Real-Time Strengthening of Natural Convection and Dendrite Fragmentation During Binary Mixture Freezing

Natural convection is a density-driven flow within the fluid in the presence of gravity. In thermally stable systems, i.e., those with a colder temperature at the bottom, natural convection is primarily driven by dissolved solutes. The phenomenon of freezing mixtures offers an ideal case for studying such flows. The presence of small-scale solid-phase structures further enhances the complexity. The present work shows the evolution and strengthening of natural convection and its effects, such as the fragmentation of growing solids while freezing binary alloys. Furthermore, these fragmented solids remelt in bulk due to composition and temperature differences. The work highlights fundamental aspects of an interplay between the compositional and thermal distributions of freezing mixtures. In the experiment, we used an SCN-15 wt.% acetone mixture, where SCN (985 kg/m³) solidifies during bottom-cooled solidification. Acetone (784 kg/m³)-enriched liquid is rejected in bulk, which is lower density, leading to solutal convection. Convective velocity magnitude is estimated by adding neutrally buoyant particles in binary solutions. The study has contributed to understanding convective flow in solidification, which relates to fluid dynamics, experimental techniques, and metallurgical aspects.