Multiple Objects Interacting with a Solidification Front

The interaction of foreign objects suspended in a liquid melt with an advancing solidification front is of special interest in nature (e.g. frost heave) and engineering sciences (e.g. crystal growth). The front can engulf the object, trapping it into the growing crystal, or the front can repel the object, pushing it ahead of itself. Therefore, the object-front confrontation can have a strong influence on the properties of the solidified material. In particular, the spatial distribution of the objects in the resulting microstructure determines the material's structural and functional properties. The past theoretical models and experimental studies have mostly investigated the interaction of isolated, spherical, and hard objects in pure melts. However, the outcome of object-front interactions in complex (more realistic) systems, where multiple objects and solutes are present, is still poorly understood. Here we show the interaction of multiple oil droplets with an ice-water front in the absence and presence of solute effects using in situ cryo-confocal microscopy. We observe the formation of a compact agglomerated layer resulting in a force equilibrium different from the isolated object approach. We elucidate the role of solute during the evolution of a material microstructure in the presence of foreign objects. We report on how the object size, number of objects, and bulk solute concentration influence the front morphology and the subsequent object spatial distribution. Our results depict how the presence of multiple objects with varying solute concentration can modify the object-front interactions and hence, can lead to the formation of complex microstructures, difficult to predict theoretically. We suggest that the volume fraction of objects suspended in a liquid melt in conjunction with the amount of bulk solute concentration are two important criteria to be incorporated in the development of object-front interaction models. Furthermore, our simplified approach of using oil-in-water emulsions can serve as a good analogue for studying the development of material microstructure in presence of foreign objects.