Hierarchical structure formation by crystal growth-front instabilities during ice templating

Area of Science:

• Materials Science
• Physics
• Chemical Engineering

Background:

• Directional solidification of aqueous solutions and slurries is a common method for creating cellular materials.
• This process involves phase separation of solutes or particles between growing ice lamellae.
• The resulting structures, known as freeze-cast materials, exhibit unique honeycomb-like porosity and complex surface features.

Purpose of the Study:

• To elucidate the formation mechanism of hierarchical structures in freeze-cast materials.
• To understand the role of ice crystal growth anisotropy in shaping these structures.
• To develop a scaling law for lamellar spacing in freeze-cast materials.

Main Methods:

• Directional freezing of binary water mixtures with Fickian diffusing solutes.
• Phase-field modeling of the directional solidification process.
• Analysis of ice crystal growth kinetics and interface instabilities.

Main Results:

• Identified slow faceted ice growth along the c-axis as responsible for the flat side of lamellae.
• Demonstrated that weakly anisotropic fast growth in other directions creates unilateral, complex surface features.
• Revealed diffusion-controlled primary instabilities forming cellular structures and secondary instabilities creating intricate patterns.
• Derived a scaling law for lamellar spacing dependent on growth rate and temperature gradient.

Conclusions:

• The complex hierarchical structures in freeze-cast materials are a result of anisotropic ice crystal growth and interface instabilities.
• The findings provide a fundamental understanding of structure formation in freeze-casting.
• The developed scaling law offers predictive capabilities for controlling lamellar spacing in cellular materials.