Interplay of flow stratification, segregation channels, and crystal dynamics in a solidifying aqueous ammonium chloride solution

• A new phenomenon that occurs at the exit of a flow channel that has formed in a dendritic mushy region has been described: the formations of indentations (bowl-shaped depressions) in the mushy zone layer at the exits of the channels. • It was observed that two different mechanisms exist that lead to crystal agglomerations at vertical mushy regions: crystals that descend along the forefront of the mushy layer i) may either stick to protruding dendrite tips or ii) sediment onto the lower edges of the indentations. These processes lead to crystal agglomerations that collapse when getting too large. • The subsequent impact of the sliding-down crystals with lower mushy zone regions leads to further fragmentation. • The corresponding crystal sedimentation is thus irregular and layers of sediment occur rather than a uniform sedimentation bed. • These observations are relevant in fields such as metallurgy, materials and environmental science, and geology, and could enhance the design of industrial processes and contribute to natural system modeling. An experimental investigation of the solidification dynamics of an aqueous ammonium chloride solution conducted using a relatively large rectangular test cell subject to vertical side cooling reveals several interesting phenomena. Besides the well-known concentration and flow stratification at the top part of the test cell, an increasing number of equiaxed crystals affects both the growth of the vertical mushy layer as well as the flow in the bulk melt. The rising melt flow in the solidifying mushy zone creates segregation channels and leads to the formation of indentations (bowl-shaped depressions) in the mushy zone layer at the exits of these channels. Crystals that descend along the forefront of the mushy layer may either stick to protruding dendrite tips or sediment onto the lower edges of the indentations. These processes lead to crystal agglomerations that collapse when getting too large. The subsequent impact of the sliding-down crystals with lower mushy zone regions leads to further fragmentation. The corresponding crystal sedimentation is thus irregular and layers of sediment occur rather than a uniform sedimentation bed. These observations are relevant in fields such as metallurgy, materials and environmental science, and geology, and could enhance the design of industrial processes and contribute to natural system modeling.