Surface premelting of ice far below the triple point

Area of Science:

• Materials Science
• Physical Chemistry
• Nanotechnology

Background:

• Surface premelting, forming a quasi-liquid layer (QLL) below the melting point, has been studied extensively on hexagonal ice near the triple point.
• The persistence of premelting at lower temperatures and pressures, where stacking disordered ice directly sublimes, remains largely unexplored.

Purpose of the Study:

• To investigate surface premelting phenomena on ice nanocrystals at conditions far from the normal triple point.
• To characterize the nature and behavior of the quasi-liquid layer under these specific low-temperature, low-pressure conditions.

Main Methods:

• Direct observation of ice nanocrystals using in situ transmission electron microscopy (TEM).
• Utilized electron energy loss spectroscopy (EELS) to confirm the nature of the quasi-liquid layer.

Main Results:

• Directly observed surface premelting, forming a QLL at the solid-vapor interface of ice nanocrystals below sublimation temperature.
• The QLL was found to preferentially decorate specific facets, with its thickness increasing closer to the phase transition temperature.
• In situ TEM demonstrated significant diffusion of the QLL, and EELS confirmed its amorphous structure, potentially linked to ultraviscous water.

Conclusions:

• Surface premelting occurs on ice nanocrystals even at low temperatures and pressures, extending previous observations on hexagonal ice.
• The observed premelting is likely associated with metastable low-density ultraviscous water, distinct from ambient liquid water.
• This study provides new insights into premelting and glassy liquid states away from the conventional water triple point.