Transport processes and coalescence of two entrapping bubbles during upward solidification

The transport processes and coalescence of a pair of bubbles during upward unidirectional solidification of water containing carbon dioxide are numerically investigated. In addition to affecting material microstructure, functional porous materials have applications in biology, tissue engineering, food preservation, and addressing global warming through porous sea ice. In this study, transport equations within the bubble, liquid, and solid phases are solved using the commercial COMSOL software (version 5.2). The results indicate that coalescence is promoted by fluid flow after bubbles make contact. Coalescence is likely when the concentration at the midpoint between two bubbles, each with identical concentrations, matches the concentration within the bubbles over a short distance. It is observed that bubble coalescence is enhanced by increases in horizontal incoming velocity, surface tension, and liquid solute diffusivity, as well as by decreases in Henry's law constant, ambient pressure, partition coefficient, and solid thermal conductivity. The solute concentration around pores also increases with decreasing Henry's law constant and liquid solute diffusivity, and increasing horizontal velocity, ambient pressure, solid thermal conductivity, and surface tension. The predicted contact angle during solidification aligns well with Abel's equation. The development of multiple pores and solute segregation inevitably occurs, influencing the material's microstructure. This can be managed by analyzing all the transport and metallurgical properties and adjusting the related working parameters accordingly.