Best face forward: crystal-face competition at the ice-water interface

Area of Science:

• Physical Chemistry
• Materials Science
• Crystallography

Background:

• The ice-water interface is crucial for biological and environmental processes.
• Hexagonal ice (Ih) is the most stable form of ice under ambient pressure.
• Determining the surface free energy of Ih ice faces is experimentally and theoretically challenging due to near-equal face energies and water's supercooling tendency.

Purpose of the Study:

• To experimentally determine the relative surface free energies of the major hexagonal ice (Ih) faces at the ice-water interface.
• To overcome challenges associated with water supercooling and kinetic control during ice crystallization.
• To identify the most stable ice-water interface at 0°C.

Main Methods:

• Circumventing supercooling by using a polycrystalline seed for isothermal, equilibrium growth.
• Employing natural selection among seeded faces to achieve single crystal growth.
• Analyzing the frozen boule's cross-section using crossed polarizers (to locate optical axis) and etching (to distinguish prism faces) to reveal crystal orientation.

Main Results:

• The secondary-prism face exhibits the highest surface free energy (most stable interface) at 0°C.
• The primary-prism face is the second most stable interface.
• The basal face, common in snowflakes, is the least stable of the three major faces at the ice-water interface under these conditions.

Conclusions:

• The most stable ice-water interface at 0°C is the secondary-prism face, contrary to expectations based on vapor-phase freezing.
• Results contrast with vapor-phase freezing, where basal and primary-prism faces have comparable free energies.
• This study provides a new understanding of ice crystal morphology and stability at the liquid interface.