Real-time observation of the freezing stage of the mannitol solution using high-resolution optical microscopy

Abstract

Vacuum freeze-drying has been expanding its influence in many fields, and the freezing stage is of utmost importance for the freeze-drying process. However, the limited understanding of the freezing stage has posed a major challenge for developing the vacuum freeze-drying process. An experimental setup was designed and employed in this paper to experimentally investigate the freezing stage of the mannitol solution. The cooling rate and solute mass fraction are chosen as the key freezing parameters. The nucleation temperature, the morphology and velocity of the freezing front, the morphology and size distribution of the ice crystals at the combination of different freezing conditions were studied. The results show that the nucleation temperature (supercooling degree) distribution could be better described by the Gauss distribution. The morphology of the freezing front is more strongly affected by the cooling rate compared with the mass fraction. In addition, the morphology of ice crystals also exhibits differences under different freezing conditions. The average size decreases with the increase of the cooling rate, while the uniformity of particle size distribution increases when increasing the cooling rate. The research could help deepen the understanding of the freezing stage in a qualitative way, and provide a reference for the theoretical research.