliminating kinetic anisotropy and designing a step kinetic anisotropy function. It is found that only slow ice-crystal growth along the c-axis ⟨0001⟩ and fast growth in other directions can maintain the asymmetrical morphology of ice crystals. This study not only identifies uneven spacing as the key asymmetric factor but also reveals the critical role of kinetic anisotropy in determining ice crystal morphology.