Interphase mass transfer (${\partial }_t({\alpha }_i{\rho }_i)+\nabla \cdot ({\alpha }_i{\rho }_i{u}_i)={\Gamma }_i,{\sum }_i{\Gamma }_i=0$)

The source ${\Gamma }_i$ is the mass gained by phase $i$ per mixture volume and time, through processes such as evaporation or condensation. Internal exchange conserves total mass, so its phase sources sum to zero. In an immiscible material-interface model without phase change, every ${\Gamma }_i$ is zero. The interface may still move: absence of transfer means that mass does not cross it relative to its own motion.