Microscale Analysis of Crystals in Ice Slurry Made from an Antifreeze Protein Solution

Abstract

In order to clarify the crystal growth mechanism and to tealize the low-terperature heat transportation system of ice slurry made from an antifreeze protein (AFP) solution which consists of ice crystals resistant to recrystallization, a fundamental and microscale analysis has been conducted on the ice crystals. Since the thermophysical propertics of ice slurry used for ice storage applications, such as energy storage ability and flowability, depend on the shape and size of individual crystals, crystal growth patterns were experimentally investigated by changing the local supercooling temperature while neglecting the influence of heat flux. At low temperatures, when supercooling exceeded a certain transition value, dendritic crystals were generated, which were apparently unaffected by the existence of AFP. In between the transition terperature and the freezing point, needle-like crystals were observed to grow rapidly in the c-axis direction. If these needle-like crystals were held at terperatures within the hysteresis gap (between the freezing and the melting point), bipyramidal crystals within a maximum tip angle of approximately 30° were formed. Since protein adsorption to the ice crystal surface will strongly affect the crystal growth, the surface of crystals was investigated by using a Scanning Tunneling Microscope (STM) in order to determine the influence of AFP on the microscale surface struture. A systematic groov/ridge pattern that was aligned 65° (±5°) to the hexagonal side on one bipyramidal plane was observed. The grooves' length and width were similar to the length and width of AFP, indicating adsorption of single protein molecules to ice with an orientation corresponding to the alignment angle. Their depth, ranging from 2 nm to 10 nm, gives information about the surface curvature. Knowledge from microscale analysis can be used in order to create cost-effective artificial additives for ice slurry systems.