Crystallisation of undercooled aqueous solutions: Experimental study of free dendritic growth in cylindrical geometry

摘要

This paper reports a study of crystallisation kinetics in small volumes of undercooled water-MPG (monopropylene glycol) mixture. The experimental cell is a vertical cylinder (height 5 mm, diameter 2r_e = 7.5 mm); its bottom section is closed by a Plexiglas disc that transmits light from the lower part of the cylinder to a high-speed digital camera. Photographic recordings allow the determination of the crystal growth rate. When the antifreeze mass fraction is below 25 wt%, crystallisation is clearly divided into two stages: the growth of dendritic crystals in the undercooled solution followed by the passage of the interdendritic solidification front. Dendrite growth induces a sudden temperature increase in the mixture, while the passage of the interdendritic solidification front determines the time at which sensible heat effects again predominate. The results show that the dendrite growth rate is an increasing function of the degree of undercooling and a decreasing function of the antifreeze mass fraction.